N a t i v e
V e g e t a t i o n
A d v i s o r y
C o u n c i l
THE ECOLOGICAL ROLE OF
THE NATIVE VEGETATION OF NEW SOUTH WALES
A background paper of the Native Vegetation Advisory Council of New South Wales
Peter L. Smith, Brian Wilson, Chris Nadolny, Des Lang Department of Land & Water Conservation ISBN 0 7347 5128 1
Background paper number 2 November 2000 © Copyright Department of Land and Water Conservation on behalf of the Native Vegetation Advisory Council of New South Wales. This material is copyright. Any portion may be reproduced by any process with due acknowledgment.
Cover photograph by Jaime Plaza - Royal Botanic Gardens Sydney
1
The role of the Native Vegetation Advisory Council The Native Vegetation Advisory Council was
range of interests and includes rural
established in 1998 by the Native Vegetation
representatives, conservation groups, and
Conservation Act 1997 to take a pro-active role in
government agencies.
advising the NSW Government on native vegetation
2
issues throughout the State. The Council’s role is to
One of the key tasks of the Native Vegetation
foster relationships between community groups,
Advisory Council is to advise the Minister for Land
landholders and government agencies; develop
and Water Conservation on a Native Vegetation
initiatives in native vegetation management;
Conservation Strategy for NSW. This paper is one
promote the benefits of native vegetation; provide
of a series of background papers to be published by
direction and leadership in native vegetation
the Native Vegetation Advisory Council. These seek
management; and create incentives and coordinate
to stimulate discussion regarding native vegetation
funding opportunities for native vegetation
conservation and management and, in particular,
conservation. The membership of the Native
inform the development of the Native Vegetation
Vegetation Advisory Council represents a wide
Conservation Strategy for NSW.
x
CONTENTS Summary ........................................................................................... 4 1.Introduction ................................................................................... 5 2.Ecological role of native vegetation ............................................ 6 2.1 Ecosystem processes ................................................................... 6 2.1.1 Climate, water and native vegetation ................................... 6 2.1.2 Soils and native vegetation ................................................... 9 2.2 Biodiversity and native vegetation ............................................. 12 2.2.1 What is ‘biodiversity’? ........................................................ 12 2.2.2 Ecosystem function ............................................................ 14 2.3 Other values of native vegetation .............................................. 15 2.3.1 Native vegetation and connectivity ..................................... 15 2.3.2 Productive benefits ............................................................. 15 2.3.3 Grazing .............................................................................. 15 2.3.4 Forestry .............................................................................. 15 2.3.5 Genetic material with economic potential ........................... 16 3.Consequences of native vegetation loss and degradation ...... 17 3.1 Impacts on ecosystem processes ............................................... 17 3.1.1 Threats to water balance .................................................... 17 3.1.2 Land degradation ............................................................... 18 3.1.3 Loss of soil fertility .............................................................. 19 3.1.4 Increasing soil salinity ......................................................... 20 3.1.5 Increasing soil acidity .......................................................... 22 3.2 Impacts on biodiversity ............................................................. 22 3.2.1 Habitat loss, fragmentation and modification ........................ 23 3.2.2 Changes in vegetation disturbance regimes ........................ 27 3.2.3 The combined effect of habitat loss, fragmentation and modification on biodiversity ........................................ 32 3.3 Other threats to biodiversity in native vegetation ......................... 33 3.3.1 Exotic invasions .................................................................. 33 3.3.2 Pollution ............................................................................ 36 4.Conserving native vegetation and its ecological values ......... 37 4.1 The role of nature conservation reserves ................................... 37 4.2 Conservation outside the conservation reserve system ................. 38 4.2.1 Incentives and rural restructuring ........................................ 38 4.2.2 Ecological research and development ................................. 38 4.2.3 The role of legislation ......................................................... 40 4.3 The role of planning ................................................................. 41 4.3.1 Setting achievable conservation goals ................................. 41 4.4 On-ground actions to conserve biodiversity ............................... 43 4.5 Catering for the special needs of threatened species ................. 44 4.6 Restoration of native vegetation for ecosystem processes .......... 44 5. Conclusions ................................................................................. 46 6. References ................................................................................... 47
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SUMMARY Native vegetation provides many benefits principally
Degradation and loss of native vegetation resulting
through the protection of the land surface,
from human activity has altered and disturbed
amelioration or modification of local climate,
many of these ecosystem processes. Broad-scale
maintenance of critical ecosystem processes,
loss of vegetation cover has led to considerable
conservation of biodiversity, enhancement and
land degradation by exposing the land surface to
protection of cultural and aesthetic values, and the
wind and rainfall, which greatly increases soil
provision of economically important products such
erosion. These problems are exacerbated by some
as timber and grazing forage. However, significant
agricultural management practices, which cause
degradation and loss of native vegetation has taken
loss of soil organic matter and nutrient decline and,
place since European settlement, principally as a
in some places, increasing soil salinity and acidity.
result of human activity. This paper discusses native
For example, it has been estimated that some
vegetation with respect to the ecological processes
120,000 ha of NSW are currently affected by
that it supports and the benefits derived from its
salinity and that 7.5 million ha could potentially
retention in the landscape.
become salt affected (PMSEIC 1999). Disturbance of ecosystem function, fragmentation
By sheltering the land surface, vegetation cover moderates local climate and water movement. Vegetation cover absorbs some of the wind’s energy and consequently, wind speeds under the canopy of dense vegetation are usually very low. Even scattered trees contribute to the reduction of wind speed and a 10-15% scattered tree cover can
of habitats, the introduction of foreign species and ecologically unsympathetic agricultural systems has been widespread. As a consequence, species decline and extinction has been marked, while altered community balance has frequently led to the unchecked and damaging spread of exotic plants and animals.
protect livestock and pastures from climatic extremes without reducing overall productivity (Bird
Halting the decline in native vegetation cover and
et al. 1992). Reduction in wind speeds from tall
rectifying some of the damage that has been done
and dense native vegetation significantly reduces
is not an easy task, but is possible. Nature reserves
wind erosion on cropping and grazing lands and a
have a vital role in this recovery but are only part of
dense and species diverse ground layer of summer
the solution. We also need sympathetic
and winter active grasses and herbs greatly reduces
management of privately owned native vegetation,
soil erosion from raindrop and surface water flow.
and it should be recognised that such management can offer production benefits by preserving land
Native vegetation plays an important role in many ecosystem processes. These processes include nutrient retention and cycling, carbon storage, purification of water and the maintenance of viable
4
and water quality. Restoration of vegetation cover also provides considerable potential for improved ecosystem function, increased biodiversity and better health for the wider environment.
and diverse populations of important components
Funding to restore and manage native vegetation is
of biodiversity such as detritivores (organisms that
scarce while the problems are large and complex.
break down organic matter) pollinators and
Detailed scientific knowledge and information on
parasites and predators of farm pests. For example
the management and restoration of native
the life cycle of some parasitic wasps and flies
vegetation will be essential if we are to effectively
depend on nearby sources of food found in native
target the problems on a landscape level. Finally, a
vegetation. Some species of these parasites seldom
concerted effort from both government and the
travel more than 200 metres from such sources of
community will be necessary to reverse native
food (Davidson and Davidson 1992).
vegetation decline in NSW.
SECTION ONE
INTRODUCTION This background paper is the second in a series of
also constitutes a significant component of the
seven that are intended to inform the development
biodiversity of the State in the habitats it provides.
of a Native Vegetation Conservation Strategy for
However, current management of the landscape is
NSW. The first background paper (Benson 1999)
threatening these values. European settlement has
provided an overview of the status of native
resulted in the clearance of extensive areas of the
vegetation. This second paper explores the role of
original vegetation (Benson 1999) and significant
native vegetation in terms of ecosystem processes
disturbance (through grazing, thinning and so on)
and the conservation of biodiversity. Threats to
of the remainder (Conacher and Conacher 1995).
native vegetation are considered, as are the ecological implications of its degradation and loss for plants, animals and the wider landscape. Ecological principles underlying vegetation planning, management and restoration are discussed along with the actions required to ensure conservation of native vegetation and its ecological values.
The loss and disturbance of native vegetation has significant effects. It has been linked to most forms of land degradation including soil erosion, loss of soil fertility and dryland salinity, as well as to the alarming decline in biodiversity (see Benson 1999). In many cases, these effects have occurred slowly over time but there are instances where change has
The values of native vegetation are many and
been catastrophic. The production, economic and
diverse. Native vegetation plays a major ecological
biodiversity benefits of native vegetation have often
role in maintaining the quality of soil, water and air
been ignored in the agricultural and urban
and also has value for production by providing
development of NSW. Action is urgently needed to
shade and shelter for livestock while supporting
arrest and, where possible, reverse the negative
wildlife that helps control pests. Native vegetation
effects of native vegetation disturbance and loss.
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SECTION TWO
ECOLOGICAL ROLE OF NATIVE VEGETATION 2.1 Ecosystem processes
shrubs can greatly influence local light penetration and wind speed. For example, shading by
2.1.1 Climate, water and native vegetation
vegetation usually reduces light intensity in the
The diversity of native vegetation communities in
depends on the density and characteristics of foliage
NSW reflects the wide range of climatic conditions.
and the size and position of gaps in the canopy.
Temperatures in NSW range from alpine in the
Shading is generally limited in eucalypt forests
Snowy Mountains to sub-tropical in the northeast,
where trees have vertically hanging leaves and their
while annual rainfall progressively decreases from
crowns seldom touch. Direct radiation in an open
east to west (Figure 1). Although the moisture
eucalypt forest in Queensland was reduced to 38%
regime is characterised by broad seasonal trends, it
of that outside the forest and diffuse radiation was
is neither well defined nor predictable. Rainfall is
reduced to 66% (Turton and Duff 1992), compared
erratic and episodic (Figure 2), and regular, balanced
with 3% and 10% for direct and diffuse radiation in
moisture regimes are relatively rare in NSW. Less
adjacent mature rainforest. Such differences are
than 200 years of climatic data exist in Australia and
ecologically significant because plant and animal
this lack of definitive documentation of historical
species vary in their sensitivity to and requirements
climatic fluctuations limits a proper understanding
for light.
of the climatic regime, in particular the occurrence and frequency of extreme events.
understorey although the extent of this reduction
Vegetation cover can absorb some of the wind’s energy and reduce its speed and consequently, wind
Much of the Australian vegetation, and the life
speeds under the canopy of dense vegetation are
forms associated with it, have developed highly
usually very low. Even in grazing country, scattered
specific ecological relationships for growth and
trees contribute to the reduction of wind speed and
survival as an adaptation to wide climatic
a 10-15% scattered tree cover can protect livestock
fluctuations and lack of regular and distinct
and pastures from climatic extremes without
seasonality. Their sclerophyllous, drooping leaves are
reducing overall productivity (Bird et al. 1992).
well adapted to abundant solar radiation and
Although remnant vegetation in cropping areas is
frequent irregular dry periods. Leaves may also be
largely confined to linear strips on roadsides,
shed during severe droughts to reduce transpiration.
riverbanks, and stock routes, it can considerably
In the past, the description of climate and
reduce crop damage by reducing wind speed, while
assessment of ecological relationships has been
marginally improving crop growth rates (Cleugh et
governed by traditional analytical techniques and
al. 1998). A well-designed windbreak can protect
approaches. This has hindered the development of
an area up to 20 times its height and significantly
appropriate sustainable land-management strategies
enhance production (Bird et al. 1992, Gregory
which lead to the appreciation and conservation of
1995, Reid and Thompson 1999).
native vegetation. Only recently has it been recognised that there is a need for more appropriate climatic analytical techniques and for a better analysis of ecosystems function (Mussared 1997).
Vegetation has a moderating effect on air temperatures by limiting the occurrence of extreme high or low temperatures. For example, under rainforest near Dorrigo, temperatures were generally 5-25°C over a four-year period, and never exceeded
6
Local climate: vegetation interactions
30°C or fell below 0°C (Nadolny 1999). Outside the
Despite the broad influence of climate on vegetation
winter minimums below zero were common.
distribution, climate can itself be affected by
Vegetation cover also reduces the severity and local
vegetation at a local scale (microclimate). Trees and
distribution of frosts. For example, trees and dense
forest, summer temperatures above 30°C and
Lismore Bourke
Broken Hill
Dubbo
Wagga Wagga
Figure 1 Average rainfall in NSW (mm)
Lismore Bourke
Broken Hill
Dubbo
Wagga Wagga
Figure 2 Rainfall variability in NSW
vegetation can block the flow of cold air and thus
Vegetation cover influences local rainfall inputs
reduce the incidence of frosts on most sites,
through a number of mechanisms (see Figure 3).
although it can cause pockets of cold air to form in
Some of the rainfall is intercepted by the canopy
bare areas just up-hill from bands of dense
and evaporated directly back to the atmosphere,
vegetation.
thus reducing the incidence of rainfall at the soil
7
surface. The proportion of rain intercepted in this
modelling is required to examine the extent to
way depends on the rainfall event and the
which the same trends occur in Australia.
characteristics and density of the foliage. By contrast, plants can increase ‘effective precipitation’
Water and native vegetation
(that is, the amount of water reaching the ground)
The water-use characteristics of native vegetation
when water from fog, mist or dew condenses on
are quite different to that of many introduced crops,
plant leaves and falls to the ground (fog drip). This
pastures and trees. Water use by native species is
process provides an important source of moisture
adapted to the episodic and erratic NSW climate,
for many vegetation types.
and to maintaining stable and robust land and aquatic ecosystems. Prior to European settlement,
Regional climate: vegetation interactions
the non-arid sections of the Australian landscape
There is evidence that wooded areas of even a few
perennial woody, and perennial and annual
square kilometres can influence the distribution of
herbaceous species (Oxley 1820, Cunningham
rainfall, probably by increasing the upward flow of
1824). These communities appear to have
moisture-laden air due to the greater absorption of
efficiently utilised available water (Timms 1998,
solar energy by the forest. However, it is uncertain
Hayman 1998), and water added to the surface in
whether regional rainfall is just displaced or actually
rainfall was largely used in plant growth. Additions
increased (Cotton and Pielke 1995). For example, in
of water to the water table by deep drainage would
the Western Australian wheatbelt, average winter
therefore have been limited, so water tables would
rainfall decreased by 4-5% over cleared portions in
have remained largely at a depth below the root
the last 70 years, and simultaneously increased by
zone.
about 6% over uncleared, semi-arid woodlands to the east (Pockley 1993). In India, studies comparing recently deforested and intact catchments reported
appear to have been dominated by mixtures of
Vegetation cover has a significant role in protecting the soil against the erosive forces of rainfall and in
that, while total rainfall was not significantly affected by deforestation, the number of rain events was reduced and their intensity increased, leading to more droughts and more floods (Meher-Homji 1988). The possibility of such a trend in Australia has not been properly investigated. Computer simulation models have also been used to study the possible effects of changing vegetation cover on climate. For example, a computer-based study of the Amazon basin in South America showed that where rainforests were replaced with grassland, total rainfall would be expected to decline to the extent that the re-establishment of rainforests would be impossible in most areas (Shukla et al. 1990). This is because most of the rain that falls in inland areas originates from water transpired by existing forests rather than moisture derived from the sea. If the forests were cleared, most of this water would run off or drain to a depth where it could not be used by plants, and so it would no longer be available for rapid circulation back to the atmosphere (Hayden 1998). Similar
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Figure 3 The water cycle at the atmospheric-land surface interface
Box 1: Vegetation and climate change Long-term climatic trends in NSW, as elsewhere, have been subject to considerable variability in the past (for example, Torok and Nicholls 1996, Lavery et al. 1997) and this variability makes it difficult to quantify the extent of contemporary climate change. Most scientists agree that recent increases in greenhouse gas concentrations will have a significant effect on the earth’s climate although the magnitude of the effect and its likely impact are still uncertain. Quantification of effects is based on computer models that can only account for some of the many complexities of the climatic system, but these suggest that extreme rainfall, flooding and drought events may become more common in NSW. Other aspects of climate change, such as impacts on diurnal temperature fluctuations, incidence of El niño and the resultant sea level rise, are harder to predict (Hennessy et al. 1995). Vegetation clearance throughout the world probably contributes somewhere between 9 and 40% to emissions of carbon dioxide caused by human activity (Houghton 1997) and in Australia, native vegetation clearance probably contributes between 13 and 18% of total CO2 emissions (Rawson and Murphy 2000). Native vegetation has considerable potential to accumulate and store carbon both in plant biomass and in organic matter accumulations in soil. This means that retention or expansion of native vegetation cover may have considerable value with regard to carbon storage and carbon trading (see Rawson and Murphy 2000).
minimising erosion and loss of nutrients in runoff
l
textural contrast in ‘duplex’ soils, which have a
water. It plays a dominant role in determining the
sub-surface, clay layer that impedes water
quantity of rainfall incident on the soil surface and
movement and root growth
hence the amount that is available in the soil or
l
overland flow. Excessive loss or disturbance of
weathered parent materials that produce highly erodible, compact or cracking, fine textured
native vegetation may increase runoff rates,
soils
decrease landscape stability, increase nutrient loss in runoff water, and seriously affect ecosystem function due to an altered water regime.
l
calcareous, sodic or saline soils in which plant growth can be restricted by surface salt accumulations.
2.1.2 Soils and native vegetation Australian soils differ from those of the northern hemisphere mainly because they are very old (Lindsay 1985) and reflect millions of years of weathering and reworking (White 1986, 1994). With a few notable exceptions, such as the black
However, poor soil quality has led to a number of unique adaptations in Australian plants that enable native species to efficiently use the limited resources that are available. (see Figure 4 Soil Nutrient Cycling)
Soil organic matter
soils of the Liverpool Plains or the basalt soils on the
Vegetation cover usually contributes organic
east coast, Australian soils tend to be of relatively
matter to the soil. This organic matter is a vital
poor quality for plant growth. Poor soil quality can
component of the land surface and contributes
be caused by many factors including:
to the biodiversity and health of the soil. For
l
strong leaching, low-nutrient status and high acidity, which are common in soils in the east of the State
l
example, the water-holding capacity of organic matter is up to 100 times that of mineral soil material and so, retention of a significant organic component in soil greatly enhances its water-
parent materials that are coarse, weakly
holding capacity. Soil organic matter also
structured and low in nutrients and moisture
represents a significant store of carbon and
9
Box 2: Soil water Soil water exists in three states: l
Hygroscopic water is water that is tightly bound to fine soil particles. This water is generally unavailable to plants and most soil organisms and is removed only by evaporation.
l
Capillary moisture (Plant Available Soil Moisture) is held by surface tension to the surface of soil particles and organic matter. Such moisture can be moved within the soil by tension exerted by plant roots or capillary action and is available for plants and other soil organisms.
l
Gravitational water is water not linked to the soil and is free to move downwards under the influence of gravity. Where drainage is restricted, and water inflow exceeds drainage capacity, the soil becomes waterlogged and anaerobic conditions can result. Anaerobic conditions have a profound effect on soil development, nutrient availability and cycling, as seen in natural wetlands, and on the plants and organisms that can survive in affected sites.
nutrients (Keith 1997) and protects the soil
being more typical. This figure declines significantly
surface by reducing the effects of soil erosion.
westward with decreasing moisture availability. In the
Organic matter is added to the soil principally through the addition of litter at the soil surface. Annual litterfall varies enormously between vegetation types and depends on nutrient supply, moisture regime and other factors. However, estimates suggest that average litterfall can range between 1 tonnes per hectare in mallee woodland
rangelands of the western region, for example, soil organic matter content is usually below 1%. In a few instances (such as the Liverpool Plains), native grassland soils can contain 5-10% organic matter and values as high as 20% have been recorded. Such high values are not, however, typical of the bulk of the State.
up to nearly 11 t/ha under rainforest (Attiwill and
10
Leeper 1987, Attiwill 1992, Ashton and Attiwill
Soil structure and stability
1994). The amount of litter tends to increase with
Plant cover protects soil structure and stability by
time and older woodlands and forests generally
acting as a barrier to water movement while adding
contain larger quantities of organic matter (Polglase
organic matter to the soil surface, which helps
and Attiwill 1992). When a steady state is achieved,
develop robust soil aggregates that resist erosion.
the litter layer on the soil surface may contain as
Most plants also produce a root network which
much as 20 t/ha (Handrek 1997).
binds the soil.
Litter is incorporated into the soil (along with dead
Erosion of soils under dense vegetation cover is
roots and other organic debris) through the process of
usually low because the canopy and foliage of a
decomposition. The rate of decomposition depends
stand reduce rainfall penetration to the soil surface
on the nature of the litter, the microclimate (especially
and reduce the velocity of raindrops hitting the soil
moisture availability) and the degree of soil biotic
surface (Conacher and Conacher 1995, Cumming
activity. In native vegetation communities dominated
and Elliot 1998). Trees and shrubs, however, may
by trees, the incorporation and accumulation of soil
not be sufficient to limit erosion and, in some
organic matter is particularly efficient and in moist
instances, drips falling from the canopy can be
forest soils, organic matter contents of 4-6% are not
more erosive than raindrops. In other instances,
uncommon in surface soils (see Spain et al. 1983,
‘woody weed’ growth can produce a large number
Maggs and Hewett 1993). In drier grass-based native
of stems and an almost complete canopy cover but
vegetation communities, however, organic matter
erosion can still take place since ground vegetation
contents are usually much lower with perhaps 2-3%
is largely absent. A full and diverse canopy that
includes ground-level vegetation and litter is
contribute greatly to decomposition in most
therefore essential to protect the soil. A dynamic
Australian environments (Spain and Hutson 1983,
grass sward offers particularly effective protection,
Lobry de Bruyn and Conacher 1990). Herbivores
with summer and winter active species that provide
may also be significant in organic matter
ground-cover all year round. In NSW rangelands
decomposition under native vegetation but their
where plant cover is sparse, ‘microbiotic’ crusts
effect has not yet been fully quantified and requires
that develop in association with mosses and
further study.
lichens may also have considerable importance in soil surface protection (Eldridge 1993, Eldridge and Green 1994).
Native plants have evolved to efficiently conserve nutrients (Handrek 1997) and many have developed specific mechanisms to maximise nutrient
Wind erosion can be greatly reduced by vegetation,
availability. These include the development of
which reduces wind speed and hence particle
specialised root structures such as the proteoid roots
movement. Interception of wind-blown material
of Proteaceous species (for example, Grevillea spp,
can also lead to an accumulation of mineral particles
Hakea spp). In addition, some native trees and
(Leys and McTainsh 1999) which alters soil surface
shrubs (such as Acacia, Casuarina, and Macrozamia)
texture and potentially enhances surface nutrient
and native herbaceous species or genera (such as
availability. The nature of the vegetation, whether
Glycine, Desmodium, Rhyncosia) develop root
native or exotic, is probably of little consequence in
associations where bacteria and/or fungi assist the
this context. For example, the sieving effect of
transfer of nutrients (especially nitrogen and
some exotic species in removing windblown
phosphorus) to the plant. Physiological mechanisms
material may be greater than equivalent native
also help to conserve nutrients. In many eucalypt
species (Virginia and Jarrel 1983). The relative
species, for example, significant quantities of nutrient
effectiveness of various species in limiting erosion
(up to 70% phosphorus, 50% nitrogen) are
requires further investigation.
withdrawn from foliage and redistributed within the
Soil nutrient cycling
tree prior to litterfall, which effectively conserves these within the biomass of the tree (Attiwill et al. 1978,
Plants use nutrients only when they are available in
Attiwill 1991, Lambert and Turner 1991). Soil organic
soluble, mineral forms in the soil. Inputs of these
matter also represents a significant store of nutrients
inorganic nutrients are therefore essential for plant
under native vegetation (Hingston et al. 1979,
nutrition and the availability of nutrients (especially
O’Connell 1990) and, in some cases, the quantity
phosphorus) has been shown to be an important
stored can exceed that in the growing vegetation.
influence on vegetation distribution across NSW (Beadle 1966, Lambert and Turner 1991, Neave et al. 1995, Handrek 1997).
The amount of mineral nutrient in the soil therefore represents only a minor component (perhaps as little as 0.13%) of the total present in a native vegetation
Soil nutrients are derived from a number of sources
system (Guthrie et al. 1978), with most nutrient
(as shown in Figure 4). Under native vegetation,
being stored in the plant or soil biomass (Maggs and
inputs from weathering, rainfall and leaching of
Hewett 1993). This has considerable ecological
nutrients from the canopy are usually relatively small
significance since much of this nutrient (especially
and organic matter decomposition is of greater
phosphorus) is liable to be rapidly lost where native
importance to the nutrient cycle. However,
vegetation is cleared (Ahern and Macnish 1986,
decomposition is often largely a microbiological
Maggs and Hewett 1993, Wilson et al. 1997).
process and low moisture availability and/or high temperatures in many parts of NSW can inhibit
Effects of isolated trees on soil
decomposition, leaving litter unaltered on the soil
Isolated native trees are common on cleared land in
surface for some time. Under these circumstances,
NSW and have a significant effect on the local
other mechanisms can contribute to decomposition.
environment. They add value to otherwise
Ants and termites, for example, have been shown to
degraded environments and many studies have
11
2.2 Biodiversity and native vegetation Native vegetation is a major component of Australia’s biodiversity. To date approximately 18,000 species of vascular plants and over 20,000 non-vascular plants, lichens and funghi have been described in Australia (Table 1). Native vegetation provides the food, shelter and habitats required by the vast majority of native animals. Loss of native vegetation leads to loss of the plants and consequently the loss of most native animals. These species are often important for the maintenance of vital ecological functions. Australia contributes greatly to global biodiversity. Figure 4 Nutrient inputs and cycling routes under vegetation
Its contribution is especially significant because of the high proportion of endemic species (that is,
assessed the effects of single trees on soil
species only found in Australia, see Table 1), which
properties. For example, enrichment of soil organic
has resulted in Australia being named one of only
matter and nutrients have been observed under
12 ‘mega-diverse’ countries. The uniqueness of
isolated native trees and shrubs (Sinclair 1983,
Australia’s biota relates to the great variety of
Pearce 1999, Lowry et al. 1988, Wilson et al. 1990).
environmental conditions present and to the
Trees and shrubs tend to attract bird and other
continent’s evolution in relative isolation from other
vertebrates and a wide range of invertebrates, all of
continents for more than sixty million years.
which add nutrients to the soil surface in their droppings. This usually leads to a net import of
2.2.1 What is ‘biodiversity’?
nutrients since many of the animals feed elsewhere
Biodiversity is conventionally partitioned into three
and use the plant during rest or reproduction. Small
components: (1) genetic diversity, (2) species
patches enriched with organic matter and nutrient
diversity, and (3) diversity of ecosystems.
can therefore result in the midst of otherwise degraded environments.
Genetic diversity Genetic diversity is normally considered to be the
12
In grazing lands, isolated trees may also attract
range of genetic information present within a
livestock in ‘camps’ that are preferentially enriched
species. Genetic information is passed on to
with organic matter and nutrients. Although this
successive generations either directly in asexual
enrichment may result simply from nutrient
reproduction or by mixing of genetic material from
redistribution, positive growth responses of ground
both parents in sexual reproduction. Genetic
vegetation have been reported under eucalypt
diversity increases the diversity of form and
canopies in grazed lands. These responses have
behaviour within a species, which provides it with
been linked to shading effects, moisture retention
greater capacity to cope with changing
and availability of soil nitrogen (Wilson et al. 1990,
environmental conditions or make use of completely
Wild et al. 1993, Chilcott 1996). Soil and grasses
different environments. For example, Ribbon gum
under individual trees have been found to have
(Eucalyptus viminalis) can grow on either calcareous
larger concentrations of nitrogen, phosphorus and
or acid soils, but the populations differ genetically
potassium than areas beyond the radius of the tree
and seedlings of plants from more acidic soils
canopy. However, a full analysis of the costs and
cannot become established on calcareous soils
benefits of isolated trees in the economics of
(Ladiges and Ashton 1977). Genetic diversity may
grazing in NSW has not yet been undertaken.
be expressed in the form of genetic variation within
individuals, within populations and/or between
NSW contributes significantly to Australia’s
populations. Genetic diversity can decline if
biodiversity. Nearly 60% of bird, 27% of reptile and
populations are lost or if the total population of the
34% of vascular plant species known from Australia
species is drastically reduced. If populations become
have been recorded in NSW. The proportion of
fragmented into very small sub-populations,
Australia’s invertebrates that occurs in NSW is
inbreeding depression may cause the genetic
unknown.
diversity to decline further. Estimates of the total number of species present in
Species diversity
the world usually vary from 3 to 30 million (May
The number of species present in a location
1990). Vertebrates are relatively well known,
depends on the type of ecosystem. Subtropical
although new species are still occasionally
rainforests usually contain over 100 vascular plant
discovered. In contrast, invertebrates are poorly
species in a hectare, including 30-40 tree species,
known. About 1.8 million species on earth have
whereas a hectare of cool temperate rainforest may
been formally named (Stork 1988), but a large
contain only 5-10 tree species. Grassy woodlands
proportion of species has never been collected and,
and heaths may also be diverse with more than 100
of those represented in collections, many have not
species in a hectare, although sometimes these
been named. Some habitats that were poorly
vegetation types contain relatively few species.
explored in the past have proven to be species rich.
However, ecosystems with relatively few species,
One example is invertebrate communities associated
such as temperate rainforests, can be important for
with tree canopies. Systematic collections of canopy
species diversity if those species are unique. The
insects in Australian rainforests (Kitching et al. 1993)
diversity of various animal species assemblages
and eucalypt forests (Majer et al. 1994) found that
varies even more than that of plants, and some
they were unexpectedly diverse and indicated that
groups also show distinct seasonal variation.
the total Australian fauna may be larger than
Table 1 Numbers of species from various groups in Australia and NSW
Group
Amphibians
No. of species present in NSW
No. of described species in Australia
Estimated number of species in Australia
Percentage endemic to Australia
70
203
~ 230
93
Birds
462
777
~ 780
45
Mammals
147
268
~ 268
84
Reptiles
208
770
~ 770
89
84
195
?
~125,000
~280,000
?
6,363
~ 18,000
~ 21,000
85
Fungi1
?
15,000
250,000
?
Mosses & liverworts
?
1,400
?
Lichens
?
2,500
38
Micro-organisms
?
105,000
?
Freshwater fish Total insects Vascular plants
90
(Sources: State of the Environment Advisory Council 1996, McLennan 1996, Orchard 1999, 1 May and Simpson 1997).
13
previously estimated. For example, 977 species of
Ecosystem productivity
insects were found on 80 trees from two Eucalyptus
The diversity of functional groups in an
species in NSW (Majer et al. 1994). Other groups
ecosystem also appears to influence its
such as mites and nematodes have not been
productivity. For example, in prairies in the
intensively studied and could also be extremely
United States the most productive swards always
diverse.
contained several species (Tilman et al. 1997).
Diversity at higher taxonomic levels is also important. Related species are aggregated into genera and related genera into families, then into orders, classes and phyla. From this perspective, a species that is the unique representative of a higher taxonomic group has special conservation significance. For example, peripatus (Peripatoides species) are cryptic, multi-legged animals that live beneath bark in wet forests (Hardie 1975) and are the sole representatives of a phylum that is intermediate between annelids (segmented worms) and arthropods (insects, spiders, crabs).
This greater productivity was better explained by presence or absence of key functional groups rather than by the number of species. If a single representative of a key group, such as a perennial winter-active grass or a legume, was missing then productivity fell. These findings have implications for the management of Australian pastures. Native pastures contain a variety of functional groups including warm-season perennial grasses, cool-season perennials, annual grasses, legumes and various types of forbs (Lodge and Whalley 1989, Trémont 1994). Further efforts to quantify the role of these various functional groups in maintaining sward
Diversity of ecosystems
productivity could be beneficial.
NSW contains a wide range of ecosystems that
Predators and parasites of pest species, which
occur in contrasting environments. Benson (1999)
affect plant production, also increase the
estimated that between 600 and 1,000 plant
productivity of ecosystems (including agricultural
communities could be recognised at a regional scale
ecosystems). Parasites and predators may kill a
in the State. These include communities adapted to
large proportion of insects and thereby reduce
climates that vary from alpine to sub-tropical to
their rate of population increase. Native
desert, and soils that vary from highly fertile to
vegetation is vital to retaining habitat and other
those of very poor quality.
resources required by these natural enemies of potential pests, but this importance is difficult to
14
2.2.2 Ecosystem function
quantify. Birds tend to feed on localised
Loss of biodiversity can result in reduction or loss
concentrations of insects, but their effectiveness
of ecosystem function. For example, in the
in controlling insects has been demonstrated only
highly fragmented box woodlands of northern
in a few cases (Ford 1981). Many insect-feeding
Victoria, the ground-active invertebrate faunas of
animals have disappeared from or become sparse
grazed woodlands were found to be significantly
in rural areas, chiefly as a result of the loss of
different to those that occurred in ungrazed
essential habitat. Bandicoots, which eat scarab
woodlands (Bromham et al. 1999). Although
larvae and other insects, have disappeared from
grazed woodlands contained more invertebrates
much of the grassy woodlands. The activities of
than ungrazed woodlands, most were from
some parasitic wasps and flies (which parasitise a
common groups characteristic of disturbed sites,
range of pest species) depend on nearby sources
such as Iridyromix ants (Andersen 1990), while
of food, such as nectar from tea trees
functional groups, such as detritivores,
(Leptospermum) or blackthorn (Bursaria spinosa).
considered important for ecosystem functions
Some species of wasp and fly seldom travel more
such as nutrient cycling, were found in
than 200 metres from such sources of food
significantly reduced numbers.
(Davidson and Davidson 1992).
2.3 Other values of native vegetation
small in comparison with its dispersal ability or the intervening area provides micro-patches or cover to aid dispersal. On the other hand a population of a
2.3.1 Native vegetation and connectivity
forest-dependent mammal such as the Greater
As well as providing essential habitat, native
the removal of the tree canopy between remnants.
vegetation, including small isolated remnants and
A population becomes isolated only if the gaps
scattered trees, has an important role in providing
between patches of remnant vegetation exceed the
connectivity across the landscape. Connectivity
ability of the species to cross them, so it is highly
concerns how easily the landscape allows plant and
likely that the structure, botanical composition and
animal species to disperse or move through it.
configuration of the land surrounding remnants
Adequate connectivity in the landscape reduces the
affects dispersal success. For example, a study of
Glider (Petauroides volans) is likely to be isolated by
probability of small isolated populations occurring,
the response of the Hazel Grouse (Bonasa bonasia)
allows mobile species to access essential but
to fragmentation confirmed that increasing distance
dispersed resources, and may be important for
from core habitat had an impact on the occupation
species migration. Connectivity needs to be
of small patches of suitable habitat. However,
considered on a whole-of-landscape basis. This
within an agricultural matrix the grouse was absent
is because species movement can occur from any
from suitable patches that were more than 100 m
patch or island that is either large enough or of
from the forest edge. However, when the patches
sufficient habitat quality to support a breeding
occurred within a managed plantation forest matrix
population or among a number of smaller
the grouse was still found more than 2 km from the
patches that combine to provide suitable habitat
source habitat (Aberg et al. 1995). Additionally the
for a population.
grouse was found in significantly smaller patches
Corridors are generally considered important for providing connectivity in highly cleared and fragmented landscapes (Fahrig and Merriam 1985,
within the forested, as opposed to the agricultural, landscape.
2.3.2 Production benefits
Downes et al. 1997). However, there is only limited proof of their efficacy in allowing species movement and they may indeed negatively affect individual species by promoting transmission of diseases and disturbances (Simberlof and Cox 1987, Hess 1994). Nevertheless, it is generally agreed that species response to fragmentation is individualistic and that corridors enhance landscape connectivity for many species (Saunders et al. 1991, Dawson 1994, Beier and Noss 1998).
Native vegetation has many economically important uses that may conflict with conservation objectives. Nevertheless, these are usually less destructive than vegetation clearance and provide incentives for landholders to better manage vegetation. Three of the more common uses are grazing, forestry and apiary (see Box 9).
2.3.3 Grazing Native or semi-natural pastures or rangeland occupy
The degree of isolation and connectivity that results
about 62% of NSW, compared with about 7% for
from clearing and fragmentation is not uniform for
sown crops and 5.5% for sown pastures (Bray 1999).
all species. For example, a population of the small
Native plants dominate grazed land in almost all of
woodland herb Glycine clandestina existing in a
the Western Division, much of the slopes and
wooded remnant is not isolated if the grassland
tablelands, and rougher country on the coast.
between the remnant contains habitat suitable for its germination and survival. Similarly, a population
2.3.4 Forestry
of a woodland bird, such as the White Throated
Most forestry in NSW is based on native species and
Treecreeper (Climacteris leucophaea) is not likely to
some 20,787,000 ha (26%) of NSW is native forest.
be isolated if either the distance between patches is
Of the 270,250 ha of plantations that were
15
established by 1995, some 12% have been planted to
plant that had been commercially exploited as a
eucalypts (McLennan 1999). Although natural forests
source of food. Other species with commercial
managed for timber production are likely to remain
potential such as lilypillies and black apples have
economically important, plantations based on both
not yet been developed as horticultural species.
eucalypts and native softwoods such as Hoop Pine
Medicinal uses of Australian plant and animal
(Araucaria cunninghamii) are being established at an
species have also hardly been explored. An
accelerating rate. Many productive forests are
industry based on tea tree oil (Melaleuca spp.) has
privately owned and occasional sales of timber are
been established and several other species, such
often important for the financial viability of grazing
as corkwood (Duboisia), the vine Tylophora and
properties in predominantly forested land, even
kangaroo apple (Solanum aviculare), are used to
though there are difficulties in sustainably combining
produce various medicines, while many other
forestry with grazing.
species contain potentially useful substances. For
2.3.5 Genetic material with economic potential
example, the rainforest plants, native pepper (Piper novae-hollandiae) and blackbean (Castanospermum australe), both offer potential
The economic potential of the Australian biota has
as cancer treatments, and secretions from the
hardly been explored (see Gillespie 1999). Until a
glands of bulldog ants have potential as
few years ago the macadamia was the only native
antibiotics (Biodiversity Unit 1993).
Box 3: Structure of ecosystems Interactions of individuals from various species with each other and their physical environment result in the development of a community structure, part of which is evident from the food web. Large top predators, such as powerful owls, that feed on animals such as possums and gliders, are always uncommon and require large foraging areas to maintain viable populations. Species can interact in other ways, too, by competing for the same resources, such as food or access to shelter for animals, or mineral nutrients, water or light for plants. One species may benefit from the presence of a second species without harming it, or two species may be dependent on each other, such as specialist pollinators and the plant species they pollinate. In this case the loss of one species will cause the loss of the second. Ecosystems change over time so that the species within them and their structure reflect the environment. Change occurs through progressive introductions and local extinctions of species. New species arriving in an ecosystem generally compete with those already established – and most fail. Both increased species-richness and the presence of species that are morphologically similar tend to reduce the likelihood of the successful establishment of an introduced species (Pimm 1992); for example, establishment is more likely on islands than on the mainland (Newsome and Noble 1986). However, there are exceptions and more work is required to understand this trend (Tilman 1997). Both the establishment of new species and the loss of existing ones can lead to extinctions, which in turn may cause further extinctions. Some species appear to have a larger influence on the species composition of a community than others. Removal of a top predator may have unpredictable impacts on the whole community, affecting both the relative abundance and the survival of many species. The loss of a species usually affects the rest of the community negatively but, in some instances, removal of a species can actually enhance local biodiversity. For example, the noisy miner actively excludes small bush birds from isolated woodland remnants. Artificial removal of noisy miners from a remnant can result in a rapid increase in bird diversity (Grey et al. 1998).
16
SECTION THREE
CONSEQUENCES OF NATIVE VEGETATION
LOSS AND DEGRADATION Modern Australian land management has led to
Native perennial species transpire throughout the
profound and lasting changes in the Australian
year provided that moisture is available. Water
environment. The magnitude, intensity and rapidity
usage by these species is therefore limited only by
of the impacts have arguably led to far greater
their ability to extract and transpire water.
change in the Australian landscape than the
Introduced crops, pasture and trees, however, are
changes that resulted from Aboriginal land
dominated by annuals and deciduous species (for
management. These management practices have
example, wheat, sorghum, ryegrass, sub-clover,
radically altered landscape processes operating over
willows). Water use by these species is only about
most of the continent.
half that of the original vegetation and is wholly concentrated during a particular season. For example, introduced grasses are often winter-active
3.1 Impacts on ecosystem processes
and water use by these species depends principally
3.1.1 Threats to water balance
groundwater. These differences have contributed to
Disturbance of the original vegetation of NSW has had a profound effect on ecological, hydrological and geomorphic processes. Many of these impacts have been via the surface component of the water cycle, in particular the partitioning of rainwater at the ground surface and movement of water in the soil. Clearing of trees has implications for hill-slope runoff mechanisms and the distribution of water in the soil mantle. Through-flow mechanisms dominate in forested catchments, while surface overland flow typically dominates in cleared catchments. Although the various through-flow models are reasonably well defined, they are highly variable and their role and function in the Australian landscape are not well understood.
Water use by plants Rising water table levels and increased runoff rates following clearing are well documented. A long-term impact of tree clearing results from the different water-use characteristics of trees and herbaceous species. Trees and shrubs draw water from deep in the soil because their roots can penetrate to depths of tens of metres under certain conditions in order to locate water. The roots of herbaceous species, in contrast, are usually concentrated in the surface 10-
on the length of the growing season, with excess water mostly lost as runoff or recharge to significantly altered stream flow regimes (Feller 1981), soil acidification, salinisation and groundwater pollution.
River health Disturbance of native vegetation has had an acute effect on river water flow and river health. Widespread clearing of upland catchments has resulted in greatly accelerated rates of erosion and sedimentation. Coarse sediments have filled in many of the deep water holes in riverbeds, while finer sediments settled on the floodplains (Mussared 1997). These sediments probably helped smother river and floodplain water plants last century, and may still be suppressing them today. In addition, turbid water limits sunlight penetration to submerged plants, disturbs fish physiology and puts pressure on a range of other river species. Changed runoff patterns, along with artificial regulation from dams and rivers, have altered river flow regimes and this has had significant impacts on river ecology and those species that depend on the natural flow regime. It is also possible that increased sedimentation and related changes in the aquatic environment of NSW have played a role in the increased incidence of algal blooms.
20 cm and are therefore unable to utilise water that
Removal and overgrazing of riparian vegetation has
periodically saturates the topsoil and drains beyond
contributed to widespread erosion of river and
their root zone. This water becomes available for the
stream banks, the clogging of flow lines, and
recharge of the groundwater system.
undercutting and collapse of remaining riparian
17
vegetation. Introduced willows have replaced native
following clearance and a few years of agricultural
trees on large stretches of inland rivers and these
activity (Anderson and Flanagan 1989, Congdon
shed their leaves at the one time in autumn,
and Herbohn 1993). Even where selective logging
providing an enormous input of nutrient-rich
occurs in rainforests, significant organic matter loss
material into the river. Native riparian trees, in
has been observed (Gillman et al. 1985, Congdon
contrast, usually retain their leaves for longer
and Herbohn 1993).
periods. It is believed that many aquatic species could be sensitive to such sudden high nutrient inputs, especially of nitrogen, and may not be able to effectively utilise the soft leaf material of the willow. Such disturbance may have significant implications for the food chain and the ecosystem.
3.1.2 Land degradation
Similar patterns of organic matter loss have been reported following the clearance of eucalypt forests/ woodlands (Valentine 1976) and replacement of old-growth stands of eucalypts by regenerating forest or plantations can also lead to decline in soil organic matter (Grierson et al. 1993, Raison et al. 1993, Attiwill 1994b, Weston and Attiwill 1996). Soil organic matter loss would therefore seem to be
When early European settlers arrived in Australia,
an inevitable feature of native forest/woodland
they found an apparently productive mosaic of
disturbance. Where soil organic matter is lost, soil
forest, woodland, shrubland and grassland.
degradation takes place. Not only is the store of
Understandably, they assumed that the soils
nutrients lost but water-holding capacity and soil
supporting this vegetation were rich and fertile and
aggregate stability are reduced. The resulting loss of
well suited to agricultural activities. On clearing the
soil structure leaves a more friable and erodible soil
vegetation, however, they found that soil fertility
leading to increased soil loss by erosion.
was rapidly lost and certain nutrients (especially phosphorus) soon became deficient.
Loss of surface organic matter is also a problem in grazing lands with high stocking rates. Grazing at
In the last two centuries, the native vegetation of
continuously high levels of intensity has been shown
Australia has been subject to very rapid and intense
to reduce organic matter inputs (Jones 1999) while
disturbance from a variety of management regimes.
increasing organic matter loss from soils (Holt
Loss or modification of native vegetation has
1997). Grazing also reduces the proportion of
contributed to virtually every aspect of land
ground cover, which can further increase erosion
degradation in Australia (Bird et al. 1992), including
(Cumming and Elliot 1998) although pasture usually
organic matter and fertility loss, soil erosion and
retains more soil organic matter than equivalent
compaction, soil salinisation and increased soil acidity.
cropped soils (MacLeod and Lockwood 1997).
Although we understand many of the processes and
Organic matter levels usually decline dramatically
effects of degradation, we lack information as to
over the first few years of traditional cultivation
whether the degradation is continuing, stable or
practices and continue to decline more slowly
reducing (Woods 1984, Burch et al. 1987).
thereafter. However, the evidence now suggests that minimum or conservation tillage techniques
Loss of organic matter
can help to stabilise or increase soil organic
Removal of native vegetation usually results in loss
matter content and maintain soil structure
of soil organic matter (Plamondon et al. 1991).
(Charman and Roper 1993) and interest in these
Australian soils are typically low in organic matter
techniques is growing.
where native vegetation has been removed and soil
18
concentrations of less than 1% are common over
Soil erosion
about 75% of the continent (Spain et al. 1983).
Erosion occurs naturally under native vegetation but
Where clearing occurs, the soil surface is disturbed
the arrival of European settlers undoubtedly
and exposure results in more rapid organic matter
accelerated erosion rates (Wasson and Galloway
decomposition. In rainforests, for example, as much
1984). Massive hill-slope erosion and down-slope
as 60% of soil organic matter can be rapidly lost
deposition phases have been documented from the
time of early settlement (Gale et al. 1995) and
Removal of vegetation for cropping has an even
extending well into the 20th century (Condon and
more significant effect on soil erosion. The size and
Stannard 1957, Dyson 1966).
stability of soil aggregates is reduced rapidly
In many places, the topsoil has been significantly eroded as a result of vegetation clearance and agricultural activities and up to 30 cm of soil has been lost from some hill slopes (Elliot et al. 1986). Loss of surface soil exacerbates nutrient deficiency in soils, leaving a sub-soil with lower inherent fertility. Exposure of sub-surface can reveal new material without the cohesion or water-retaining
following clearance and cultivation, leaving loose surface particles that are easily eroded (Hunt 1980, Harte 1984). Conacher and Conacher (1995) estimated that less than one tonne per hectare per year under pasture would be lost to erosion. This figure, however, reaches up to 50 t/ha/yr under various cropping systems and up to 100 t/ha/yr from bare fallow.
characteristics of the original layer, further
Land management however need not lead to erosion.
enhancing the erosive effects of wind and water.
Declining rates of erosion in the latter half of this
Such soils are therefore degraded in both ecological
century were indicated by Wasson and Galloway
and production potential.
(1984) and Condon (1986) and attributed largely to
Many of the effects of soil erosion are observed offsite and the transport and re-deposition of material can result in excessive sedimentation (Rowan 1986). Examples of fences being covered or the sedimentation of dams and watercourses are common in NSW. Transportation of eroded nutrients, and deposition and enrichment of downstream locations, can also represent an environmental problem. Downstream nutrient enrichment commonly results in burgeoning populations of unwanted or, in some cases, hazardous plants, bacteria and algae and may contribute to the growing problem of algal blooms. Land degradation, therefore, need not be restricted merely to specific erosion sites but can have significant implications at a landscape scale.
reduced stocking rates and improved methods of land management, including minimum tillage. Several management techniques also allow for grazing while retaining and, in some cases, enhancing soil properties. For example, maintaining both summerand winter-active species retains ground cover all year round, while reduced grazing pressure can increase organic matter content and surface cohesion. Through careful grazing management it is also possible to add organic matter and nutrient to soils while providing a rest period in which grass species recover their biomass (Earl and Jones 1996). It may be possible therefore to manage pastures in such a way that soil structure is preserved and erosive effects minimised (Puvaneswaran and Conacher 1983), although a good deal of work remains to be done to optimise pasture management in this respect.
Grazing has its own specific problems and damage tends to increase with more intense grazing (Holt
3.1.3 Loss of soil fertility
1997). Packer (1988) concluded that compaction of
Removal of native vegetation can also result in loss
soils resulted from livestock traffic on virtually all
of soil nutrients. In some instances, increases in soil
grazed lands and was a particular problem on
nutrients have been found at the soil surface
medium to fine textured and moister soils. Since such
following clearfelling of Eucalyptus forest
compaction leads to reduced infiltration rates and
(Weston and Attiwill 1993, Raison et al. 1993),
more efficient runoff, grazed plots are frequently
presumably as a result of rapid organic matter
found to yield more sediment than ungrazed
decomposition. However, this flush of nutrient is
equivalents. Wind speed and hence wind erosion also
normally short lived. In the longer term, loss of
tends to be higher for areas with grass cover than for
organic matter and nutrients (especially nitrogen,
areas with taller vegetation. This is especially so in
phosphorus and potassium) will generally result
arid and semi-arid regions where this combination of
(Plamondon et al. 1991, Hingston et al. 1979,
factors makes erosion much more likely and
1980) and can persist for considerable time
damaging under these conditions.
periods (Maggs and Hewett 1993).
19
Although there is debate as to whether logging will
Australia. The effects of fire on the ecosystem are
result in a long-term decline in soil fertility (see
complex and have received considerable attention in
Freedman 1981, 1991, Hopmans et al. 1993,
the literature (see, for example, Biodiversity Unit
Handrek 1997), forestry operations can undoubtedly
1996). Fire affects the soil by changing its physical,
result in large nutrient exports. This is especially true
chemical and, possibly more important, its
of short rotations where a higher proportion of
microbiological properties (Gill 1975, Hall 1996). The
nutrient-rich components of the tree is removed
mechanisms of soil change resulting from fire include:
relative to tree parts with lower nutrient contents (for
l
organic matter mineralisation
l
nutrient liberation through ashing of organic
example, Birk and Turner 1992). In mixed eucalypt forest it has been estimated that clearfelling and
material
timber extraction could remove some 14% of the phosphorus and 20% of calcium, magnesium and potassium from the system (Turner and Lambert 1986, Conacher and Conacher 1995). Where fire is a component of the management, nutrient loss can be
l
material losses through smoke
l
change in soil nutrient store and availability
l
subsequent loss of nutrients from runoff or wind erosion
even greater (Hopmans et al. 1993) and it might be predicted that, in the absence of compensating inputs of nutrients, these losses to the system will result in
l
soil sterilisation by temperature increase and post-fire recolonisation by vegetation.
long-term nutrient depletion of soils in NSW forests. The productivity of highly fertile sites may not be affected immediately (Hopmans et al. 1993, Attiwill 1994b) but they will probably still be depleted to some degree. On less fertile sites, productivity decline may result more rapidly. It has been suggested that phosphorus lost from the system might be replaced by adding superphosphate, but given the experience of fertiliser use elsewhere in the Australian environment, this remedy may have as yet unknown detrimental effects.
Fires in native vegetation often result in short-term higher soil nutrient availability (Raison et al. 1990, Tomkins et al. 1991, Polglase and Attiwill 1992, Attiwill 1992, 1994b, Bauhus et al. 1993). This encourages regeneration because a flush of available nutrient allows rapid germination and seedling growth. This nutrient enrichment, however, is fairly short-lived. Significant loss of nutrients (especially phosphorus) is usually the long-term outcome (Hingston et al. 1979, Raison et al. 1985, Stewart et al. 1990) since fire can remove nutrients from a site by
Removal of plant and animal materials in the form of
volatilisation, loss of particles in the wind, and
farm products such as crops, wool or meat may also
through the leaching of soluble mineral ions. Given
reduce the levels of available plant nutrients in soils
the limited supply of phosphorus in Australian soils,
(Wild et al. 1993). Such removals have been the basis
such nutrient losses from burning could be important.
for maintenance fertiliser applications for many years.
Despite this, studies on the effects of fire regimes on
Although grazing animals undoubtedly redistribute
soil nutrient storage and cycling have been limited
nutrients in grazed lands, it is uncertain whether
and further research is needed to fill gaps in our
grazing in itself significantly depletes soil nutrients.
understanding (Williams and Gill 1995).
Research results have typically found little difference between the overall nutrient status of grazed and ungrazed sites (Packer 1988). However
3.1.4 Increasing soil salinity
greater organic matter and soil loss from grazed
Soil salinity and salinisation is a growing problem in
lands compared with ungrazed may contribute to
NSW and can be directly linked to the clearance of
nutrient depletion.
native vegetation and associated agricultural activities (Cumming and Elliot 1998, MDBC 1999).
20
Impact of fire on soil fertility
Most soils in NSW contain salts derived from mineral
Fire, whether natural or human-induced, is a common
weathering, deposition in ancient inland saline
feature of many native vegetation systems in
waters or, more recently, inputs from wind and rain.
The existence of this salt in itself is not damaging.
that will be affected by salinity in NSW is very
Salinity becomes a problem where these salts are
significant and exceeds that of the land currently
mobilised and enter the root zone where they can
under cropping in the State.
impair a plant’s ability to absorb water and cause physiological damage or ultimately premature death. Such salt-affected soils therefore become unsuitable for the growth of plants
In addition to its detrimental effects on agricultural plants, salinity may also represent a threat to the remaining native vegetation where salinity levels exceed salt tolerance, and further loss of native
Soil salinity is largely absent in catchments where
vegetation from this problem is highly likely. It has
native vegetation cover remains intact. This is
been demonstrated for example, that salinity
because deep-rooted native vegetation utilises most
contributes significantly to dieback (Heatwole and
of the water that falls in precipitation and, as a
Lowman 1986) and the impact of salinity in this
result, salts remain largely isolated in the layers
respect is likely to become more severe.
below the root zone. In more arid regions, where salts do reach the surface, native species such as the shrubs of the extensive mulga, mallee and saltbush communities, are well adapted to coping with salt stress. A complete native vegetation cover therefore has considerable value in limiting the incidence and damaging effects of soil salinity.
Dryland salinity also affects other components of the environment. For example, erosion of saline soil and the through-flow of water can flush considerable quantities of salt into freshwater systems causing increased stream salinity, which is now of considerable concern in NSW (MDBC 1999). For example, it is estimated that the salinity of the
Clearance of native vegetation and associated
Namoi River in northern NSW might rise from its
agricultural activities have significantly altered the
1998 value of 680 EC to 1550 EC by 2100. At such
fine balance between native vegetation and
high salinity levels, changes will undoubtedly be
hydrology, resulting in artificially raised water tables
detected in the biota and these could potentially
that bring salts to the surface. Salinity from
become significant (MDBC 1999), although the
irrigation is of considerable importance in some
nature of these changes has not yet been assessed.
sections of the State. In NSW, for example, MDBC (1999) estimated that some 412,000 ha in the Murray and Murrumbidgee catchments may require draining to control salinity. Dryland salinity, which will potentially affect larger areas, is caused by incomplete water use by shallow rooted, introduced plant species, which leads to enhanced deep drainage, raised water tables and the influx of salt into the root zone. Increased instances of dryland salinity are now being observed in NSW as a result of vegetation clearance and agricultural activity and the area affected is likely to increase due to a delayed response to clearing (MDBC 1999).
The effects of salinity are therefore considerable and mitigation strategies are now being actively sought. The retention of native vegetation is an option to prevent or at least limit the incidence of salinity. The retention of tree cover in upper catchments, where recharge and salinity pose a problem, can greatly reduce leakage to the water table. For example, it has been suggested that a 30% tree cover in upper catchments can reduce or prevent the effects of salinity on lower catchments (Zhang et al. 1999). This option is especially suited to high rainfall, salt-prone areas where neither annual nor perennial pastures can sufficiently limit deep drainage (MDBC 1999).
Estimates vary as to the area currently and
Planting native trees and deep-rooted perennial
potentially subject to salinity in NSW. PMSEIC (1999)
pastures to limit dryland salinity has considerable
estimate that some 120,000 ha of NSW is currently
potential if it is done appropriately with regard to
affected by salinity and that 7.5 million ha could
catchment hydrology and hydrogeology (Farrington
potentially become salt affected. MDBC (1999)
and Salama 1996, Stirzaker et al. 1999). For example,
estimates are however slightly more conservative at
maximum benefit might be derived where vegetation
potentially 2 to 4 million ha in the NSW section of
planting is targeted at strategic locations such as high
the Murray-Darling Basin. Nevertheless, the area
recharge zones on upper catchment slopes.
21
3.1.5 Increasing soil acidity Soils under native vegetation in NSW vary in their level of acidity and tend to be more acid (at least in their surface layers) where rainfall and organic matter content are high (see Table 2). For this reason, the soils of moist woodlands and forests in the eastern zone tend to be relatively acid (Charman 1993), with pH in the range 4.0-6.5. To the west,
older farming areas, pH has already dropped below pH 5.0. At this pH, phosphorus becomes less available and aluminium and manganese compounds become more mobile/soluble, so toxicity to plants can result. Currently 13.7 million ha of NSW is affected by soil acidification which amounts to a considerable limitation on pasture, and hence livestock, production.
however, soil acidity tends to decrease and surface
Acidification progresses more slowly where
acidity is unlikely to occur under native grass and
adequate cover of native, deep-rooted perennial
rangeland due to the absence of leaching and
pasture is retained (Blair 1992) since this recycles
limited organic matter incorporation. These soils
calcium more effectively to the soil surface. Surface
typically fall within the pH range 6.5-7.5, but where
acidification has also been overcome by applying
calcareous or sodic conditions exist, soil pH in excess
lime, but acidification is of particular concern where
of 8.0 is not uncommon.
it affects the sub-soil, which is beyond the reach of effective treatment. More recently, it has been
Increased soil acidity has been observed where
suggested that tree planting might reduce soil
native vegetation has been cleared and replaced
acidification (Australian Landcare 1999) and native
with improved pasture. It is thought that this
vegetation restoration may also offer some potential
acidification results from intense grazing and
to limit this damaging process.
removal of nutrients in crops or livestock, combined with fertilisation, increased soil organic matter content (Ridley et al. 1990) and the addition of
3.2 Impacts on biodiversity
nitrate by leguminous plants, which is readily
Most native species have been negatively affected
leached with an acidifying effect. Although the rate
by habitat loss, fragmentation and modification,
of acidification is typically slow, perhaps one pH unit
altered disturbance regimes, introduced weeds and
in 50 years (Cumming and Elliot 1998), the problem
feral animals, disease, pollution, climate change and
is cumulative and is of growing concern. In some
commercial harvesting, although a few species have
Table 2 Typical surface soil acidity levels in a range of vegetation types
Author
Maggs and Hewett (1993)
Granites and meta-sediments
Vegetation type/ Location
Primary and secondary rainforest, Queensland Cleared, unmanaged grass, Queensland
Burrough et al. (1977)
Hawkesbury Sandstone
Open eucalypt forest
Maggs and Hewett (1993)
Basalt
Primary and secondary rainforest, Queensland Cleared, unmanaged grass, Queensland
Pearce (1999)
Eldridge (1999)
22
Parent material
Granite
Cracking Clays
pH (in water)
4.0-4.3 4.7 4.9-5.3
5.7-6.8 5.4-6.3
Isolated tree in fertilised pasture, Armidale, NSW Fertilised pasture, Armidale NSW
6.0 6.2
Native grassland, Walgett, NSW
7.7
gained advantage from these changes. These
loss of forests and woodlands across Australia to be
processes do not occur in isolation, but interact in
about 50 million ha. This means that, since 1788,
ways that are often difficult to understand or
over half of Australia’s forests and woodlands have
predict. Extinction or population decline of any one
been cleared or severely modified (Young et al.
species cannot be directly attributed to any one
1990). This disturbance has not been even across
cause. What we can say is that the combined
the continent and some regions have been more
effects of these disturbances have led to levels of
severely affected. For example, over most of the
species extinction that cause concern. In Australia
central west of NSW, between 60 and locally up to
over the last 200 years we have had the highest rate
90% of the pre-settlement native vegetation has
of mammal extinctions in the world, with 20
been cleared (Benson 1999, Sivertsen 1995). This
mammal and 97 plant species now considered
level of habitat loss has serious effects on the
extinct (Endangered Species Advisory Committee
diversity of species that can survive in the landscape
1992). In NSW, over 80 species of plants and
and there is currently a significant amount of
animals are extinct and more than 600 species are
research work directed towards what levels of
considered either endangered or vulnerable (NSW
habitat loss can be sustained without significant
Biodiversity Strategy 1999).
impacts on biodiversity (see Box 4).
The following section deals with some of the factors
Genetic diversity, population diversity, ecosystem
that potentially threaten biodiversity. This is not an
and functional diversity are all adversely affected as
exhaustive list or a comprehensive treatment of the
habitat is lost. Genetic diversity is lost primarily
threats. The intention is to overview the range of
through the loss of individuals and individual
problems and outline a number of more
populations, while ecosystem diversity is generally
comprehensive treatments.
lost even at relatively low percentages of habitat
3.2.1 Habitat loss, fragmentation and modification
clearing. This is because habitat loss is not a random process and is primarily targeted towards sites of high productive potential. This generally
The removal of native vegetation usually results in
leads to landscapes where the remaining native
localised extinction of the plant and animal species
vegetation occurs on steep slopes and relatively
dependent on that vegetation. Habitat loss can be
infertile soils. In the Bega valley, for example, 92%
defined as the broad-scale removal of native
of the native vegetation on arable land has been
vegetation or other plant or animal habitats
removed, but most vegetation on steep slopes
resulting from human activity (Sivertsen 1995).
remains (Sivertsen 1995). Those species and
Habitat alteration, on the other hand, occurs when
ecosystems that occurred in these environments are
the broad structure of the habitat remains the same
now either extinct or have suffered serious declines
but components are lost or modified. For example,
in distribution and populations.
heavy grazing removes or depletes the understorey layer of a forest or woodland.
The full effects of habitat loss are not always apparent immediately after clearing has taken place.
Habitat loss
Decline in biodiversity resulting from habitat loss
On a global scale the cumulative effect of habitat
often continues for many centuries after the major
loss is one of the major threats to biological diversity
disturbance has occurred. Continuing species loss,
(Fahrig 1997). The process of habitat loss is not
long after original clearing, has been observed in
new, and clearing for agricultural development has
ecosystems as diverse as grasslands in Switzerland
been occurring in many parts of the world for
(Fischer and Stocklin 1997) and tropical forests in
millennia. There are no definitive measurements
Singapore and Peru (Turner and Corlett 1996,
that quantify the habitat loss that has occurred in
Kattan et al. 1994). The effects of habitat loss may
Australia, but in 1992 the Resource Assessment
take a number of generations before becoming
Commission (as cited in Norton 1997) estimated the
evident, so they may not be detected if we
23
concentrate on monitoring only long-lived dominant
1910 and many of the species were considered to be
species. This effect is apparent across broad areas
quite common up until the 1940s. Surveys in the late
of the grazing lands of NSW where much of the
1980s found that six mammal species had become
existing native vegetation consists of mature and
locally extinct and a further four were in danger of
senescent woody species.
extinction. Importantly many of the mammal species found in the regional survey were in very low
Australian examples of studies that look at long-term
numbers and are considered to have suffered
species loss are rare. Nevertheless the following
significant population declines.
example shows similar trends. In the Bega valley in southern NSW an historical analysis of mammal
Habitat fragmentation
species records and settlement patterns were
Fragmentation is the cumulative process of habitat
compared to a recent survey (Lunney and Leary 1988).
loss and isolation (Forman and Godron 1986).
In this valley, the majority of the clearing on the valley
Habitat loss and fragmentation are often considered
floor and lower slopes had been completed by about
to be the most significant threats to the
Box 4: Thresholds of habitat loss The study of critical thresholds of habitat loss is a new and rapidly developing field. As a guiding principle, loss of biodiversity occurs as any habitat is lost or degraded (Freudenberger et al. 1997). Generally, at low levels of habitat loss, the decline in biodiversity is approximately proportional to the area of habitat lost (Andren 1994). As the amount of habitat loss increases there appear to be thresholds beyond which biodiversity decline accelerates and then becomes very rapid. These thresholds are difficult to delineate and may vary between differing landscapes. Nevertheless there is growing evidence that in a landscape where between 40% to 70% of the original habitat area has already been lost, further losses of habitat result in proportionally greater species loss than would be expected from the same area of habitat loss in landscapes where more than 30 to 60% of habitat remains. In landscapes where less than 30% of the original habitat remains species loss is particularly rapid (Andren 1994, 1997, With 1997). The value of this threshold is also influenced by the type of habitat remaining, the species involved and the composition of the landscape between the remnants (known as the matrix) (Andren 1994, 1999, Monkkonen and Reunanen 1999). If the matrix surrounding the remnants is of a more intensive land use such as cropping, the effect on species diversity from habitat loss can be greater and the critical threshold of remaining suitable habitat will be substantially higher (Monkkonen and Reunanen 1999). Thresholds can also be affected by a range of other factors. In Australia, most of these tend to raise this threshold. Most northern hemisphere studies such as those of Andren and Monkkonen and Reunanen were from areas where the climate is relatively predictable and environmental variability is low. However, when the climate becomes more erratic and the environment more complex, it is possible that the effects of fragmentation could be felt at much lower levels of clearing. These models also assume that habitat loss is a random process across the landscape. In reality it is usually targeted at specific landscape types (Sivertsen 1995). Finally the modelling assumes that the quality of the habitat is uniform (that is, it is either habitat or it is not) but in real landscapes the quality of the habitat is variable. In landscapes where the quality of the habitat is poor, species are likely to experience more extreme responses to fragmentation and in regions such as the Western Division of NSW, where climate is erratic and habitat alteration has been significant, these critical thresholds could be significantly lower.
24
conservation of biodiversity (Fahrig 1997, NSW
l
Biodiversity Strategy 1999). Fragmentation has three major geographical components, including loss of habitat, reductions in average patch or
disruptions in landscape-scale ecosystem processes such as the hydrological cycle
l
modification of environmental conditions within the remaining vegetation.
habitat size, and increasing isolation of remaining patches (Andren 1994).
The effects of fragmentation are not uniform for all species. Some species are advantaged, others
Overviews of the state of knowledge on the impacts of fragmentation have been provided by Saunders et al. (1991, 1993) and Andren (1994). Fragmentation can lead to:
disadvantaged and a third group seems to be unaffected. For example, in a fragmentation study of Eucalyptus forest in the southern highlands of NSW, Margules et al. (1995) found that the effects of fragmentation varied enormously between species.
l
reduced area of native vegetation
l
isolated and small populations
l
increased risk of species extinction
and another three experienced population increases.
l
reduced population sizes of sensitive native
If the degree of habitat loss and fragmentation is
biota
severe it can result in a number of small isolated
increases in ‘weedy’ species in remnant
populations (Forman and Godron 1986). Small
l
vegetation
Within a group of eight Carabid beetle species, three suffered population declines, two showed no effect
isolated populations are at greater risk of extinction simply because of the higher probability of a
l
l
l
reductions in genetic diversity of sensitive
catastrophic event destroying the entire population
species
(Farhig and Merriam 1994). For example, the Long-
increased predation on species surviving in the
nosed Potoroo (Potorous tridactylus) is dependent
remnant vegetation
on dense understorey vegetation for cover (Bennett
increased competition from species favoured by the altered environmental conditions
l
1987). If, in a small remnant, wildfire or intensive grazing destroys all of the ground cover then the population is likely to become extinct in that patch.
lower levels of connectivity between
If the remnant is truly isolated there is little
populations
probability of subsequent re-invasion from another
Box 5: Island biogeography Traditionally the decline in species diversity resulting from habitat loss and fragmentation has been described in terms of MacArthur and Wilson’s 1967 Theory of Island Biogeography. This model describes species loss as a function of isolation and size of habitat patches. Others have argued that the reason lower species diversity is found in smaller areas is simply that smaller areas contain less environmental diversity than larger areas. Therefore, species loss is simply a matter of a proportional loss in species in relation to the area available. This is known as the random sample hypothesis (Connor and McCoy 1979). Both of these theories have been developed for true island environments and assume that the intervening matrix between habitat patches is hostile to terrestrial biota. The reality is that the response of species to habitats created by the urban, agricultural and forestry development matrix are highly variable. Remnant habitat may act like an island of suitable habitat surrounded by inhospitable territory for some species. For others, the pattern of remnant vegetation and other land use may be more akin to the variegated landscape model of McIntyre and Barrett (1992), where the landscape is a mosaic of differing qualities of habitat suitability.
25
sub-population elsewhere. If extinction of small
susceptible to the physical effect of desiccation
isolated populations is repeated within all of the
brought about by fragmentation (Margules et al.
remnants then regional extinction of the species in
1995). There have been similar findings of reduced
the fragmented environment is likely (Farhig and
populations for tropical Amazonian birds, butterflies,
Merriam 1994).
dung beetles and tree species in response to a
Less obvious but still of concern is that small populations may have reduced fitness. For example, small populations of plants are often found to have reduced seed set and germination. Further reductions
complex series of altered environmental conditions resulting from fragmentation (Lovejoy et al. 1986, Bierregaard et al. 1992).
since reduced germination rates may result in a
Fragmentation and its effects on competitive interactions and predation levels
competitive disadvantage with other species in the
Species exist in a dynamic and competitive
patch. For example, the germination success of the
environment, so changes in habitat conditions can
Royal catchfly (Silene regia) is related to the
alter the competitive interactions and species
population size (Menges 1995). Populations of less
composition of an area. Cleared lands provide no
than 100 plants had germination rates of less than
suitable habitat resources for many species, while
50% while populations larger than 150 plants had
reduced habitat area leads to a decline in overall
much higher germination rates.
population size and, in the case of some fauna, a
in the population size of those species are possible
Altered environmental conditions resulting from fragmentation
temporary increase in population density in the remnants (Lovejoy et al. 1986).
Clearing and fragmentation significantly alter the
In fragmented landscapes, competition between
environmental conditions that occur in native
species is significantly altered through a number of
vegetation remnants. Generally fragmentation
mechanisms. These include:
leads to a number of changes in the microclimate
l
altered environmental conditions in the
of the remnants as well as in soil nutrient status,
remnants which advantage some species over
nutrient cycling and soil moisture balance
others
(Saunders et al. 1991). l
elevated populations of, and increased
Native vegetation in fragmented areas is subject to
competition from, species that can survive in
different environmental conditions compared with
the modified areas
those originally encountered. Fragmented vegetation generally has increased wind speeds and solar
l
either excluded previously by some other local
radiation, a locally altered radiation balance, higher
competitor or were not favoured by the pre-
maximum and lower minimum temperatures, higher
cleared environment
incidence of damage due to frost and windthrow, altered nutrient and water balance and cycling, lower
invasion of remnants by species that were
l
competition from species introduced into the
humidity and altered habitat structure and diversity
modified environment surrounding the
(Saunders et al. 1991). These changes have a
remnant
profound effect on the type of biota that can exist
l
within these fragmented environments. In a review of
increased predation by native and introduced predators as a result of the changed
long-term fragmentation studies, Margules (1996)
environmental conditions and an increased
suggested that many ‘species which decline, do so in
abundance of some exotic prey species, such as
response to physical changes in habitat and changes
rabbits and mice
in ecosystem processes brought about by fragmentation’. He went on to demonstrate that
26
l
reduced predation by some native species as a
Amphipods and some Carabid beetles from his own
result of the remaining patch sizes being too
fragmentation experiments in southern NSW are
small to support a viable predator population
l
altered plant-pollinator relationships affecting
The nature, extent and timing of many types of
the competitive ability of some species.
disturbance have changed since pre-European times.
There is evidence that changed environmental conditions can significantly affect competitive interactions of plant species. In a central NSW study on the reproductive ecology of four plant species in fragmented mallee woodlands, overall seed production in small linear strips as opposed to larger areas was reduced in two species, Acacia brachybotrya and Dianella revoluta, increased in Senna artemisiodes and unaffected in Eremophila glabra (Cunningham 1999). In another study, Bell et al. (1999) found that increased light intensity reduced germination rates in several native Western Australian perennial species, but promoted the germination of some native, exotic and annual species of the Asteraceae family. Evidence of reduced seed set and variable germination rates are not conclusive proof that the species composition of these woodlands will alter as a result of fragmentation. It does, however, indicate that fragmentation can have significant effects on a number of important ecological functions that may alter the competitive interactions between species.
Habitat modification Habitat alteration has occurred across all ecosystem types in NSW and none could be considered to be unaffected by some form of degrading influence. Within NSW, the major forms of habitat modification result from changes to natural disturbance regimes caused by grazing of domestic stock, commercial logging of timber, and fire hazard management.
Mechanical disturbances caused by earth-moving and forestry operations now occur. Total grazing pressure has increased, with herbivores being more controlled by supply of forage and less controlled by water supply (in semi-arid and arid areas) and predators (Wilson 1990). Types of herbivores have changed, with sheep and rabbits grazing swards more closely than macropods. Damming or draining has altered the water regimes of many natural shallow wetlands and new types of generally deeper artificial water bodies, such as farm dams, have been created (Brock et al. 1999). New disturbances or modification of existing disturbance regimes pre-disposes communities to invasion by exotic plants (Hobbs and Huenneke 1992). These exotic plants are often ‘weedy’ disturbance specialists that are able to quickly colonise an area devoid of vegetation.
Fire Much of the Australian environment has been shaped by the interaction of fire and native vegetation. In many areas of Australia, natural wildfires have always been highly seasonal and episodic events and the native vegetation evolved in their presence. Some plants undoubtedly benefit from fire and indeed some (for example, Eucalyptus regnans: see Attiwill 1994a) seem to depend on fire to germinate and regenerate. The impact of human-induced fire on such a dry continent must have been marked. Some evidence suggests that Aboriginal use of fire was already sophisticated 60,000 years ago (Gill et al. 1981).
Other habitat modifying disturbances include
Fire is used for a variety of purposes in land
commercial apiary (see Box 9), recreational activities,
management. It has been used simply to remove
hunting and harvesting of wildlife, commercial flower
unwanted vegetation, to prepare seedbeds for
picking and harvesting of other bush products.
sowing crops, to liberate nutrients at the soil surface
3.2.2 Changes in vegetation disturbance regimes
and to reduce fuel in forests. In fuel reduction burns, relatively frequent, low-intensity fires are used to prevent less frequent, high-intensity fire
While most native vegetation communities require
events. In some areas, fires have been actively
some form of disturbance to enable continued
suppressed, resulting in fewer (but probably more
regeneration, changes in disturbance regimes since
intense) fires. In other areas vegetation that was
the arrival of Europeans threaten the character of
originally protected from fire is now at risk; for
many communities. Types of disturbances range from
example, an area of rainforest on the north coast
sudden and severe impacts, such as fire or land-slips,
adjacent to cleared grazing land that is maintained
to chronic stresses such as grazing pressure.
by fire. The overall extent of changes to fire regimes
27
remain a matter of controversy (Benson and
treated in other ways) to reduce its biomass (Lunt
Redpath 1997, Conacher and Conacher 1995).
1991). Increased fire frequency may simplify
Absence of fire is associated with an increased density of woody shrubs in many areas, and with
vegetation leading to, for example, greater cover by grasses and ground ferns and fewer shrubs.
rainforests plants colonising wet eucalypt forests. Groundcover may also be affected. For example,
Rural tree decline
the species richness of grassland on fertile soils
Trees on much of the better grazing country have
dominated by kangaroo grass (Themeda australis)
been thinned, creating a park-like landscape of trees
may decline rapidly if a site is not regularly burnt (or
and pasture. Often the pasture has been cultivated
Box 6: Predicting changes in plant communities Changes in vegetation composition following a disturbance is often referred to as succession. An older theory that succession always led towards a climax community which was predetermined by the climatic environment (Clements 1916) has lost favour because it failed to explain many observed trends in vegetation change. Most now believe that succession can generally be explained by competitive interactions of individual species (Gleason 1926). Understanding the mechanism of these changes will greatly assist in the management of native vegetation. Modes of succession observed in nature can be classified into three types (Connell and Slatyer 1977). (1) Early colonisers modify the environment so much that they are replaced by other species that are more suited to the new environment (facilitation). This is commonly observed in primary successions in which new soil is formed, but is less common in secondary successions. (2) The first organism to colonise a site has a competitive advantage over newcomers that allows it to dominate the site while it survives (inhibition). This is commonly observed, for example, in species-rich rainforests or eucalypt forests where any one of a number of tree species could successfully grow in a newly formed gap and the first tree to become well established occupies the gap. In these cases species composition is at least partly determined by historical accident. (3) Some species become established and grow on a site despite the presence of the early colonisers and, in time, eventually replace them (tolerance). One example is rainforest understoreys in wetter eucalypt forest which grow into rainforest that eventually replaces the eucalypt forest in the prolonged absence of a major disturbance. Often the major vegetation changes that result from a particular type of disturbance can be predicted using a small number of vital attributes (Noble and Slatyer 1980). These vital attributes relate to (i) the species’ method of persistence during a disturbance and/or its arrival in the aftermath; (ii) its ability to establish and grow following the disturbance; and (iii) the time required to reach critical stages in its life history. Computer models based on these attributes have been developed to predict vegetation changes that follow the creation of canopy gaps in vegetation types such as sub-alpine eucalypt forest (Shugart and Noble 1981) and subtropical rainforest (Shugart et al. 1980). These models are not well suited to predicting vegetation changes in grasslands which result from minor disturbances or climatic events. Grassland is best seen as vegetation that can be in various states and that where certain conditions are necessary to enable transitions between those states (Westoby et al. 1989). This concept appears useful, for example, in explaining vegetation changes that occur in pastures on the northern tablelands (Whalley 1994).
28
and sown to exotic grasses. This destroys existing
reduce percentage ground cover. Variations in the
juvenile trees which are crucial in ensuring the survival
grazing regime (that is, the intensity, duration,
of woodlands, while livestock grazing prevents the
grazing species and seasonal variability of grazing)
establishment of new seedling trees. Even when
can alter a range of ecosystem functions and
stock are excluded, establishment of eucalypts in
significantly affect the structure and botanical
some heavily disturbed environments is an infrequent
composition of vegetation. The impacts of altering
event that usually requires the coincidence of a heavy
grazing regimes can be severe and can vary for each
seed-fall, adequate rainfall and a seedbed that is
species depending on their palatability and sensitivity
relatively free of competing vegetation (Curtis 1990).
to grazing. For example, grazing has been implicated
In more marginal grazing country, where livestock and
as the partial or primary cause of the extinction of 34
feral animals have had a greater impact, regeneration
plant species in Australia, while a further 55 are
has been virtually eliminated from some areas.
considered at risk (Leigh and Briggs 1992).
Natural deaths of aging trees and lack of tree
The sensitivity of plant species to, and their capacity
replacement has resulted in a general loss of trees
to recover from, grazing varies:
from many of the landscapes of the sheep/
l
wheatbelt of central NSW, which is often
Some native grass species increase in abundance as grazing intensity increases
exacerbated by dieback (see Section 3.2.5). The
(Lodge and Whalley 1989)
natural life span of trees varies for different species and depends on environmental conditions. Most
l
Even light grazing can eliminate sensitive
species of eucalypts can live 200-300 years under
species such as some ground orchids and
good conditions, but some are more short-lived and
Microseris lanceolata, (see Box 7) (Prober and
there are individual River Red Gums (Eucalyptus
Thiele 1995)
camaldulensis) over 1,000 years old. The life
l
Many native species (palatable shrubs, forbs
expectancy of trees is reduced by injuries such as
and grasses) decline, either slowly or rapidly, if
repeated scarring by fires. Larger, older trees have a
grazing is intense and prolonged (Lodge and
reduced capacity to recover from physical injury,
Whalley 1989, McIntyre and Lavorel 1994,
wood decay and the risk of windthrow increase.
Prober and Thiele 1995).
If the failing condition of the remaining trees with
Prolonged access of livestock to an area allows stock
short-lived, poorly dispersed seeds, prevents them
to select and repeatedly graze preferred species
from reproducing, the species will die out from the
without giving them time to recover. In contrast,
area over time. Eucalypts that grow along
areas that are grazed only infrequently, such as
watercourses where seeds can be dispersed in
some travelling stock reserves, have greater native
floodwaters are an exception. Species of trees and
species diversity (Benson 1994).
shrubs with seeds that are readily dispersed and can establish despite grazing (for example, Callitris, Dodonea, Cassinia, Eremophila) often increase in abundance while more sensitive species die out.
Grazing by domestic stock per se is not always deleterious to native vegetation communities. Grazing can sometimes enhance local diversity by reducing the competitive advantage of fast-growing species. The disturbance caused by grazing was
Grazing impacts
found to allow regeneration of some rare or
Grazing is the most widespread land use in NSW
threatened species in remnants of native vegetation
with over 70% of the State being utilised for
in Tasmania (Kirkpatrick and Gilfedder 1995). These
domestic stock grazing. In addition to domestic
species lacked mechanisms to ensure their
stock, feral and native herbivores are distributed
persistence without disturbance and could
across all bioregions and land tenures in NSW.
otherwise have become extinct. Other disturbances
Grazing is a powerful ecological force mainly
such as fire might also create the appropriate
because it reduces overall plant biomass and can
environment for regeneration. This is important
29
Box 7: Pre-European grazing regimes and an example of the potential effects of their alteration The extent to which Australian ecosystems were grazed in pre-European times remains controversial. Large herds of herbivores were probably uncommon as the relatively infertile soils and irregular and unpredictable climate made it difficult to sustain large populations. As such much of the grassy vegetation was maintained by climatic factors and fire (Benson and Redpath 1997). Some authors, however, believe that prior to Aboriginal occupation, herds of very large herbivores may have existed (Flannery 1994). In addition, early European settlers observed large populations of small herbivores such as bettongs, which have since become regionally or totally extinct (Jarman and Johnson 1978). Therefore, over most of central and western NSW the grazing regime was probably one of intermittent high levels of grazing by soft footed small-to-medium size herbivores for relatively short periods of time. The native daisy, Microseris lanceolata, was once an abundant and widespread species that has suffered a major decline and is now considered comparatively rare (McIntyre 1995). The recent decline has been attributed to the effects of modern agriculture and in particular grazing by domestic stock. Microseris lanceolata is susceptible to grazing because it is palatable to most herbivores and is preferentially grazed during key times of the year. In a recent survey in southern NSW the plant was considered relatively common in sites that had been lightly grazed or where grazing was excluded but was uncommon or absent on sites that had been moderately and heavily grazed (Prober and Thiele 1995). This sensitivity to domestic stock grazing appears to be uniform across its range. McIntyre et al. (1993) found that on the northern tablelands the plant occurred in only 4% of sites surveyed.
because grazing by domestic stock at these sites
grazing resulted in a reduction in the amount of
also reduced the integrity of the remnants, making
vegetation in the ground layers as well as a
them more liable to invasion by exotic plants. In this
reduction in habitat complexity. As grazing
case, an intermediate level of disturbance associated
increased, the removal of vegetation and the effects
with light, intermittent grazing or possibly fire may
of cattle trampling were found to lead to significant
result in a better conservation outcome than either
changes in beetle species richness and community
heavy, continuous grazing, which would threaten
composition.
the community, or no disturbance at all, which would cause the rare species to become extinct.
Within these ecosystems it is difficult to separate the effects of repeated burning and grazing because fires are frequently lit to encourage new, palatable
30
Forest ecosystems and grazing
growth of herbaceous material and to remove
Many forest ecosystems of the east coast are grazed
unwanted older vegetation. The resultant
by domestic stock. Although the impacts of grazing
simplification in forest structure appears to
have not been extensively studied there are
significantly affect the faunal assemblage (Smith
indications that it may have significant though
1994). Areas that are frequently burnt and grazed
subtle effects on these ecosystems. In a study on
have lower biodiversity than areas that are unburnt
the effects of grazing on forest biodiversity, York
or infrequently burnt (Smith 1994, York 1998).
(1998) found that light grazing by cattle in forest
Smith (1994) found that 22 species of birds were
ecosystems had little effect on invertebrate diversity
less abundant on sites frequently burnt and grazed
in most forest areas. However, cattle preferentially
and only four species were favoured by the more
grazed in the more open sites where intensive
open, grassy forested areas.
Grazing of the rangelands of the arid/semi arid inland
while the lack of herbaceous ground cover leaves the
Grazing is the principal land use in the arid and
It is important to note that the lack of a herbaceous
semiarid rangelands of NSW where the most
ground layer is often due to the combined effects of
significant impact on native vegetation has been the
competitive interaction of the shrubs and continued
increase in grazing pressure. This has arisen from
stock, feral and native grazing pressure.
the introduction of exotic herbivores and increased populations of some native and feral herbivores as a result of the provision of stock watering points. Many authors now believe that grazing at the current intensity across the entire landscape is not sustainable (James et al. 1995).
topsoil without adequate protection from soil erosion.
It is in the arid zone that most of the mammalian extinctions in Australia have occurred with most of the small-to-medium size ground-dwelling species now either extinct or occurring only on small offshore islands (James et al. 1995). Many of these extinctions are related to fox and cat predation.
Rainfall in the arid zone is characterised by sporadic,
Nevertheless, it is difficult to separate the effects of
patchy and small rainfall events and less common
reduction in habitat complexity and critical resources
widespread large flood events. Before the
such as adequate cover from predation. It is
development of stock watering points, grazing
interesting to note that some of the species that
animals either moved to other areas or simply died
have become extinct on mainland Australia now
out during droughts. The provision of water
occur only on offshore islands that are free from
increased the pressure on native vegetation, especially
both predators and domestic stock grazing. Further
during the critical period of re-establishment after rain
incidental but not conclusive evidence for the
events. Additionally, because of the availability of
interaction of grazing and predation can be found
water, the numbers of native, feral and domestic
in the forested lands of eastern Australia. In this
herbivores stay higher for longer periods and
higher rainfall zone, where the impacts of
therefore there is greater grazing pressure on the
modification from grazing are less pervasive, many
palatable perennial native vegetation that occurs
small-to-medium sized ground-dwelling mammals
within the area (James et al. 1995). This again
are still extant. Nevertheless, many of these
reduces the ability of the ecosystem to recover after
mammals, such as potoroos and bandicoots, are
scarce rainfall events. This pattern of elevated grazing
absent from heavily grazed remnants (Bennett 1987)
and reduced habitat heterogeneity is implicated by
and are less common in forest areas that have been
many authors in the decline of a number of native
burnt and heavily grazed (Smith 1994).
vertebrate species (Smith et al. 1998).
Grazing in temperate woodlands and its interaction with fragmentation
In much of the arid zone this depletion of perennial groundcover resulting from altered grazing regimes is attributed as the cause of the widespread growth of woody native shrubs, the so-called woody weeds (Hodgkinson and Harrington 1985). The exact mechanism of how formerly perennial grass/ chenopod rangelands become dominated by nonpalatable woody shrubs is uncertain but is thought to relate to reduced fire frequency, reduced competition from herbaceous species as a result of grazing, and changed soil moisture regimes in response to the altered herbaceous layer. Woody shrub invasion is considered to be a significant cause of land
The temperate woodland areas of NSW primarily exist as small areas of remnant woodland vegetation in a matrix of agricultural crops, introduced grasslands or derived native grasslands. Very few of these woodlands are not grazed by domestic stock and almost all of them are or have been heavily grazed by feral herbivores, most notably the rabbit. In these ecosystems the effects of the interaction between fragmentation and grazing are most pronounced. It is often difficult to determine whether species loss in these landscapes is a result of fragmentation or the influence of elevated grazing pressure.
degradation in the arid zone. Areas dominated by
The grassy White Box (Eucalyptus albens) woodlands
woody weeds tend to be of low grazing productivity,
were the dominant vegetation type over a vast area
31
of the NSW western slopes, stretching from just
forestry operations on biodiversity (adapted from
over the border of Victoria to just past the border of
Norton and Kirkpatrick 1995):
Queensland. Over much of this area, and in
l
particular the southern half of the distribution, these
biological and ecological processes are significantly altered by forest fragmentation
woodlands now remain only as small scattered
and modification, which may be irreversible
remnants, isolated trees in agricultural paddocks and highly modified derived native grasslands. In a
l
can lead to reductions in genetic diversity
study of the plant species richness within White Box remnant woodlands, Prober and Thiele (1995)
l
found that species richness declined with the size of the remnant so there was a significant effect of habitat loss and fragmentation. Additionally, they
to changes in dominant grass species; Themeda
reduced viability of threatened plant and animal species
l
reduced water quality resulting from forestryinduced erosion
found that domestic stock grazing greatly affected plant species richness and composition. Grazing led
reduced populations of plants and animals that
l
reduced ecosystem and landscape diversity and connectivity.
australis and Poa sieberana were the dominant
Nevertheless, it can be argued that these impacts on
grasses in the ungrazed remnants, but declined
biodiversity are related to the type of logging and are
rapidly with intensification of grazing and were
not the inevitable outcome of forestry. The intensity
replaced mostly by exotic annual grasses such as
and spatial distribution (and timing) of forestry
Bromus hordeaceous and Hordeum leporinum, and
operations are important (Law and Dickman 1998).
the native Bothriochloa macra. Three native species
Intensive logging can lead to temporarily simplified
were not recorded in remnants that were subject to
habitat. If the whole, or even significant proportions,
grazing and a further two showed a marked decline
of the forest or isolated remnant forest were logged in
in abundance when subject to any grazing.
this manner, the impacts on biodiversity would be
Forestry
severe. If logging is carried out in a diffuse manner with return logging cycles that match the recovery
The impact of forestry operations on biodiversity
patterns of the native forest, and sufficient areas of
and conservation values is a subject of intense
interconnected forest are left unlogged, then at a
debate and conflict within Australia. Considerable
regional scale the impact of forestry operations is
resources have been directed towards research and
likely to be less severe.
planning for sustainable forestry as is illustrated by the recent federally sponsored Regional Forest Agreement process. A full discussion on the effects of forestry is not possible in this paper and the reader is directed to other works such as Norton and
In much of eastern Australia the remaining
Kirkpatrick (1995) and Norton (1997).
biodiversity must simultaneously contend with the
Logging is a widespread and highly variable disturbance that can lead to losses and, in certain circumstances, increases in the total biodiversity of a site. Losses of biodiversity can arise from the impact of logging management and intensity on key habitat resources such as tree hollows and oldgrowth trees (Law and Dickman 1998). The loss of such habitat resources can lead to localised extinctions and population declines of some
32
3.2.3 The combined effect of habitat loss, fragmentation and modification on biodiversity
effects of fragmentation, grazing, introduced competitors and predators, logging, firewood collection, herbicide and pesticide spray drift, and other altered environmental conditions. To date only a handful of studies have explicitly attempted to look at combinations of even two of the above factors, for example, grazing and fragmentation (Prober and Thiele 1995). Studies that examine how these various factors interact in real environments are required.
arboreal marsupials and hollow-dependent bats and
In highly fragmented environments the impacts of
birds. The following are potential impacts from
localised habitat modification can lead to the
decline of species able to survive the initial impact of habitat removal. For example, in grazed remnant woodlands in the fragmented Western Australian wheatbelt, only two of the originally occurring seven species of geckos are now found (Sarre et al. 1996). All seven species still exist in the region in the ungrazed conservation reserves within the same fragmented environment. The prime difference between the remnants on private land and the conservation reserves is the intensity of grazing. The conservation reserves are not grazed and are structurally more diverse than the grazed woodlands. Therefore fragmentation alone cannot be considered the reason for extinction in the
3.3 Other threats to biodiversity in native vegetation There are a number of processes that can affect biodiversity in native vegetation but may not be directly related to native vegetation loss or alteration. Nevertheless, they contribute to the current pressures on native biodiversity. For example, climate change is potentially a significant threat to terrestrial ecosystems. However, this issue is fully discussed elsewhere (see Rawson and Murphy 2000). Exotic species invasions and pollution are two processes of more immediate significance.
grazed remnants.
3.3.1 Exotic invasions Dieback
The spread of exotic species is a poorly appreciated
Dieback is an extreme example of how
but major threat to biodiversity. Modern
fragmentation and habitat modification can
transportation, and the movement of people and
combine to affect biodiversity and ecosystem
items of trade, has increased the rate of spread of
function. Dieback occurs when trees die or lose
species to new locations. This means that exotic
condition prematurely and often rapidly (Landsberg
introductions are an issue of global significance which
and Wylie 1991). Often large numbers of trees are
requires concerted international attention (Vitousek
affected at once. In NSW, dieback is common in
1992). Australia, as an island continent, has been
fragmented remnants of woodlands on the
relatively isolated from the rest of the world, and is
tablelands and in River Red Gum and Box
particularly vulnerable to introductions of exotic
communities further inland, with numerous
plants, animals and microbes. The consequences of
localised, but severe, occurrences elsewhere
introducing a new species to an ecosystem are
(Nadolny 1995). The causes of dieback are varied
difficult to predict and it is still uncertain whether an
and complex and vary in importance from region to
introduced species will become established, or if its
region. It is common in rural areas, where it
presence will cause existing species to disappear.
appears to be linked with land-use practices
In Australia, a large number of exotic species has
(Heatwole and Lowman 1986, Beckmann 1990).
become established over the last 200 years, and
These include chronic insect infestations (see Box
more are still arriving. These include 25 mammals,
8), increased salinity, waterlogging, lack of water
32 birds, 2 reptiles, 1 amphibian, 21 freshwater fish,
along inland streams, and prolonged inundation
1,500-2,000 vascular plants, and an unknown
due to river regulation. At a more local scale, tree
number of invertebrates and microbes (State of the
deaths can result from fungal diseases, mistletoes,
Environment Advisory Council 1996). At least 290
herbicide spray drift or spillage in waterways,
taxa of vascular plants became naturalised in
girdling of trees by livestock, earthmoving
Australia between 1971 and 1995 (Groves 1997).
(particularly the burial of tree roots), fire, frost, wind pruning, hail and self-thinning in dense
Exotic plants
regrowth stands. The long-term effect of herbicide
Most vegetation in NSW, particularly in agricultural
spray drift on trees is one matter of recent public
areas, consists of an artificial combination of native
controversy that requires investigation. Diagnosing
and exotic species (Bridgewater 1990). For
the causes of dieback in particular instances can be
example, of 1,041 vascular plant taxa (1,016
very difficult (Bacon 1995).
species) known from the Tamworth district, some
33
Box 8: Insects and dieback Dieback in rural areas often involves repeated defoliation by a variety of native insects. Severely defoliated trees become susceptible to fungi, which produce lesions (cankers) that cause branches to die back (Old et al. 1990). Many types of insects are involved, including psyllids, scarab beetles, chrysomelids (leaf beetles), leaf hoppers, sawfly larvae, scale, gall-forming insects and skeletonising caterpillars. Their relative importance varies over time and, since different insects prefer different species of eucalypts, the relative susceptibility of eucalypt species to dieback changes. We are far from fully understanding this type of dieback but several factors appear to contribute to insect outbreaks. l
Insect outbreaks generally occur when the weather favours the insect’s breeding and survival. Many species of insects involved in dieback are susceptible to extremes of heat or cold or prolonged wet or dry spells (Davidson and Davidson 1992).
l
Clearing and pasture improvement can favour scarab beetles that feed on eucalypt foliage. Their larvae feed on roots of pasture species and on soil organic matter. Populations of scarab larvae can build up to greater densities in improved pastures than in native pastures. However, improved pastures usually dry out faster than native pastures, causing populations to collapse to a greater extent during dry spells. This means the relative abundance of scarabs in improved versus native pastures depends on the weather.
l
Effectiveness of control by natural enemies is reduced due to habitat loss. Densities of birds in areas affected by dieback are about 10% of those in healthy woodland (Ford and Bell 1982). The number of bird species is dramatically reduced in small patches of bushland that have been taken over by aggressive birds such as Noisy Miners.
l
More nutritious foliage can increase the number of young each female insect produces and may increase the growth rates and survival of the young insects. More nutritious foliage can result from increased soil fertility beneath trees due to livestock excretions and pasture improvement (Landsberg et al. 1990). More nutritious foliage can also result from trees being under stress (White 1978) or having a greater proportion of young, nutritious leaves because older leaves that have been damaged by insects are shed. More work is required to quantify these effects (Landsberg and Cork 1997). In particular, eucalypts respond to fertilisers and, if insects do not eat them, fertilised eucalypts grow faster than unfertilised ones (Fox and Morrow 1992).
38% are exotic (Hosking and James 1998). Riparian
Most plant species that have recently become
and flood plain areas, with rich soils that are
naturalised were deliberately introduced to
frequently disturbed, often have an especially high
Australia. Introductions of ornamentals accounted
proportion of exotic species.
for 65% of the taxa, agricultural species for 7%,
In NSW, multi-species infestations of exotic plants are more common than those caused by a single species.
34
and only 2% are known to have arrived as contaminants in some product (Groves 1997).
Invasive plants include vines (particularly in sub-
Continuing efforts to find exotic pasture species, or
tropical environments), perennial shrubs and grasses,
cultivars, capable of long-lasting and productive
and wetland and aquatic plants (Humphries et al.
growth on poor soils, present a problem.
1993). Benson (1999) compiled a list of the most
Characteristics that enable a species to persist in a
serious environmental weeds (that is, plant species
pasture are often similar to those that could also
capable of invading native vegetation) in NSW.
enable invasion of conservation areas and other non-
productive sites. Of 463 introductions of exotic
from spectacular successes such as the control of
pasture species in northern Australia attempted since
prickly pear and Noogoora burr (Xanthium
1947, a total of 60 became listed as weeds, while
occidentale) (McFayden 1998) to complete failures.
only four were useful and never became a weed
The potential of biological control of many weeds
problem (Lonsdale 1994). Clean up of experimental
has not been fully evaluated but could offer a
sites, which could reduce the spread of such plants, is
prospect for cost-effective control.
not mandatory in either Queensland or NSW.
Exotic animals Some exotic plant species that were introduced for soil conservation have also become weeds. For example, Coolatai grass (Hyparrhenia hirta) was introduced over 50 years ago and persisted in relatively small areas on the western slopes. However, the species is now spreading rapidly, apparently after becoming acclimatised, and now dominates extensive areas on the northern slopes and tablelands (Nadolny 1998). Such a lag period before rapid spread is common in plant invasions (Hobbs and Humphries 1995). The use of exotic pasture species is still standard practice for soil conservation works in many
Some introduced animals such as the rabbit have already been responsible for major environmental destruction and loss of biodiversity. Rabbits have reduced regeneration of native plants, and their presence has probably artificially increased the populations of predators that also prey on vulnerable native animals. Foxes and cats are associated with the decline of many native animals. In parts of western NSW goat populations are greater than those of domestic livestock. Other feral species are of concern in particular areas.
districts and contributes to the spread of exotic
Introductions of exotic invertebrates also pose threats
species throughout the landscape.
to the Australian biota. There are many prominent
Certain disturbance regimes favour certain weeds. Roadsides are an important conduit to the spread of weeds. Ground disturbance, especially heaping soil onto a site, with the associated nutrient enrichment, and the presence of excess water from runoff, favours many weedy species. Vehicles and earthmoving machinery also spread weed seeds along roads. However, eliminating disturbance will not prevent the spread of all weeds. Some weeds, such as privet, can become established beneath intact open forest.
examples (see Box 9). Currently there is pressure from horticulturalists to introduce bumblebees to mainland Australia, even though they appear to directly compete with native bees in Tasmania (Hingston and McQuillan 1999). Intentional introductions of exotic earthworms are of concern because their effects on the native soil fauna and nutrient cycling are uncertain. The impacts and possible extent to which many naturalised species can invade native vegetation remains uncertain. For example, cosmopolitan springtails (Collembola) dominate the soil meso-fauna
Once established, many exotic weeds are difficult to
beneath most exotic pastures (Chilcott 1996) but it is
eradicate. In particular, there is no effective control
not known whether these are recent introductions or
for exotic perennial grasses such as Serrated Tussock
just cosmopolitan species that have resided in
(Nassella trichotoma) and Giant Parramatta Grass
Australian ecosystems for an extended period.
(Sporobolus indicus var major). Repeated spot spraying may work if the infestation is small, but can kill surrounding native grasses (Campbell and van de Ven 1996), resulting in bare patches that can be recolonised by the problem species. Alternatives, such as altering grazing regimes to discourage problem species, require further investigation.
Exotic microorganisms Exotic microorganisms can cause major impacts on the Australian biota. The Cinnamon Fungus (Phytophthora cinnamomi) causes serious disease in eucalypts and associated vegetation in Western Australia, Victoria and Tasmania. The fungus is water-
Being in an environment without a full complement
borne and most serious infestations are associated
of their natural enemies can advantage exotic
with periodically wet sites. It is present in NSW, but
species. Control of exotic pest weeds by
native sclerophyll forests in NSW (and Queensland) do
introductions of their natural enemies has ranged
not seem to be seriously affected except in localised
35
Box 9: Honeybees and apiary The impacts of the introduced European Honeybee (Apis mellifera) are a contentious issue. The commercial honey industry relies exclusively on production from this insect, which gathers pollen and nectar from both native and introduced plant species. Honeybees appear to effectively compete with and have a deleterious effect on the population of at least some native pollinators (Pyke and Balzer 1985). Their different foraging behaviour may lead to reductions in pollination success in some native plant species (Paton 1993), while in others their effects on pollination are either beneficial or not significant (Paton 1997). Bees appear to be more efficient pollinators of some exotic plants such as clovers and medics and their presence may favour the relative competitiveness of those species. In addition, feral honeybees are widely dispersed and often nest in tree hollows where they could displace native species of bees or other hollow-dwelling animals. To date no localised extinctions attributable to the Honeybee have been documented, but this may be the result of our poor knowledge of invertebrates. Possible effects on the pollination success of rare or threatened plant species and on species with small populations resulting from fragmentation are, however, of concern (Paton 1996).
areas on the southern tablelands and in Silvertop Ash
not have regional or continental scale impacts.
(Eucalyptus sieberi) forests on the south coast.
However, in areas affected by serious pollution,
Localised patches of heath vegetation and sclerophyll
the effects on biodiversity can be substantial. A
understorey are also affected (Environment Australia
notable case is the almost total destruction of
1999). It is likely that Cinnamon Fungus behaves as a
vegetation surrounding Queenstown in Tasmania.
newly introduced species in Victoria and Western
The bald hills surrounding the town are the result
Australia, but could have been present in NSW for
of a combination of removal of trees to provide
long enough for sensitive species to have already
props for the mine and fuel for the smelters. The
been displaced from high-risk sites, such as along
smelters in turn produced water and air pollution
gullies, or to have developed some resistance.
that further affected the native vegetation
However, some forms of dieback in NSW, particularly
through acid rain. The high rainfall of this part
dieback of Antarctic Beech (Nothofagus moorei) and
of Tasmania eventually washed the topsoil and
coachwood (Ceratopetalum apetalum) associated
seedbeds from the hillsides leaving a bare
with road work and logging operations, have not
landscape (Woodside and O’Neil 1995).
been adequately explained and may involve a fungal pathogen such as Cinnamon Fungus (Nadolny 1995).
3.3.2 Pollution
36
The most significant threats to native vegetation and biodiversity from pollution sources are spray drift of pesticides and herbicides, sulphate deposition surrounding large smelters, nutrient
In Australia the effects of pollution on terrestrial
pollution and subsequent weed infestation in
ecosystems tend to be localised and usually do
urban bushland and wetlands.
SECTION FOUR
CONSERVING NATIVE VEGETATION AND ITS ECOLOGICAL VALUES The early settlers were driven by an honourable
Representative (CAR) reserve system is a major
vision of transforming the bush into a productive
element of the Government’s strategy for the
landscape (Davidson and Davidson 1992). This
conservation of biological diversity (NSW Biodiversity
vision drove them through adversity to clear land
Strategy 1999). This process aims to set aside, in
and develop it for farming. Unfortunately (and
formal conservation reserves, representative examples
understandably) this was not based on an
of all ecological communities and individual species
appreciation of the values of native vegetation or
occurring in the State. To be considered adequately
the effects their practices would have on important
conserved, a number of criteria including at least 15%
ecosystem processes. Additionally, Government
of the areal extent of the pre-settlement distribution
policies often encouraged overclearing. Things have
of any particular vegetation community (taken as the
changed and farmer-led movements such as
extent in 1750) is to be included within conservation
Landcare are becoming important vehicles for both
reserves (Young and Howard 1996).
the development of a conservation ethic and the coordination of on-ground actions aimed at conservation. However, ecological information and understanding of the role of native vegetation is often lacking. The challenge now is to motivate efforts to ensure long-term sustainability of agriculture, in conjunction with the retention, careful management and, where appropriate, restoration of remaining native vegetation. To achieve this, ecological requirements must be considered within a socio-economic context (Lambert and Elix 2000, Gillespie 2000). Falling commodity prices and increasing input costs have created a cost-price squeeze affecting many agricultural sectors. Landholders who depend on an income derived from a given parcel of land may be tempted to develop more of their land for production to compensate for declining returns, even though the net economic gains are usually small (Lawrence 1987). The instability of agricultural markets may prompt farmers to change what they are producing, say from livestock to crops, which can involve clearing of native vegetation with all its associated problems.
NSW is far from achieving a CAR reserve system, particularly in the more productive lands such as the southwestern slopes and central wheatbelt, mainly because these areas were developed for agricultural production before conservation of biodiversity became a priority (Pressey et al. 1999). For example in the South Western Slopes Bioregion less than 1% of the land surface area is within conservation reserves (Benson 1999). One of the difficulties of implementing this strategy is that information on the composition and classification of ecological communities (based on the full assemblage of species) is difficult to collect and consequently is lacking. As a result, a number of surrogate methodologies have been tested (Norton 1997) and no systematic process of classification is universally accepted. The most common approach has been to use vegetation communities, as described by dominant canopy species, as surrogates for ecological and biological diversity (Margules 1989, Hager and Benson 1994, Pressey et al. 1996). Unfortunately, the use of dominant canopy species as a surrogate for other biota is questionable because these species alone appear to be poor predictors of understorey plant
4.1 The role of nature conservation reserves
species and invertebrate assemblages (Norton 1997,
Conservation reserves are an essential component of
Active management of these areas is often required
any strategy to conserve biodiversity. Without
because much of the Australian biota requires some
reserved areas many of the effects outlined in Section
form of disturbance regime. Many areas have been
3 will continue across the whole landscape. In NSW
disturbed in the past in a way that allowed exotic pest
the development of a Comprehensive, Adequate and
species to become established, so management may
Kirkpatrick 1997, and Majer et al. 1997).
37
be required to prevent further spread. Reserve
l
encourage and support management of native
managers may need to strategically manipulate some
vegetation for conservation within private and
areas to protect and enhance the survival prospects of
leasehold land (see Gillespie 2000, Sheahan
particular threatened species and communities.
2000 for mechanisms) l
4.2 Conservation outside the conservation reserve system
develop a policy on carbon credits which ensures that credits contribute to conservation of native vegetation or are located to help prevent dryland salinity or erosion (see Rawson
Conservation reserves can only be part of the
and Murphy 2000)
strategy to protect biodiversity. Conservation of biodiversity requires that the task be viewed from a
l
continue to support removal of unnecessary
whole landscape context. Attempting to achieve
watering points and bore drains in western
complete conservation of all species in conservation
NSW (see Section 3.2.2 on grazing impacts)
reserves alone is unachievable and unwise. This is
l
continue to provide establishment funds for
because the funds and resources available to
native vegetation restoration (see Sheahan
acquire and then manage the reserve system are
2000)
limited and, unless viable populations of all species and combinations of species are adequately
l
support adoption of stocking rates that minimise loss of native biota and ecosystem
represented in reserves, the system will not be
function.
adequate. More typically, the continued occurrence of species within the reserves is dependent on ecological processes and habitats outside reserves. Therefore, biodiversity conservation requires the
4.2.2 Ecological research and development
planning of land use at regional, district and
Research and development is a powerful tool for
property scales and the adjustment of some
shaping our future and a sound understanding of
management practices to prevent further net losses
ecological process is necessary for sustainable
of biodiversity and damage to ecosystem function.
natural resource management. Well-directed
These actions should also aim to maximise
research can benefit both human welfare and the
production benefits conferred by native vegetation
environment. Possible options for research and
and to minimise production losses associated with
development are to:
achieving long-term sustainability.
l
This section presents some options for policies or
research and development project
initiatives that could enhance the prospects for
recommendations are assessed in terms of
improving conservation of biodiversity and ecological
environmental risks
processes within the rural landscape. This is not an
l
ensure experimental sites involving
exhaustive list, and focuses on ecological issues.
introductions of exotic species are cleaned up
However, many solutions involve economic incentives
at the end of the project
and policy revision which are discussed in other Background Papers of this Native Vegetation Advisory
l
develop mechanisms to involve land managers in research and the setting of priorities
Council series (Gillespie 2000, Sheahan 2000). l
38
establish a review process to ensure that
encourage and fund innovative research and
4.2.1 Incentives and rural restructuring
development projects that increase the
Incentives and direct Government support for
vegetation in agriculture, while ensuring the
actions that benefit the environment are needed
projects fully evaluate environmental risks
(see Gillespie 2000). Some suggestions are to:
(see Box 10)
prospects for more sustainable use of native
l
identify and address gaps in our understanding
(v) processes that control vegetation change, in
of how to best achieve conservation of
particular,
biodiversity in rural landscapes.
l
processes that lead to excessive woody regrowth (especially in western NSW)
Priority research for rural landscapes 1. Develop inventories of the current distribution
l
and abundance of native plants and animals in all
extent and rate of spread of invasive exotic plants, and the invasion processes
ecosystem types (including wetland and aquatic l
systems) to:
critical values of native vegetation disturbance where significant land
(i) determine the current status of our biota as a
degradation occurs
basis for assessing future trends l
(ii) more objectively assess which, and to what
reversibility of land degradation and the value of native vegetation in its reversal
extent, species and communities are threatened (vi) links between vegetation, water quality and
(iii) provide spatial information to assist the
river health
development of vegetation management plans.
(vii) links between terrestrial and wetland
2. Monitor changes in native vegetation condition
vegetation types
and the status of fauna.
(viii) effects of vegetation on regional climate
3. Develop a better understanding of: (i) critical thresholds of habitat loss,
(ix) effects of vegetation on catchment hydrology
fragmentation and connectivity
(especially in salt-prone areas)
(ii) conservation values of native vegetation used
(x) natural control of pest species.
for productive purposes
4. Develop better methods to:
(iii) the combined effect of fragmentation and habitat alteration on biota and ecosystem
(i) re-establish native vegetation
processes
(ii) assess risks of degradation
(iv) the various causes of eucalypt dieback and
(iii) reduce impacts of grazing by both native and
possible preventative actions
exotic herbivores.
Box 10: Example of an innovative approach Farmer-led development of techniques to directly sow winter-active wheat or oats into degraded summer-active native pastures shows promise as a means of reducing soil erosion and loss of soil fertility on wheat-sheep farms in the central west (Jones 1999). The native pastures are dormant when the wheat is sown and do not compete with the wheat until it is almost ready for harvest. However, this technique, and the purpose-built machinery it requires, needs extensive fieldtesting and fine-tuning, which is not possible without Government support. This example also illustrates the limits of relying on improved technology to achieve sustainability and how research and technology can lead to unforeseen consequences. If successful, the technique would improve sustainability on a given site in comparison with conventionally sown wheat, which requires complete destruction of all existing vegetation. But how would regional biodiversity be affected if the technique were used to expand wheat production to areas not considered suitable for conventional cropping?
39
5. Develop integrated solutions to landscape
financially in the short-term. This leads to the
conservation problems by forming multi-disciplinary
possibility of individuals overclearing and profiting at
teams led by people capable of developing solutions
the expense of the community at large, unless
that are compatible with both socio-economic and
clearing is regulated (Possingham et al. 1995).
biophysical reality. To protect the ecological and biodiversity values 6. Resolve issues where landholders receive
of native vegetation, legislation to regulate
conflicting advice (for example, in what circumstances
clearing should:
should fallen timber be retained for wildlife habitat or removed to eliminate rabbit harbour?)
l
protect ecosystems of high conservation value and cater for threatened species
4.2.3 The role of legislation
l
ensure that clearing does not cause
The environmental consequences of clearing, as
unacceptable damage to ecological
discussed in Section 3, are long term and their costs
processes (for example, increase dryland
are often borne by the community as a whole,
salinity, soil erosion, waterlogging or
whereas the individual that clears may benefit
extreme loss of soil fertility)
Box11: Case study – the challenge of utilising native pastures Historically the merits of native grasses were not appreciated in terms of their resilience to climatic extremes, capacity to regenerate, and suitability for grazing systems with lower environmental impacts and desirable agronomic qualities. Intensification of grazing generally relied on introduction of exotic plants and greater use of chemicals and machinery. Sown pastures gained prominence because many of the original grasslands were degraded by continuous grazing or were otherwise not entirely suited to the needs of livestock. These sown pastures often failed, especially on poorer soils or during periods of drought. Sowing was not always of economic benefit, especially on poorer soils when commodity prices were low (Pitt 1995). Intensive pasture development also resulted in unforeseen environmental problems, such as tree decline (Nadolny 1998). Much of the evidence for superior performance of exotics came from trials that compared newly established, well-fertilised, carefully selected cultivars of exotic pasture species with unfertilised, degraded native pastures, without even distinguishing which species were present (Jones 1995). It is now clear that many native pasture species are highly productive and respond well to fertiliser (Lodge and Whalley 1989, Lodge 1994). Native grasses did not fit in well with the philosophy of treating pasture as a crop to be sown. Wild varieties of native grasses are difficult to sow and harvest because of problems such as a prolonged period of seed set and dormancy (Lodge 1994). Until recently, native grasses were excluded from research related to pasture establishment and consequently seed has not been commercially available. Several promising native grasses, including species of Danthonia and Microlaena, are now being domesticated and have been found to compare well with currently used exotics in terms of persistence under grazing, productivity and nutritive value (Jones 1995). An alternative approach of manipulating the species composition of pastures by using grazing management and completely avoiding the need to sow is gaining interest (Lodge and Whalley 1985, Earl and Jones 1996). Knowledge about the best ways to manage native grasses is accumulating but is still in its infancy compared to knowledge about favoured exotic species (FitzGerald and Lodge 1997). Interest in native grasses is now increasing and, in areas such as the northern tablelands, fewer exotic pastures are being sown.
40
l
l
provide for planning which enables limitation
to be carried out at various scales: continental,
of losses of native vegetation at landscape and
bioregional (or regional), sub-catchment (or
regional scales and minimises the extent of
community group) and individual property.
fragmentation.
Community and landholder ownership and support
reduce the rate of native vegetation loss, especially where ramifications of clearing are uncertain and thresholds are unknown
of plans is essential to ensure willingness to implement them. Plans initiated and developed by groups of landholders are to be encouraged. Planning should also be based on a common
l
be flexible enough to allow for a diversity of
understanding of the ecological principles set in the
management solutions and be accommodating
context of current socio-economic constraints and
of new scientific information.
opportunities. Plans require clear goals and
The NSW Native Vegetation Conservation Act 1997
mechanisms that prioritise actions and ensure that
includes provisions aimed at achieving these
these are implemented by the reviewing progress,
objectives. A review of the effectiveness of the NSW
which should adjust the plan as conditions and
legislation is beyond the scope of this paper.
knowledge change.
However, the following issues have been the subject
Many Landcare and other community groups have
of recent debate and should be considered when
developed district or sub-catchment plans that
the legislation is reviewed:
include vegetation management (Prior and Browne
l
l
clarification of the definitions, particularly those
1999). These plans embrace the local community
relating to sustainable grazing, native
vision and are usually developed in consultation
groundcover and what constitutes clearing in
with advisers, including nature conservation
native grass pastures and wetlands
specialists. Building on this foundation, Regional
a review of the exemptions to ensure that definitions and interpretation is consistent with the objectives of the Act.
Regarding other legislation: l
l
Vegetation Committees can achieve a great deal in conservation terms.
4.3.1 Setting achievable conservation goals
Zoning provisions could be used to distinguish
Conservation goals need to be achievable. There is
lands intended for different agricultural (or
limited benefit in setting goals based on a concept
private conservation) uses with different
of what the landscape was like before European
impacts on native vegetation and ecological
settlement. We need to accept that significant
processes (for example, land intended for
change has occurred within local ecosystems, plan
cropping could be distinguished from
to retain what remains, and restore those processes
rangeland grazing areas, and irrigated lands
that are critical. For example, a study of the
could be distinguished from those intended
prospects for bird conservation on the northern
only for dryland cropping).
tablelands showed that some regionally threatened
Regulations regarding legal imports of exotic organisms have not prevented the recent introduction of potentially threatening species and may require review to prevent the importation of species that are potentially ecologically damaging.
species were typically forest dwellers that were unlikely to persist in the now fragmented forests and woodlands (Barrett et al. 1994). Planning in this region can now cater only for woodland birds, which can survive well in bushland remnants over about 20 ha in size, rather than the forest specialists, which generally require habitat patches of over 200 ha.
4.3 The role of planning
One approach is to use a process of regionally specific
Planning is essential to optimise the conservation of
targets for native vegetation retention and/or
native vegetation and the resulting benefits. It needs
restoration and for the degree of protection for 41
various ecosystems and landscape features. Target setting is a related but separate concept to that of
l
rate of clearing of native vegetation
l
annual increase in area of native vegetation
critical thresholds discussed earlier. A threshold is a
protected (including high conservation value)
point where further change in the landscape, such as the amount of land cleared, results in a
l
percentage of pre-clearing distribution of
disproportionate or critical change in an
vegetation communities maintained in
environmental variable such as the size of an animal
statutory reserves and outside reserves
population or the number of species that can occur.
l
area of land revegetated for the purposes of
Targets, on the other hand, are goals that can be set
conservation, production, rehabilitation,
as a standard for landscape management or planning
amelioration of land degradation and carbon
and ideally should be well within the expected critical
sequestration
thresholds. These targets should attempt to integrate socio-economic, landscape and environmental
l
vegetation
aspirations. Specific targets should also address the intensity of land use in the matrix, for example, the
improvement in the condition of native
l
reduction of degree of fragmentation and
proportion of an area to be maintained as native
isolation of native vegetation, including the re-
pasture rather than developed for intensive grazing.
establishment of landscape linkages.
This can be important, for example, because the
It is expected that statewide and bioregional targets
incidence of dieback has been linked to the intensity
will be developed in 2000 (DLWC unpub.).
of pasture development. Properties with more than 50% improved pastures were found to have a greater incidence of dieback in southeastern Queensland (Wylie et al. 1993), while on the northern tablelands the incidence of dieback was related to how intensively the district was developed for pasture production (Morgan and Terrey 1999).
There have been several attempts to develop targets for native vegetation retention (Table 3). One of these studies (McIntyre et al. 1999) attempted to estimate the physical constraints on land use consistent with sustainable agriculture, as well as the ecological constraints on native vegetation removal (see Table 3). These estimates were based
Since the science of determining these targets, and
on the deliberations of 11 experts from various
the critical thresholds underlying them, is new and
natural resource management fields and should be
subject to much debate, there is only a limited
regarded only as a first, best-guess for property-
number of examples where targets have been
scale planning in the region specified. Other targets
developed for any region. The NSW Government
have been identified in the literature and are listed
has acknowledged the need for vegetation retention
in Table 3.
and revegetation targets to be developed, agreed on and implemented across the State (NSW Carr Labor Government 1999). As a first step, the Department of Land and Water Conservation has begun to develop a set of targets based on performance measures that have been endorsed by the relevant natural resource agencies (DLWC unpub.). These targets can be linked to other initiatives such as the statewide Native Vegetation Conservation Strategy, currently under preparation
42
The targets in Table 3 are not definitive and have been included for discussion purposes only. There is still much research required on a number of the issues, such as minimum patch size, and thought is required on how the targets relate to the management of the whole landscape. An additional note of caution is that targets developed specifically for one region may not be applicable to other regions.
by the Native Vegetation Advisory Council.
In regional plans, the evaluation of numerous
Although statewide and bioregional targets are yet
conservation planning options, and balancing them
to be finalised, the draft performance measures
against other considerations, becomes problematic.
which underpin target-setting for vegetation
A computer-based method using Geographical
retention and revegetation include:
Information Systems has been developed to allow
planning groups to rapidly evaluate the implications
responds to actions and to be able to adapt
of various conservation strategies (Pressey and
management actions in response to new
Logan 1997). The system could be useful wherever
information (Clarke and Davison 1997).
sufficient resource information is available in a digital form.
Many recent publications provide nuts-and-bolts advice for conserving native vegetation in the rural landscape (for example, Recher 1993, McIntyre
4.4 On-ground actions to conserve biodiversity On-ground actions to conserve biodiversity need to be based on a clear appraisal of the existing vegetation condition, and an understanding of the ecosystem and how vegetation responds to
1994, Wakefield 1997, Nadolny et al. 1997, DLWC 1998, and various extension notes by Greening Australia). The following principles for conserving native vegetation and wildlife at a property or district scale are based on these sources: l
Retain, or where possible enlarge, existing
management actions. It is also essential to monitor
remnant vegetation, especially where the native
how the vegetation and distribution of animals
vegetation is already fragmented.
Table 3 Example targets suggested for various regions
(a) Sub-tropical woodland/ Black Speargrass country in South East Queensland (based on McIntyre et al. 1999) Forest and woodland retained
30% min
Proportion of landscape managed for conservation
10% min
Bare ground exposed (at any one time)
30% max
Tussock grasses maintained (low grazing intensity)
60-70% min
Intensive pasture and cropping
30% max
Existing woodland patch size not to be reduced below
5-10 ha min
b) Other suggested targets Native vegetation cover (see Box 5, Section 3) 70% min1,
(Western Division – each vegetation type)
20% conserved1, Existing woodland patch size not to be reduced below Tableland woodlands Tableland forests
20 ha min2 5-10 ha min2
Width of important district connective links not to be reduced below Western NSW tablelands and coast
500 m min3 100 m min2,4
(1= Freudenberger et al.1997, 2= Barrett et al. 1994, 3= Smith et al. 1998, 4= C. Catterall, pers. comm.
43
l
Manage to improve the condition, in terms of
accommodate in a fragmented rural environment.
habitat value, of remnant vegetation.
Dispersed, restricted habitats, such as tableland lagoons or relict grasslands that occur within an
l
Link up remnants.
l
Ensure on-going regeneration (or replacement)
only way to conserve those species is to protect the
of tree cover.
habitat that contains them.
Protect watercourses, including vegetation
The NSW National Parks and Wildlife Service is
along drainage lines, ephemeral streams,
developing recovery plans for all endangered, and
wetlands, ponds and rivers.
ultimately all vulnerable, plant and animal species in
Protect vegetation on unusual or interesting
NSW. There is considerable debate as to how many
sites, such as rocky outcrops, caves and patches
resources should be allocated to individual
of unusual soil.
threatened species rather than to broader
agricultural setting, can contain unique species. The
l
l
l
Exclude introduction of, or where necessary control, exotic organisms.
conservation processes (for example, McIntyre et al. 1992). Special efforts, such as captive breeding, to prevent extinction of highly endangered species are
l
Reduce disturbance of groundcover.
prohibitively expensive. Earlier recognition and
l
Minimise edges.
action to eliminate threats is a more efficient use of
l
Retain dead standing trees and wood.
l
Separate high-intensity land use from areas
objectives. For example, actions to assist the Regent
where nature conservation is a priority.
Honeyeater (Xanthomyza phrygia), a threatened
resources. Sometimes action to protect threatened species is compatible with other nature conservation
migratory bird, may include protection of remnant
l
Minimise nutrient enrichment.
l
Retain vegetation in clumps rather than as
sideroxylon), a species that flowers prolifically, and
scattered trees.
contiguous River Oak (Casuarina cunninghamiana)
l
Maintain all strata of native vegetation.
communities. Such an action would advantage
l
Protect old trees.
l
Encourage a diversity of management
vegetation containing Mugga Ironbark (Eucalyptus
approaches to increase habitat diversity in the landscape.
4.5 Catering for the special needs of threatened species
4.6 Restoration of native vegetation for ecosystem processes Repair of native vegetation can range from partial ecological rehabilitation through to complete ecological restoration. The need for restoration of
Threatened species require special protection. For
native vegetation and rehabilitation of degraded
this to happen the most threatened species need to
land has growing importance in NSW, although
be identified, the reasons for their decline identified,
resource constraints and specific goals will
and a plan for their recovery devised and
determine what is attempted.
implemented.
44
many birds and small mammals in the locality.
Restoration does not simply involve the planting of
Various types of threatened species have special
trees. In many cases, restoration may be achieved
requirements. For example, small-to-medium
through improvements or modification of
ground-feeding mammals require thick ground
management practices such as grazing or fire
cover and protection from predators. Tree-hollow
regimes. For example, ecosystem processes and
dependent fauna are at risk because of the decline
biodiversity in degraded grassy woodland may be
of old hollow-bearing trees. Species with extensive
improved by reducing grazing pressure. The
home ranges are also more difficult to
increase in vegetative cover may reduce soil erosion
and the improvements in soil biological productivity
surface within a relatively short period of time.
and biodiversity would result in a build up of soil
Although, the values found under restored
organic material and reduction in the loss of
vegetation cover are generally a good deal lower
nutrients from the system.
than the original, undisturbed sites, it is possible
It is important, however, to be clear about the goals of any restoration project and the
that, over longer time periods, improvements of greater significance might be achieved.
limitations of the techniques that are to be used.
Nutrient loss would also seem to be slowed or
Ideally, restoration should be strategically
reversed where native vegetation cover is restored
planned on both farm and regional basis to
and, given sufficient time, it may be possible to
achieve maximum benefit for ecosystem
accumulate soil nutrients on degraded sites.
processes and biodiversity. Simple planting of
However, the time periods necessary for recovery are
non-local trees in degraded landscapes may not
uncertain and probably vary between different soil
have any effect on important ecological processes
properties. Some properties will undoubtedly
and is of little benefit to biodiversity.
recover rapidly following the re-establishment of
Alternatively, improvements of native grass cover
native vegetation but others (such as phosphorus)
on degraded hillsides through pasture
seem to be much more sensitive to disturbance (see
management could have significant benefits for
Wilson et al. 1997). It would therefore be unwise to
regional hydrology and provide better quality
imagine that native vegetation can restore land
habitat for grassland-dependent biodiversity.
surface quality to its original condition.
Appropriate restoration can benefit land quality and
Native vegetation also has considerable value in an
productivity. By restoring habitat and ecological
agricultural context by stabilising the soil surface
processes, restoration can also enhance biodiversity.
and reducing soil erosion. Shelter belts and
The extent to which degraded vegetation can be
remnant native vegetation have demonstrated
restored depends on a range of factors and much
benefits both for crop and livestock production (for
research is now examining best practice for
example, Reid and Thompson 1999) and the
restoration planning. Bodies such as Greening
protection and improvement of the land surface
Australia (NSW) have developed revegetation
(Bird et al. 1992). However, uncertainty remains as
policies that explore these ideas in more detail but
to the competitive effect between trees and ground
much work needs to be done.
vegetation in pasture systems. Retention or planting
The importance of vegetation cover in soil
of native trees and grasses strategically (for example,
conservation is widely recognised (Cumming and
with regard to catchment hydrology) may also limit
Elliot 1998). Native vegetation can increase soil
the incidence of dryland salinity and/or soil acidity
organic matter content (Ward and Koch 1996) and
and native vegetation restoration would seem to
offers some degree of rehabilitation of the land
offer both ecological and production benefits.
45
SECTION FIVE
CONCLUSIONS Native vegetation plays a vital role in the natural
water quality. Research needs to be directed to
environment of New South Wales and its presence
better understand ecological processes and to
in the landscape provides numerous benefits. These
develop better ways to sustainably manage and
include the conservation of land and water quality
utilise native vegetation in agricultural systems.
and the provision of much of the State’s native
Planning is essential to optimise the conservation of
habitat and biodiversity. However, significant
native vegetation and the resulting benefits and
degradation and loss of native vegetation has taken
must involve setting achievable goals for
place since European settlement, principally as a
conservation and restoration.
result of human activity with resultant degradation of soil and water resources and loss of biodiversity.
46
Funding to restore and manage native vegetation is scarce while the problems are large and complex.
Halting the decline in native vegetation cover and
Detailed scientific knowledge and information on
rectifying some of the damage that has been done
the management and restoration of native
is not an easy task, but is possible. Nature reserves
vegetation will be essential if we are to effectively
have a vital role in this recovery but are only part of
target the problems on a landscape level. Finally, a
the solution. Conservation of biodiversity requires a
concerted effort from both government and the
whole-of-landscape approach that embraces the
community will be necessary to reverse native
socio-economic context. We also need sympathetic
vegetation decline in NSW. The development of
management of privately owned native vegetation,
strong partnerships and well-directed action will
and it should be recognised that such management
result in tangible improvements in the NSW
can offer production benefits by preserving land and
environment.
SECTION SIX
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This background paper is one of a series produced by the Native Vegetation Advisory Council of New South Wales Other papers in the series are: Background Paper 1: Setting the Scene: Native Vegetation in NSW, John Benson, National Herbarium, Sydney, 1999. Background Paper 3: Social Values of Native Vegetation, Judy Lambert and Jane Elix, Community Solutions, Sydney, 2000. Background Paper 4: Economic Values of Native Vegetation, Rob Gillespie, Gillespie Economics, Sydney, 2000. Background Paper 5: Aboriginal Cultural Values of Native Vegetation, Stephan Schnierer, Southern Cross University, Lismore, 2000. Background Paper 6: Arrangements and Opportunities for Native Vegetation Management in NSW, Mark Sheahan, Department of Land and Water Conservation, Sydney, 2000. Background Paper 7: The Greenhouse Effect, Climate Change and Native Vegetation, Andrew Rawson and Brian Murphy, Department of Land and Water Conservation, Sydney, 2000.
Further copies of this paper and copies of the other background papers are available from: The Department of Land and Water Conservation Information Centre, ph: 02 9228 6415 or your nearest DLWC office. Should you wish to make any comments on this paper please write to: Executive Officer, Native Vegetation Advisory Council, Level 3, Bridge St DLWC, GPO Box 39 Sydney NSW 2001 email:
[email protected]
Acknowledgments The authors gratefully acknowledge the help of members of the NVAC steering committee and also Ian Oliver (DLWC), Col Rosewell (DLWC), Rob Banks (DLWC) and Rebekah Gomez-Fort for constructive comments on early drafts of this paper, and Irina Dunn for editorial work.
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