Evolution and ecology of coral range dynamics under climate change

Abstract

The geographic distribution of a species represents its ecological niche and spatial variability in fitness. A species’ fitness peaks where the environment and biotic interactions provide optimal conditions for a given combination of traits and ebbs as the conditions become less suitable. As such, spatial differences in trait expression provide a framework on which species range dynamics can be evaluated. Over time, the boundaries of species ranges can undergo expansions, contractions, and shifts. The labile nature of species ranges has been attributed to the spatial tracking and/or changes in the extent of niche space. The ability of a species to track and/or modify its niche is strongly determined by the degree and rate of climate change. While some species may track their environmental niches, many others are likely to suffer local/regional extirpations since their ability to track or modify species characteristics to changing conditions lags behind the rates of climate change. At present, even the most rudimentary information on spatial variations in species traits and fitness is lacking for many taxon groups. In addition, current research is heavily focused on species range cores, with range dynamics under climate change in peripheral regions remaining underexplored.In this thesis, I focus on scleractinian corals as study organisms. Scleractinian corals form a pivotal taxon group that sustains reef ecosystem diversity, show historical and contemporary shifts in species ranges, and are under substantial threat from changing climatic conditions. I recognise substantial data gaps in the global repository of coral traits, use statistical modelling to quantify the risk involved in using sparse trait datasets, and introduce a phylogeny-informed imputation approach to reduce inferential errors relevant to range dynamics research (Chapter 2). I then examine the spatial variation in taxon-specific environmental suitability and habitat degradation under climate change (Chapter 3). I further assess if and how taxonomic variability in response to environmental disturbances is likely to change range dynamics (Chapter 4).The raw material to examine the linkage between spatial variability in species-specific biological features and range dynamics was gathered from various sources, including literature records of coral distribution and trait information from the global Coral Trait Database. The compiled dataset highlighted that trait information for numerous taxa was missing. This was mainly due to trait information being collated from field observations or via experiments, which by nature could not cover the geographic extent of all coral taxa. Past studies handled such data gaps by removing missing information. However, several studies cautioned that omission of missing data could result in misinterpretation of ecological patterns. In order to overcome this limitation, I applied a phylogeny-informed imputation approach to estimate expected values for missing trait modalities or expressions (Chapter 2). The resulting publication shows that a big obstacle in accurate prediction of large-scale ecological patterns is data gaps in global trait datasets and provides a novel approach to reduce inferential errors in ecological studies.Using distributions of scleractinian species and environmental conditions in which each species occurs, I quantified the breadth of niche space for scleractinian species that occur in eastern Australia (Chapter 3). Overall, the size and centroid location of niche space differed widely among taxa. Using a multivariate characterisation of environmental conditions and ocean warming simulations, I also show that warming ocean impairs the environmental suitability of eastern Australia for many resident corals.Finally, I examined the implications of taxonomic variability in response to environmental disturbances for assemblage structure, and how taxonomic variability in disturbance sensitivity might lead to potential changes in species ranges (Chapter 4). Using an extensive bleaching survey dataset from eastern Australia, I compared the severity of stress response among scleractinian corals, exhibited as coral bleaching, and investigated if environmental differences were linked to the variable responses of abundant genera in the region. The five most abundant genera in the region differed in overall bleaching severity. Two of the genera, Pocillopora and Porites, suffered severe bleaching, while Acropora, Goniastrea, and Turbinaria were much less affected. Overall, Turbinaria became increasingly dominant in the assemblage structure with reduced per capita bleaching toward higher latitudes (winner). In contrast, bleaching impact on each individual increased with latitude for Pocillopora (loser). Severe bleaching seen among two abundant genera, Pocillopora and Porites, is particularly concerning as they contain species endemic to the region, or less abundant on tropical reefs.Together, this thesis provides crucial insights into the factors that determine species range boundaries for scleractinian corals, and how these are likely to be altered under climate change. The taxonomic variability in the degradation of habitat quality and sensitivity to recurrent disturbance events found in this thesis implies that coral assemblages will become increasingly non-analogous to the contemporary configurations.