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Macroecology of the Australian Wet Tropics

To develop an integrated theory on the determinants of Australian Wet Tropics (AWT) vertebrate assemblage structure, we plan to examine patterns of diversity over a range of spatial scales and to include perspectives from traditional ecology and the related fields of evolutionary biology, population genetics, phylogeography, landscape ecology and biogeography. Over the past decade, there has been renewed interest in how historical biogeographic and evolutionary processes, together with local ecological interactions, affect the richness and structure of species assemblages (Ricklefs & Schluter 1993; Brown & Lomolino 1998; Hubbell 2001; Jansson & Dynesius 2002; Chase 2003; Ricklefs 2003). In general, the more specialized, lower vagility and smaller-area-requiring species are expected to be the most prone to isolation during periods of habitat contraction. In turn, the richness and composition of a local species assemblage should be influenced by the dynamics of local extinction, speciation and colonization from adjacent areas (MacArthur & Wilson 1967, Rosenzweig 1995, Hubbell 2001).

Most analyses of diversity in this system have been at a subregional scale and focused on the upland (>300m) areas (FU to EU, Fig, 2) across which the endemic species are concentrated. The richness and structure of rainforest vertebrate assemblages vary among subregions, with the richness of all groups being influenced positively by area and negatively by geographic shape (convolution) of rainforest (Williams & Pearson 1997; Williams 1997), and there is significantly nested species distributions between putative refugia and expansion areas (Williams and Pearson 1997; Williams and Hero 2001). Further analysis, using spatial predictions of upland forest types under various paleoclimates, showed that patterns of subregional richness and turnover were better explained by including the “stability surface”, i.e. areas inferred to be continuously suitable for upland rainforests (Graham, Williams & Moritz, in prep.; Table 1). Overall, these statistical analyses of vertebrate species diversity suggest a strong influence of history, particularly for the more dispersal limited amphibian and reptile species, with rainforest contraction at the LGM causing local extinctions, some of which were recovered by recolonisation from adjacent refugia during the cool-wet phase of the early Holocene (Williams and Pearson 1997; Schneider and Williams, in press; Graham, Williams & Moritz, in prep.). In conjunction with data on the current distribution and abundance of species across habitats and climatic zones, we propose to use a novel molecular approach - multi-locus nuclear estimates of historical demography from co-distributed species that vary in both vagility and degree of climatic/habitat specialisation, to test two key hypotheses; That species which are the most restricted to specific habitats and which have the narrowest current climatic range have been the most sensitive to climate-induced habitat contraction; and That the richness and structure of assemblages at a sub-regional scale should be determined by responses of individual species to historical fluctuations in habitat area.

Resolving the history of specific assemblages within and across subregions where they occur provides a powerful way of focusing and interpreting research on local-scale interactions. The AWT meets this challenge by providing a system with the following attributes; (i) high species richness and endemicity at a regional scale, (ii) knowledge of key ecological attributes of species and of spatial patterns of assemblage structure, (iii) a well characterized history of spatial fluctuations in habitat distribution, and (iv) estimates of population responses of species to those changes in habitat distribution.




Craig Moritz Research Group - Home

Museum of Vertebrate Zoology
Department of Integrative Biology
University of California, Berkeley