Using species traits to understand reef coral distributions and responses to stress

Coral reef ecosystems provide important ecological goods and services, but are threatened by climate change and other anthropogenic impacts. The numerous scleractinian coral species that build coral reefs vary in tolerance to environmental conditions and stressors. Therefore, in response to environmental change, coral reefs are likely to change significantly rather than disappear completely. Explicitly quantifying relationships between corals and their environment will provide a better understanding of the mechanisms influencing species distributions and local persistence. However, the lack of overlap in the composition of coral assemblages across different geographic regions has limited the development of a broad-scale understanding of global coral-environment relationships. In this thesis, I therefore focused on species traits to describe biogeographical patterns over large spatial scales, which enabled an integrated, systematic approach for explicitly linking environmental conditions with species distributions. This thesis specifically aimed to: 1) quantify multi-decadal change in coral assemblages within a climate change hotspot, 2) characterize species traits that dominate marginal environments globally, 3) model multiple species distributions in relation to species traits along environmental gradients, and 4) quantify how species traits mediate the responses of corals to a key environmental stressor (increased sea surface temperature, SST). I used a variety of techniques including conducting field surveys, collating published coral distribution data, analyzing bleaching surveys, and applying statistical modeling techniques to investigate coral distributions and responses to stressors. My research resulted in several key findings. First, I unexpectedly found little evidence of change in assembly structure of high latitude reef corals in southeast Australia over the last 20 years where SST has increased. Second, reef corals found at high latitudes - beyond reef growth - were generally found to share traits associated with environmental tolerance, as opposed to dispersal potential. Third, I incorporated coral traits into a multispecies distribution model to quantify how three traits modulate responses to three environmental gradients across northeast Australia. The strongest link was a positive response between species depth range and SST variability. Finally, I demonstrated that growth form and family explain more variation in global coral bleaching responses than other morphological or physiological traits; hence including these traits in bleaching surveys will increase predictive power of surveys. My thesis has generated a richer understanding of coral trait-environment relationships, which ultimately allows for more accurate predictions of the ways in which future environmental changes will impact different coral species.