Biological invasions of Southern Ocean islands: the Collembola of Marion Island as a test of generalities

It has been suggested previously that the presence and abundance of indigenous species have a marked influence on the likelihood of invasion of a community. It has also been suggested that such biotic resistance has a negligible influence on the outcome of an invasion, but that the abiotic characteristics of the environment being invaded are more important. The latter has been claimed to be especially important on the islands of the Southern Ocean. In order to test these competing hypotheses we examined the distribution and abundance of indigenous and introduced springtails across 13 habitats, which differ considerably in the properties of their soils, and soil temperature, on the eastern quarter of sub-Antarctic Marion Island. There was no evidence of negative abundance covariation or species associations within habitats, nor were there significant relationships between species richness or abundance of the indigenous as opposed to the introduced collembolans across habitats. Interspecific interactions thus seem to have played no readily identifiable role in the outcome of invasions by Collembola on Marion Island. In contrast, the indigenous and introduced species responded very differently to abiotic variables. The indigenous Collembola prefer drier, more mineral soils with a low organic carbon content, and species richness tends to be highest in cold, fellfield areas. On the other hand, the introduced springtails prefer moist, warm sites, with organically enriched soils. Introduced species richness was negligible in cold, fellfield areas. Disturbance also appeared to influence positively the species richness and abundance of introduced species at a site. These results provide independent support for the idea that abiotic factors, especially temperature, significantly influence the likelihood of biological invasions on Southern Ocean islands. They also suggest that predicting the outcome of climate change on community structure in this region is likely to be problematic, especially in the case of the Collembola.