Influences of past, present and future climate on the structure and diversity of rainforest bird assemblages in north-eastern Australia

In this thesis I endeavour to advance our understanding of the influence of climate on patterns of diversity and assemblage structure of rainforest birds in north-eastern Australia. In chapter 3, I apply a distance sampling method to quantify the factors influencing detectability of rainforest birds. In doing so I aimed to extend basic measures of abundance of species at sites to estimates of absolute density. Recognising that distance sampling presents a significant logistical challenge, particularly for rare species, in chapter 4 I develop a compromise approach to estimating density that involves modeling the detectability of species as a function of their characteristics. In chapter 5, I use the improved density estimates to test predictions of Species-Energy Theory using the More-Individuals Hypothesis as a framework. This analysis shows a strong contribution of historical climate change in shaping contemporary patterns of energy flux, and hence density and diversity of birds, particularly among insectivores. These results highlight an ongoing influence of long-term environmental instability on patterns of energy uptake in this system, along with secondary effects of resource seasonality. In chapter 6, I apply the refined density estimates to a space-for-time substitution analysis of the influence of temperature on the elevational density profiles of rainforests birds. Temperature is shown to be a strong correlate of elevational patterns of density across the bird community, validating a key assumption of species distribution modelling, used to predict impacts of climate change on biodiversity. Based on this, in chapter 7 I extend previous species distribution modeling work undertaken in the Australian Wet Tropics with the addition of new data from lowland sites, and with data from neighbouring rainforest regions to the north and south, including both species whose ranges extend outside the AWT, and some endemic species restricted the Central Queensland Coast and Cape York Peninsula. The results of these analyses are consistent with previous predictions of biodiversity losses of upland endemic species, as their preferred cool, moist environments contract up-slope, and also indicate extensive reshuffling of assemblage composition across the elevation gradient as lowland species expand upslope into previously unsuitable climates. Crucially, however, predicted impacts on patterns of species richness are strongly influenced by underlying assumptions about dispersal between regions. Under a scenario of free dispersal, lowland biotic attrition predicted in the Australian Wet Tropics is completely offset by an influx of warm-adapted species with New Guinean affinities expanding southwards from Cape York Pensinsula lowland rainforests. This result suggests that dispersal limitation as well as historical biogeography continue to play an important role in defining the realised distributions of many rainforest species, and that habitat changes will play a critical role in determining the composition of future assemblages. These key findings are discussed in terms of their significance for broader ecological theory, and their relevance to the identification of suitable environment for future translocation of upland endemic species.