Life history and ecophysiological responses to ...

21 downloads 0 Views 2MB Size Report
Two communities are being studied: a polar semi-desert community {Dryas octopetala zone) and tundra heath (Cassiope tetragona zone) (Coulson et al., 1993).
Section 2

Ecosystems Research Report

Life history and ecophysiological responses to temperature in arctic terrestrial invertebrates N. R. Webb,1D Hodkinson, S Coulson, J S Bale, A T Strathdee and W Block 1

2

2

3

3

4

'Furzebrook Research Station, NERC, Institute of Terrestrial Ecology, Wareham, Dorset BH20 5AS, UK. Division of Biological and Earth Sciences, Liverpool John Moores University, Byrom Street, Liverpool L3 3AF, UK. ^School of Biological Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK. 'British Antarctic Survey, NERC, High Cross, Madingley Road, Cambridge CB3 OET, UK. 2

Abstract

In a field-temperature manipulation experiment on two tundra communities summer field temperatures were elevated by 5°C in the vegetation and 2°C at 30mm in the soil. Cumulative day degrees above zero were enhanced by 35% in the vegetation and 9% in the soil. The life cycle of the aphid Acyrthosiphon svalbardicum is genetically determined and is unique with the asexual fundatrix giving birth to both males and females. Elevated field temperatures advanced the phenology of the aphid and led to the completion of 3 generations per season and an eleven-fold increase in the number of overwintering eggs. The collembolan Onychiurus arcticus showed locomotor activity down to -4°C and supercooled to -6°C. Survival at high temperatures was humidity dependent and animals survived for up to 3h at 30°C. Respiration showed a non-linear response to temperature with Q at low temperatures (0-10°C) as high as 7.0. This stabilises at 1.6 in the range 10-30°C. 10

Introduction The short cold summers of the arctic regions are an important constraint on the growth and development of invertebrates and this has resulted in modifications to their life cycles (S0mme and Block, 1991). Like temperate species, some polar species complete their life cycle within one season. Such species are highly adapted and able to respond rapidly to changing thermal conditions. Additionally, herbivorous species, exhibit a high degree of synchrony with their host plants (Hodkinson et al., 1979). More commonly, species respond to the low temperatures by extending their life cycles over two or more years (S0mme and Block, 1991) and this is characteristic of soil dwelling forms. Where life cycles are prolonged, species face the additional problem of surviving low winter temperatures often in different life cycle stages. The responses of these two types of invertebrates to higher temperatures are likely to be different.

-51 -

Temperature response in arctic invertebrates Webb et al. In this contribution we report preliminaryfindingsfrom a three-year invertebrate study of tundra communities at Ny-Alesund, Svalbard. Firstly, we report results from afieldtemperature manipulation experiment to assess the effects of elevated temperatures on invertebrate communities. Secondly, we compare the responses to elevated temperatures shown by an above ground herbivorous species (an aphid) with an annual life cycle with those shown by a soil dwelling collembolan with an extended life cycle.

Field temperature manipulation experiment Two communities are being studied: a polar semi-desert community {Dryas octopetala zone) and tundra heath (Cassiope tetragona zone) (Coulson et al., 1993). At these sites twenty geodesic transparent polythene tents each 1.5 x 1.5 were erected and maintained for the summers of 19911993 in an experiment to modify the soil and vegetation temperatures and to assess the responses of the above and below ground invertebrate populations. Temperatures within the tents and in control plots followed similar seasonal and diel patterns. Summer temperatures within the tents were increased by up to 5°C in the vegetation, and up to 2°C at 3cm in the soil. More importantly for invertebrates, cumulative day degrees above zero within the tents were increased by c 35% in the vegetation and c 9% in the soil. On occasions the maximum difference between the inside and outside of the tent was as much as 10°C. The vegetation appeared to act as a thermal insulator and prevented the conduction of heat into the soil from above and enhanced the thermal contact with the cooling permafrost below. Although temperatures in the vegetation were similar at both sites, soil temperatures were modified less in the tundra heath than the polar semi-desert with its sparser vegetation (Coulson et al, 1993). During thefirstseason there were no detectable differences in either soil mite or collembolan populations between the tented and control plots; however the total population densities of mites and collembola were higher at the polar desert site than in the tundra heath. At both sites and in both treatments large numbers of immature mites of each species were collected. This suggests that the life cycles are continuous and are not synchronised with the annual climatic cycle.

tr; te

di la vi h c b tl a g e

ti F ii a 1

I

In 1992 the mites and collembola of the tundra heath remained fairly constant, whereas in the polar semi-desert collembola numbers declined while mite numbers increased on both the tented and control plots. This coincided with an unusually warm dry spell that led to a drying out of the soil at the polar semi-desert but not at the heath site. These results were surprising; an increase in the thermal budget of 10% produced no measurable response in mite and collembola populations over two years, but an unexpectedly dry spell enhanced recruitment and increased mortality at the polar semi-desert site. The effects of temperature appear to interact strongly with soil moisture and these two factors need to be considered together when modelling the effects of elevated temperatures.

An insect herbivore

The aphid Acyrthosiphon svalbardicum (Heikinheimo) was found to be most abundant aboveground invertebrate herbivore in the vicinity of Ny-Alesund, and is monophagous on Dryas octopetala. Although this species had been previously reported from Spitsbergen (Heikinheimo, 1968), its four apterous morphs (fundatrix, vivipara, ovipara and male) have been described for

- 52-

Webb et al. Temperature response in arctic invertebrates the first time (Strathdee et al., 1993a). The annual life cycle is genetically determined unlike temperate aphids in which it is determined by environmental factors. The fundatrix gives birth directly to both sexual morphs (males and females) (unique in the Aphidinae) which mate and lay eggs thereby ensuring survival in the following season. The fundatrix also produces some viviparae which in turn produce only sexual morphs; however, under the present climate this happens rarely (Strathdee et al., 1993b). When thermal budgets are increased, this flexible life cycle has the potential to increase the number of overwintering eggs. Raising field temperatures by 2.8°C using cloches (Strathdee and Bale, 1993) advanced the phenology of both the aphid and the host-plant and enabled the aphid to complete three generations in a season; in turn, this led an eleven-fold increase in the number of overwintering eggs. Acyrthosiphon svalbardicum, with guaranteed egg production by mid-summer combined with an in-built flexibility to produce and extra generation in favourable seasons, is well adapted to respond rapidly to enhanced temperature regimes (Strathdee et al., 1993b). Retrospective analysis of climate data from Ny-Alesund (Strathdee et al., 1993c) has shown that in only 6 of the last 23 seasons was the thermal budget sufficient for A. svalbardicum to complete a third generation thereby making a greater contribution to the number of overwintering eggs. They calculated that if temperatures were elevated by 2°C the third generation would have been produced in 20 out of the 23 years and had temperature been 4°C higher then a third generation would have occurred in all 23 seasons.

A soil dwelling species Onychiurus arcticus (Tullberg) is a large species of springtail (Collembola) with a maritime distribution in the high Arctic (Fjellberg, 1980). It is to be found beneath stones on screes, particularly beneath bird cliffs and on glacial outwash fans. This species was found to be unusual among the collembola in feeding on living bryophyes (Drepanocladus uncinatus, Racomitrium lanuginosum and Polytrichum alpinum) and algae as well as organic detritus (Hodkinson et al., 1994). Detailed ecophysiological studies on animals taken from cultures maintained at 0-2°C have shown that this species is active down to -4.0°C. These animals supercooled to -6.0°C before freezing in experiments so far conducted. The variation of supercooling point with season, starvation and at other temperatures of acclimation is currently under investigation. The survival of individuals at sub-zero temperatures showed a similar pattern. At -3.0°C 60% of the individuals survived for 84 days, but at lower temperatures survival was reduced with only 35% of the individuals surviving at -5.0°C. In contrast, survival at high temperature is good and was shown to be humidity dependent. At 100% humidity over 80% of the individuals survived for 3 h at 30°C, but there was negligible survival above 32.5°C and above 35.0°C there was no survival. Oxygen consumption of O. arcticus was comparable with that of other surface dwelling species of collembola despite O. arcticus being morphologically similar to soil-dwelling species. The relationships between oxygen consumption and temperatures was non-linear over the range 0-30°C. From zero to +10.0°C the Q was as high as 7.0, falling to 1.6 over the range 10-30°C and above 30°C increasing again as the animals suffered heat stress. 10

- 53 -

Temperature response in arctic invertebrates Webb et al. The ecophysiological profile of Onychiurus arcticus reveals an animal which has physiological characteristics to respond rapidly to warmer conditions such as those following snow melt. This enables it to be active quickly, frequently taking advantage of the warmth beneath stones (Block et al., 1994, in press). Preliminary observations during the summer months suggest that in these habitats this species will need to survive temperatures of at least 25°C. Clearly this is possible and its dampened metabolic response as temperature rises is an adaptation which would leave it relatively unaffected were temperatures to increase by 2-3°C. Acyrthosiphon svalbardicum and Onychiurus arcticus provide contrasting examples of the responses shown by invertebrates to elevated temperatures. For the aphid there are both the direct effects of temperature on the animal resulting in increased growth and reproductive rates; however, increased temperature will affect the phenology, growth and food quality provided by the host plant which in turn will be mediated to the aphid. The response of the collembolan is simpler with increased rates of metabolism growth and reproduction with an evident dampening of these direct effects.

References Block, W., Webb, N.R., Coulson, S. and Hodkinson, I.D. (1994). Adaptations to temperature in the Arctic springtail Onychiurus arcticus (Collembola: Onychiuridae). Journal of Insect Physiology (in press) Coulson, S., Hodkinson, I.D., Strathdee, A., Bale, J.S., Block, W., Worland, M.R. and Webb, N.R. (1993). Simulated climate change: the interaction between vegetation type and microhabitat temperatures at Ny-Alesund, Svalbard. Polar Biology, 13: 67-70. Fjellberg, A. (1980). Identification keys to Norwegian collembola. Norsk Entomologisk Foreining, As. Heikinheimo, O. (1968). The aphid fauna of Spitsbergen. Annales Entomologici Fennici, 34: 82-93. Hodkinson, I.D., Jensen, T.S. and MacLean, S.F. (1979). The distribution, abundance and host-plant relationships of Sa/ix-feeding psyllids (Homoptera: Psylloidea) in arctic Alaska. Ecological Entomology, 4: 119-132. Hodkinson, I.D., Coulson, S.C., Webb, N.R., Block, W., Strathdee, AT. and Bale, J.S. (1994). Feeding studies on Onychiurus arcticus (Tullberg) (Collembola; Onychiuridae) on West Spitsbergen. Polar Biology. 14: 17-19. S0mme, L. and Block, W. (1991). Adaptations to alpine and polar environments by insects and other terrestrial arthropods. In: R.E. Lee and D.L. Delinger (eds). Insects at low temperatures. Chapman and Hall: New York. Strathdee, A.T. and Bale, J.S. (1993). A new cloche system for elevating temperature in polar ecosystems. Polar Biology, 13: 577-580. Strathdee, A.T., Bale, J.S., Hodkinson, I.D., Block, W., Webb, N.R. and Coulson, S.C. (1993a). Identification of three previously unknown morphs of Acyrthosiphon svalbardicum Heikinheimo (Hemiptera: Aphididae) on Spitsbergen. Entomologica Scandinavica, 24: 43-47. Strathdee, A.T., Bale, J.S., Hodkinson, I.D., Webb, N.R., Coulson, S.C. and Block, W. (1993b). Extreme adaptive life-cycle in a high arctic aphid Acyrthosiphon svalbardicum.

-54-

Webb et al. Temperature response in arctic invertebrates Ecological Entomology, 18: 254-258. Strathdee, A.T., Bale, J.S., Block, W., Coulson, S.J., Hodkinson, I.D. and Webb, N.R. (1993c). Effects of temperature elevation on a field population of Acyrthosiphon svalbardicum (Hemiptera: Aphididae) on Spitsbergen. Oecologia. 96: 457-465.

-55-