Arthropod colonisation of a debris-covered glacier

The largest debris-covered glacier in the Alps (Miage Glacier, western Italian Alps) has been studied to explore the effects of debris-cover extent and depth on the spatial distribution of ground-dwelling arthropods. A multitaxa approach has been used to compare taxa richness and distribution to the functional role (dietary habits) of each taxon along the glacier tongue. Spiders and ground beetles have been studied in detail. Taxa richness declines with distance from the wooded sites (in front of the glacier tongue) to those above the glacier tongue. At each of the supraglacial sites, spiders, ground beetles, aphids, springtails and flies were found. A change in the dominance of the different functional roles was observed along the tongue. Wooded sites are characterised by predatory (e.g. spiders, beetles), detrivore (e.g. springtails and certain flies), phytophagous (e.g. aphids, certain beetles) and parasitoid (e.g. certain wasps) assemblages, whereas at the debris-covered sites, aphids, flies and springtails are likely to be prey for spiders and beetles. The species richness of the predominant predators (spiders and beetles) shows a positive relationship with vegetation cover and debris thickness. Two mutually exclusive spider and ground beetle assemblages were found; one within the debris cover and one within the wooded sites. In our opinion, debris-covered glaciers are acting as a refuge area for the cryophil stenotherm species living at higher altitudes which descend the glacial tongue to lower elevations. A similar hypothesis supports the biogeographical interpretation of the distribution of many boreo-alpine relict species in the Alps. We discuss our results in the light of possible future scenarios which suggest an increase in debris cover with global warming.