Patterns of Mammalian Species Richness in the Australian Tropical Rainforests: Are Extinctions during Historical Contractions of the Rainforest the Primary Determinants of Current Regional Patterns in Biodiversity?

Abstract

Distribution data on the mammals of the wet tropics have been used to analyse biogeographic patterns in assemblage composition and to correlate patterns of species richness with environmental factors such as climate and vegetation. Multivariate analyses suggest five different geographically separated assemblages of rainforest mammals. The most species-rich is found in the central uplands (Atherton Tableland) with a decrease in species richness to the north and south and with decreasing altitude. The most species-rich areas are characterised by large areas of rainforest with a rounder shape (low shape index), high annual rainfall, consistent rainfall in the dry season and a diversity of rainfall regimes within the area.

Multiple-regression analysis suggests that the combination of rainforest area and shape explain the most variance (r² = 0·74) in the patterns of species richness of rainforest mammals. Various measures of habitat diversity are also highly dependent on area, and a similar degree of the variance in species richness (r² = 0·78) can be explained by using rainforest shape and habitat-diversity variables (rainfall and vegetation diversity) and excluding area. This suggests that the effect of area on the patterns of species richness is primarily due to its positive influence on habitat-heterogeneity factors in the regression.

Analysis of the guild structure (number of guilds and the species richness within each guild) indicates that it is the number of species within guilds that most strongly affects patterns of species richness in rainforest, although the number of guilds also has an effect. Most of the variance in species richness can be attributed to three (primarily arboreal) guilds that have previously been shown to be the most extinction- prone species in the wet tropics.

These patterns suggest the hypothesis that current patterns of mammalian 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 contractions on each rainforest block are indexed by current area and shape.