PATTERNS OF MAMMALIAN-SPECIES RICHNESS IN THE AUSTRALIAN TROPICAL RAIN-FORESTS - ARE EXTINCTIONS DURING HISTORICAL CONTRACTIONS OF THE RAIN-FOREST THE PRIMARY DETERMINANTS OF CURRENT REGIONAL PATTERNS IN BIODIVERSITY

Distribution data on the mammals of the wet tropics have been used to analyse biogeographic patterns in assemblage composition and to correl ate patterns of species richness with environmental factors such as cl imate and vegetation. Multivariate analyses suggest five different geo graphically separated assemblages of rainforest mammals. The most spec ies-rich is found in the central uplands (Atherton Tableland) with a d ecrease in species richness to the north and south and with decreasing altitude. The most species-rich areas are characterised by large area s of rainforest with a rounder shape (low shape index), high annual ra infall, consistent rainfall in the dry season and a diversity of rainf all regimes within the area. Multiple-regression analysis suggests tha t the combination of rainforest area and shape explain the most varian ce (r(2) = 0.74) in the patterns of species richness of rainforest mam mals. Various measures of habitat diversity are also highly dependent on area, and a similar degree of the variance in species richness (r(2 ) = 0.78) can be explained by using rainforest shape and habitat-diver sity variables (rainfall and vegetation diversity) and excluding area. This suggests that the effect of area on the patterns of species rich ness is primarily due to its positive influence on habitat-heterogenei ty factors in the regression. Analysis of the guild structure (number of guilds and the species richness within each guild) indicates that i t is the number of species within guilds that most strongly affects pa tterns of species richness in rainforest, although the number of guild s also has an effect. Most of the variance in species richness can be attributed to three (primarily arboreal) guilds that have previously b een shown to be the most extinction-prone species in the wet tropics. These patterns suggest the hypothesis that current patterns of mammali an species richness in wet tropics rainforest are primarily the result of localised extinctions in those areas most affected by Pleistocene contractions of the rainforest. The relative impacts of these contract ions on each rainforest block are indexed by current area and shape.