The macroecology of rainforest ants of the Australian Wet Tropics under climate change

Anthropogenic climate change is altering biological communities, ecosystems and their associated services. Understanding contemporary distribution and diversity patterns and accurately predicting biological responses to climate change is therefore necessary to help assess the nature of future changes and to mitigate associated negative impacts. Predicting responses to climate change is a particular challenge for invertebrates, for which distributions are often poorly known despite representing most of the Earth's species, and being especially sensitive to a changing climate. There is particular concern about the impact of climate change on the biota of tropical mountains, because tropical species often have particularly narrow geographical and thermal ranges and therefore display high levels of short-range endemism. This thesis examines changes in diversity and distribution patterns of the rainforest ants in the Australian Wet Tropics (AWT) under future climate change. The thesis addresses the question: How will diversity and distribution patterns of rainforest ant communities in the Australian Wet Tropics be affected by future climate change? It has three specific aims. The first is to document spatial variation in rainforest ant diversity and composition in the AWT, and to identify the role of climate as a driver of this variation. Ants were comprehensively sampled in leaf litter, on the litter surface and on tree trunks at 26 sites across elevational gradients spanning from 100 – 1,300 m within six montane subregions that spanned the latitudinal range of the AWT. A total of 79,853 individual ants were collected, belonging to 296 species from 63 genera. Species richness showed a slight peak at mid elevations, but did not vary significantly with latitude. Species composition varied substantially among subregions and changed markedly with elevation, with a striking disjunction at the elevation of the orographic cloud layer. The second aim is to assess the extent to what the distribution of rainforest ant species related to their physiological thermal limits. Maximum thermal limits (CTmax) and body mass of 20 species were examined along one of the elevational gradients. Phylogenic relationship was controlled, which there was no significant signal in the data. Community CTmax did not vary systematically with increasing elevation and there was no correlation between elevation and elevational ranges of species. However, body mass significantly decreased at higher elevations, but there was no significant difference in CTmax of different-sized ants within a species and models indicated that elevation and body mass had limited influences on CTmax. The results of this chapter showed direct contradictions with climatic variability hypothesis, Rapoport's rule or Bergmann's rule, which adds to the uncertainty around this issue for ants. The third aim is to investigate how projected climate change and associated changes in suitable habitat will affect rainforest ant community composition in the AWT. Compositional dissimilarity of the communities was estimated using Generalised Dissimilarity Modelling, and the models were fitted into future projections of climatic changes, incorporating projected changes in the distribution of rainforest habitat. Rainforest ant community composition was forecast to change markedly under projected climate change, primarily because of predicted changes in rainforest habitat, especially on the inland side of the region. Habitat suitability is predicted to reduce due to projected transformation of rainforest to sclerophyll forest, which will likely shift the community composition completely from the sites that the rainforest will remain intact. This is an additional change to the expected turnover in the rainforest community composition due to changing climate only. Noteworthy findings of the thesis are the importance of the orographic cloud layer between 600 m and 800 m elevation as a driver of variation in species composition and likelihood of climate change impact on ant species primarily through changes in rainfall via its effects on vegetation structure and therefore thermal microhabitats, than through direct temperature changes. These findings highlight the sensitivity of the cloud layer zone and vegetation structure in the AWT to a changing climate. Therefore, key future directions for predicting ant community responses to climate change are to incorporate changes in the cloud layer and thermal mircohabitats, via changes in rainforest habitat, into compositional dissimilarity modelling.