Corals in a stressful world: life histories, synergies, and resilience

Diverse anthropogenic and natural disturbances affect ecosystems, with the potential to provoke synergies and other ‘ecological surprises’ that may impede our ability to predict and mitigate the impacts of multiple stressors. This thesis examines the impacts of stressors on populations and communities to establish the prevalence of synergies and identify ecological factors that influence interactions between stressors. First, I quantify the magnitude of multiple stressor interactions using a meta-analysis of published factorial studies, and find that synergies are as common as two other types of interactions, antagonisms and simple additive effects. Then, I turn my focus to coral reefs, an ecosystem that is expected to be particularly vulnerable to the impacts of synergies and stressor interactions. Using a long-term time-series of coral cover data in Kenya, I quantify the impacts of two common stressors, fishing and climate change-induced coral bleaching, and conclude that these stressors do not interact synergistically but instead have a weakly additive or antagonistic effect. The next two chapters investigate the impacts of multiple stressors on coral community dynamics in Kenya. I first propose a new approach to quantitatively classify the diversity of scleractinian corals into four life-history strategies based on species traits. I then apply these life-history groupings to Kenyan corals and show that the composition of life histories can, in part, explain the long-term impacts of fishing and coral bleaching, although life histories do overlook variability within coral communities. Finally, I discuss the implications of my results for our understanding of coral reef resilience to the cumulative impacts of stressors. Specifically, I consider how managing local impacts may have counterintuitive effects on the resilience of coral reefs to global climate change. This thesis highlights the importance of community dynamics for understanding the complex interactions among stressors and provides novel insights for conservation and management actions that attempt to mitigate the impacts of multiple stressors in an increasingly stressful world.