Terrestrial isopods (Crustacea: Oniscidea) and biogeographical patterns from South-Western Australia

This thesis examines terrestn'al isopod (Crustacea: Oniscidea) diversity and biogeography from south-western Australia, a region already recognised for tls exceptional high btodtversity and concentrations of endemic species. A taxonomic investigation of tsopods in the Western Australian Museum and material colfected systematically as part of this study revealed seventy taxa, sixty of which were cons1dered native species. The thesis provides a comprehensive revtew of regional taxonomic htstory and includes a key, complete with a set of forty characters states and taxa, illustrated by a number of photographic plates that are designed for use by specialists and non-specialists alike. Prior to this study all the region's indigenous terrestrial isopods were poorly described and, while many taxa are still to be resolved at the species level, it appears at least 35 species are new to science. The distributions of indigenous taxa form some clear and consistent biogeographical patterns. Distributtons of taxa are influenced by steep rainfall gradients, the seasonality of rainfall and by biophysical characteristics of the landscape including, localised landforms and differing microhabitat utilisation. The region ts one of high diversity, with both widely distributed species and highly localised endemics. Two areas, the Perth region of the Swan Coastal Plain and the tall, wet southern forests, are particularly species rich. Beta diversity is also high. This reflects a heterogeneous landscape with profound differences in microhabitat availability, differences often due to recent exogenous disturbance. The construction of a biogeographic model showed a major fauna/ break, which created northern and southern bioregions to which 68% of the taxa were endemic. This biogeographical boundary is significant in that it had not been recognised in any previous biogeographical schemes. Data derived from the analysis of soil, leaf litter and log microhabitat samples show that the existence of terrestrial isopods in south-western Australia is intractably related to organic matter. The geographical range of species is often best explained by the types of m1crohabitat utilised and their degree of moisture dependency. The wetter forests of the region contain relictual short range endemics reliant upon surface organic matter, particularly leaf litter. Moisture is clearly a limiting factor in the distribution of all species. The five most common genera can be differentiated by their dependence upon different moisture regimes in a range of microhabitat types. The spatial and temporal continuity of different types of organic matter in forested landscapes is considered critical to the conseNation of the group in south-Western Australia. Land management practices, such as logging and burning, which influence the nature and distribution of organic matter, are likely to induce profound changes in terrestrial isopod communities. Regular and broadscale prescriptton burning and the increasing scarcity of large and decayed logs are considered to be a pnmary threat to their conservation status. Hypotheses involving the histoncal role of surface organic matter are developed to explain the speciation patterns of the group in the region. In order to test these models, and to provide an urgently needed assessment of the conservation status of relictual forest invertebrates, clear taxonomic prionties are recommended.