Invertebrate food webs in recently deglaciated alpine areas

The dramatic retreat of the glaciers over the last 150 years is one of the evident signs of climate change in alpine landscapes. Because of their relative simplicity, glacier forelands are model systems for investigating fundamental ecological processes such as colonization, population establishment and community assembly. Interestingly, early successional stages of primary successions are dominated by predatory coloniser communities, and although the rather simple faunal pioneer communities are characterised quite well, we still do not understand on which trophic interactions community assembly depends. This project directly addresses this gap of knowledge. Its aims are two-fold: 1) To determine the trophic links within macro-invertebrate communities colonizing recently deglaciated alpine terrain using, for the first time, a molecular approach. 2) To compare macro-invertebrate food webs between early and late pioneer stages and test the hypothesis that time since deglaciation impacts food web structure more than site-specific characters. There are four objectives to this three-year research program: (i) to determine relative abundances of invertebrate taxa found in the glacier foreland communities, to assess predator nutritional status and to record external input of invertebrates, (ii) to use DNA-based gut content analysis for identifying specific trophic links within the invertebrate community and to determine their strength, (iii) to compare the trophic links between communities at early, species poor and late, species rich pioneer stage and (iv) to combine data on animal abundance and trophic linking to create semi-quantitative food webs and compare webs between three adjacent glacier forelands. The work proposed here will take research on glacier forelands and primary succession one step further, as it will not only describe changes in invertebrate community structure but focus on a key functional aspect, trophic linking, within animal pioneer communities. Employing the latest approaches in molecular prey detection and merging these data with a comprehensive analysis on community composition will allow creating semi-quantitative food webs, depicting trophic linking in terrestrial invertebrate communities at an entirely new level of resolution. Moreover, the outcomes of this project will not only be highly relevant to the understanding of how pioneer communities are sustained, but will be of broad significance as this work will also serve as a primer for new avenues in food web research.

The dramatic retreat of the glaciers over the last 150 years is one of the evident signs of climate change in alpine landscapes. Because of their relative simplicity, glacier forelands are model systems for investigating fundamental ecological processes such as colonization, population establishment and community assembly. Interestingly, early successional stages of primary successions are dominated by predatory coloniser communities, and although the rather simple faunal pioneer communities are characterised quite well, we still do not understand on which trophic interactions community assembly depends. This project directly addressed this gap of knowledge and it had two aims:1) To determine the trophic links within macro-invertebrate communities colonizing recently deglaciated alpine terrain using, for the first time, a molecular approach.2) To compare macro-invertebrate food webs between early and late pioneer stages and test the hypothesis that time since deglaciation impacts food web structure more than site-specific characters.Four multiplex PCR assays were developed to test gut contents of the pioneer invertebrate predators for the DNA of 24 different prey taxa representing the most abundant ground-dwelling and aerial invertebrates found in recently deglaciated land in the western Austrian Alps. New approaches were developed to optimize the DNA-based detection systems in terms of sensitivity and evenness of prey detection, to minimize the methodological induced variation in the trophic analyses. Carabid beetles, wolf spiders, linyphiid spiders and harvestmen were collected over two summers in early (< 10 years ice-free) and late (15-20 years ice-free) pioneer sites in front of the Rotmoos glacier in the Ötztaler Alps in western Austria. Within the second summer, the two adjoining glacier valleys were investigated too. The invertebrate ground-based and aerial communities were recorded in each successional stage and valley. In carabid beetles, a non-invasive approach was used to retrieve dietary samples, minimizing the impact on the investigated populations. We found that springtails were the most frequently consumed prey, suggesting that autochthonous detritivores play an important role in sustaining the generalist pioneer predators. Contrary to our initial hypothesis, predator-predator feeding interactions were not higher in early, prey-poor, than in later, prey-rich, pioneer sites. The molecular analysis of the prey spectrum revealed that the different predator species occupied specific trophic niches, which was also reflected in the stable isotope signatures of the predators. Both the identity of the valley and the successional stage affected the food web interactions, with food webs getting more connected and complex in late pioneer sites.Our findings demonstrate that in these pioneer sites the invertebrate food webs are sustained by both allochthonous and autochthonous prey and that the changes in the food webs are primarily driven by site-specific factors and the time since deglaciation.