Avian diversity in arid Australia : patterns in species richness and composition across varied assemblages and environments

Abstract

I studied avian diversity in three regions of arid Australia, namely Cooper Creek (northern South Australia), and the MacDonnell Ranges and Lake Mackay in the southern Northern Territory. The primary objectives of the study were to document the major correlates of bird species richness and compositional turnover, as measures of alpha and beta diversity, and to see if gradients in both covaried. I tested the hypotheses that primary productivity has ultimate control on bird diversity and vegetation proximate control. Cooper Creek flows intermittently and because of large transmission losses, a naturally steep gradient of declining annual discharge occurs longitudinally, with predictable consequences for the structure and composition of riparian vegetation. A dominant riparian tree, river red gum, and at least eight species of birds reach the limits of their distribution in the study area in response to the main hydrological gradient. Bird species richness declined laterally, from riparian to outer floodplain, and longitudinally downstream, and the main trends in compositional turnover followed these gradients. The distribution and abundance of red gums and coolibah were found to be the major determinants of avian diversity on the floodplain. Bird species richness decreased, and assemblage composition changed, from upstream riparian areas to outer parts of the floodplain and lower riparian reaches. The decrease in the stature and cover of riparian vegetation with distance downstream resulted in downstream bird assemblages forming significantly nested subsets of rich upstream assemblages. Steep topographic gradients in the MacDonnell Ranges allowed bird diversity responses to be examined at small spatial scales. Bird species richness was highest in the riparian zones of major streams and lowest on the upper slopes and tops of mountains dominated by spinifex. Avian alpha diversity was also high in alluvial soils and run-on environments (foot slopes), characterised by greater moisture availability and more fertile soils, in line with the diversity-productivity hypothesis. The floristic component of vegetation explained slightly more bird species turnover than vegetation structure, but both were independently significant, also consistent with previous studies. Mulga was a major determinant of a secondary gradient in compositional turnover, and so beta diversity variation diverged a little from alpha diversity, despite productivity being the major determinant of both. In the absence of mulga, hills and streams, bird communities of the spinifex-dominated vegetation around Lake Mackay have low diversity and abundance. Preliminary analyses of the trophic structure of bird assemblages revealed that no truly granivorous species permanently occupy spinifex grasslands on sandy substrates. A null-model analysis showed the Lake Mackay avifauna is a significantly small-bodied subset of arid Australian landbirds. The low productivity of the study area is thought to account for this pattern. Many vegetated islands are scattered across Lake Mackay, and bird observations on seven larger islands revealed a significant checkerboard pattern of occurrence among two species of Maluridae. The study confirmed the dominant role of productivity, as expressed through vegetation, in determining major patterns of variation in bird species richness and compositional turnover in arid Australia. Show more