Diversity and conservation of invertebrates on the sub-Antarctic Prince Edward Islands
The Prince Edward Islands form a unique component of South Africa's natural estate. Here we present an overview of the diversity of the invertebrate fauna found on Marion and Prince Edward Islands and the conservation threats facing it. The invertebrate fauna at the islands is well known owing to a significant recent effort to sample the entire fauna, although the nematodes remain poorly known. Mite, insect and springtail assemblages differ considerably between habitats and these patterns support an earlier distinction made between the epilithic and vegetated biotopes. Seasonal variation in the abundances of the arthropods is the norm, although the form of this seasonality varies considerably between species and between habitats. From a regional perspective, the biogeographic affinities of the fauna remain enigmatic. Nonetheless, it seems likely that isolation has been an important contributor to local, indigenous species richness on Marion Island, and speciation has clearly contributed several endemic species to the fauna. Introduced insect species richness is more closely related to mean annual temperature and the number of humans occupying islands in the sub-Antarctic region, and this pattern is reflected locally in the distribution of indigenous and exotic springtails on Marion Island. The introduced species are common in warm, moist habitats, while the indigenous species prefer colder, drier sites. Local climate change, in step with global trends, seems set to have pronounced influences on the invertebrate fauna. Direct effects are likely to take the form of increased abundances of introduced species because of their shorter life cycles and greater fecundity compared to indigenous species, which tend to be long-lived with low reproductive output. Indirect effects are likely to be the result of changes in predation patterns of introduced house mice, and changes in plant communities precipitated by the spread of invasive vascular plants, which in turn have a marked influence on invertebrate assemblages. Undoubtedly the largest conservation threats at the island are the interactions between climate change, introduced species, and human use. In particular, climate change is likely to mean the ready establishment of alien species propagules, while increasing human use is likely to increase propagule pressure. Conservation of the invertebrates at the island will best be served by reduction in human use and stringent enforcement of the provisions of the management plan for these special nature reserves.