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Marine and Freshwater Research, 2009, 60, 1309–1316
Riverine macroinvertebrate assemblages up to 8 years after riparian restoration in a semi-rural catchment in Victoria, Australia Alistair BeckerA,B and Belinda J. RobsonA,C A School
of Life and Environmental Sciences, Deakin University, PO Box 423, Warrnambool, Vic. 3280, Australia. B Present address: South African Institute for Aquatic Biodiversity, Private Bag 1015, Grahamstown 6140, South Africa. C Corresponding author. Email:
[email protected]
Abstract. Willow removal followed by riparian revegetation is a widespread river restoration practice in Australia, but the ecological response to this has rarely been evaluated. We sampled river macroinvertebrates from six sites each of three riparian vegetation types: revegetated (treatment), willow-dominated (control) and native forest (reference) in the Gellibrand River catchment during austral spring 2007 and autumn 2008, and measured temperature and light intensity. Revegetated sites varied in age from 1 to 8 years since restoration. Abundances of invertebrates were similar across vegetation types, but were higher during autumn. Macroinvertebrate assemblages at revegetated sites (regardless of age) and at willow-dominated sites showed little among-site variation compared with native forest sites, which showed high site-to-site variability. Water temperatures and light intensity were higher at revegetated sites where works had recently been completed and cooler in native forest sites and long-established revegetated sites. The reduced variability in macroinvertebrate communities among revegetated sites may result from their history as willow-dominated sites or from the disturbance created by willow removal. Either way, these results suggest that longer than 8 years is required before macroinvertebrate assemblages in restored stretches of stream show the variation that appears characteristic of natural sites. Additional keywords: disturbance, multivariate dispersion, recovery, revegetation, Salix spp., willow removal.
Introduction Worldwide, river rehabilitation works are underway to minimise human impacts and restore ecological function (Bohn and Kershner 2002; Henry et al. 2002; Palmer et al. 2005). Within Australia, there has been a rapid expansion in effort towards river restoration (Webb and Erskine 2003; Lake 2005), which has included practices such as re-snagging (e.g. Lester et al. 2007), stock exclusion and riparian management (e.g. Jansen and Robertson 2001). Despite considerable effort, restoration works continue to struggle with basic questions, such as what constitutes a successful rehabilitation endpoint (Palmer et al. 1997; Giller 2005)? This is, in part, due to a lack of ecological study of restoration projects (Brooks and Lake 2007). Brooks and Lake (2007) reviewed 2247 restoration projects within Victoria, Australia, and found a poor retention of data on past works and low levels of monitoring, which are hampering our understanding of restoration ecology. They stressed the need for studies to properly evaluate the outcomes of restoration endeavours. The Corangamite Catchment Management Authority in western Victoria instigated an evaluation of their river restoration works in the Gellibrand River catchment. Therefore, the present study used existing restored sites within the catchment to evaluate the success of willow (Salix spp.) removal and revegetation with native riparian plants. © CSIRO 2009
When the outcomes of river restoration are assessed, some form of reference state or reference system is often used as a guide to an acceptable outcome (Lake 2001; Hughes et al. 2005). One problem with identifying an appropriate reference condition for riparian restoration is that there is a mismatch of scale between the area covered by most restoration works and disturbance processes occurring in the larger landscape within which they sit (Lake 2001). Agricultural landscapes contain a mosaic of riparian habitats at different stages of response to multiple disturbances, so the response to restoration practices at any single site will probably not produce an assemblage that resembles any former, less-disturbed state (Hughes et al. 2005). In addition, sites in reference condition are also likely to be fragmented in these landscapes, meaning that they too will reflect their divergent disturbance histories and specific location within the landscape. Restoration practices themselves can be viewed as a disturbance, and so river reaches where riparian restoration has occurred are on a recovery trajectory that is nested within the mosaic of multiple catchment-scale disturbances. Experimental work on grasslands has shown that apparently identical restoration ‘treatments’ can lead to widely divergent assemblages when there are multiple disturbing factors (Houseman et al. 2008). Therefore, the outcomes of riparian restoration involve considerable unpredictability (Hughes et al. 2005) as illustrated by 10.1071/MF08350
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the ‘Humpty-Dumpty’ and ‘shifting-target model’ pathways for recovery described by Lake et al. (2007). There is increasing interest in examining variability in biotic assemblages in response to disturbance or restoration (Fraterrigo and Rusak 2008), in part because it may assist in disentangling the issues of multiple disturbance, spatial scales and unpredictability described above. Disturbance is known to affect variability in assemblages (Fraterrigo and Rusak 2008), but it may either increase or decrease variation (Houseman et al. 2008). Generally, if a disturbance favours abundant and tolerant species in an assemblage, spatial variation will be reduced (Houseman et al. 2008). Conversely, a disturbance that creates opportunities for species by increasing habitat diversity may increase spatial variation (Houseman et al. 2008). Conceptual models for disturbance have struggled with those long-term disturbances referred to as ‘chronic’ or ‘press/ramp’ (Fraterrigo and Rusak 2008); however, it appears possible that reduced variation will result from repeated or continuous disturbance (Houseman et al. 2008). Based on these studies, we predict that fragmentation of native riparian forest will increase variability of in-stream macroinvertebrate assemblages because differences in size, location and disturbance history will increase habitat diversity and the range of possible response trajectories within each forest fragment. In contrast, we predict that the dominance of riparian areas by exotic willows is a chronic disturbance that will create an artificially uniform stream environment favouring abundant and tolerant species and therefore variance in invertebrate assemblages will be lower than in forest. Lastly, areas restored from willow-dominance to native vegetation have been recently disturbed and are predicted to start with assemblages with low variance. Using the model developed by Fraterrigo and Rusak (2008), riparian restoration of small spatial extent is predicted to result in little change in the variation of invertebrate assemblages, meaning that their level of variability will reflect their antecedent, willowed condition. Willows have been widely planted along riparian zones throughout Australia, often to provide bank stability following the removal of native vegetation (Holland and Davies 2007). While willows provide initial stabilisation of banks, over time they encroach into the stream, cause braiding and mid-stream islands and ultimately lead to increased erosion (Holland and Davies 2007). Leaf loss during autumn is rapid and dropped leaves are quickly broken down leading to water quality problems, particularly reduced dissolved oxygen concentrations. Willows are also highly invasive, and throughout south-eastern Australia, now line thousands of kilometres of stream (Holland and Davies 2007). Consequently, streamside willows are now being removed with native vegetation replanted in their place. The aim of the present study was to compare macroinvertebrate assemblages, stream temperature and light intensity within remnant native forest sites to sites that have had willows removed and restoration works completed (revegetated sites) and where restoration works have not yet occurred (willow sites). With this design, forest sites represent the reference condition for the physico-chemical and macroinvertebrate characteristics of a less-disturbed environment. Comparisons of forest to revegetated sites indicate if there has been a recovery of the ecosystem towards the reference state. One important aspect of our design is that we compared the revegetated sites to willow sites to estimate
A. Becker and B. J. Robson
the degree of change (if any) since works were carried out. This enables an assessment of how far the macroinvertebrate communities have diverged from their probable antecedent condition (Lake 2001). We hypothesised that macroinvertebrate assemblages would differ between all three types of sites, but that assemblage composition at revegetated sites would be more similar to forest sites than to willow sites. We predicted that willow sites would have fewer taxa than forest sites, but that macroinvertebrate abundances would not differ among site types. Also, we hypothesised that light levels and stream temperature would be higher in revegetated sites than in forest sites because the canopy was not fully developed at revegetated sites. We expected these patterns to be consistent across the two sampling times. Material and methods Study sites and design The upper Gellibrand River catchment is in the Otway Ranges, southernVictoria (Fig. 1,Table 1).The study sites were in streams and rivers that are close to one another (within a 10-km radius of the town of Gellibrand; Fig. 1), with the same climate (average rainfall 900 mm year−1 , maximum flows are in August and minimum flows in March), topography and geology (Department of Water Resources 1989). Bed materials were similar across all sites and varied from gravel and sand to fine clay within each site. Water quality is generally good, with dissolved oxygen levels near saturation, pH from 6 to 9 and low conductivity (below 0.66 µS cm−1 ). The catchment is generally forested, but contains patches of agricultural land, predominantly dairy pasture (Table 1). The Gellibrand River has a mean annual flow of ∼300 000 ML (Department of Water Resources 1989). At the areas sampled, streams were 0.16–2 m deep and 1.6–10 m wide. Continuous forest and forest fragments comprised wetsclerophyll forest or cool-temperate rainforest; dominant tree species were: Eucalyptus regnans F.Muell., Eucalyptus obliqua L’Her., Acacia melanoxylon R.Br. and Nothofagus cunninghamii (Hook.) Oerst. Six sites that had had willows removed and were revegetated were located in the study area (Fig. 1, Table 1). These sites varied in age since restoration from 1 to >8 years (Table 1). Sites restored 8 years) is required before communities within revegetated stretches of stream begin to show the variability that is characteristic of forested sites. Alternatively, other factors that are not redressed by revegetation works may be limiting recovery. Changes to riparian soils arising from past land use may not be ameliorated by revegetation (e.g. March and Robson 2006). Similarly, the willow-removal process described here does not remove the willow root mat from the riverbed and this root mat (although dead) may persist and continue to affect riverbed structure. It is also possible that impacts arising from the continuing use of adjacent land for pasture at revegetated sites are limiting in-stream recovery. Such factors may need to be addressed to improve the effectiveness of this type of riparian restoration. This study also highlights some of the difficulties in setting targets for restoration works within streams, which is often a key challenge in developing and assessing the success of such projects (Giller 2005; Palmer et al. 2005). One aim of river restoration is to increase natural variation as this is regarded as an
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attribute that has been lost from many disturbed rivers (Hughes et al. 2005). The higher variability of invertebrate assemblages at forest sites bears this out, suggesting that the presence of willows might reduce natural variability. Expecting a stream community to eventually return to a particular endpoint consisting of a particular group of taxa is perhaps not realistic and, at least within the context of this study, the variability displayed by the natural sites should in fact be the target. These results also show the value of examining variability in assemblage structure in addition to measures such as species richness and of including control (pre-restored) as well as reference sites in sampling designs for post hoc assessment of restoration success (Lake 2001). Acknowledgements This research was funded by the Corangamite Catchment Management Authority and Deakin University. The authors thank the landowners who allowed access to private properties to sample the revegetated and willow sites. Denis Lovric instigated this project and helped with logistics. Garry Matherson was valuable as a guide on the initial fieldtrip. Kerrylyn Johnston and Travis Howson assisted with fieldwork during the macroinvertebrate sampling trips. Edwin Chester provided advice on invertebrate identification and statistical procedures.
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Manuscript received 17 December 2008, accepted 3 June 2009
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