Showing papers by "Stephen E. Williams published in 1997"

Historical rainforest contractions, localized extinctions and patterns of vertebrate endemism in the rainforests of Australia's wet tropics

Abstract: The spatial patterns in the distributions of vertebrates in the rainforests of the wet tropics biogeographic region of north-eastern Australia were examined to form hypotheses on the processes that have shaped vertebrate assemblages and patterns of species richness and regional endemism. These rainforests occur in a relatively narrow and discontinuous strip along the coast of north-eastern Australia. We found that the number of regionally endemic species and the proportion of regional endemics present in each subregion are both strongly related to the geographic shape of subregional patches of rainforest, independent of rainforest area, within Australian tropical rainforests. Shape has a more significant influence on regional endemism than area, and area has a stronger influence on species richness. These patterns were congruent for all terrestrial vertebrate classes manuals, birds, reptiles and frogst, and for the four groups combined. Our results suggest that the combination of current rainforest area and shape are an index of the relative susceptibility of each area of rainforest to historical contractions, with the implication that historical habitat fluctuations, coupled with subsequent localized extinctions species sifting; have been extremely important processes in determining current patterns of endemism in Australia's wet tropical rainforests. This hypothesis is supported by the highly nested structure of the subregional distribution patterns.

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.