Direct and indirect effects of drought on community structure and ecosystem processes in an aquatic ecosystem

A major challenge of ecologists is to discover general mechanisms that explain how climate shapes ecological communities and ecosystems. Efforts have traditionally focused on direct effects, but a growing body of evidence suggests that indirect effects of climate, via altering species interactions, may be more important. Predators are often particularly vulnerable to environmental stress, thus effects of climate may cascade through ecosystems by altering top-down trophic interactions. In Costa Rica, where climate change is predicted to decrease the amount of precipitation, bromeliads contain aquatic insect food-webs largely controlled by the top predator, damselfly nymphs. Community composition varies with bromeliad size. Notably, top predators occur only in large bromeliads, possibly because the probability of drought stress decreases with bromeliad size. Thus, bromeliads are ideal systems to study the relative importance of direct and indirect effects of climate on community and ecosystem responses. To determine whether climate governs community composition directly, I regressed insect drought sensitivity, which I obtained experimentally, against insect habitat size sensitivity, which I calculated from observational data. To examine the importance of indirect drought effects from altered trophic interactions, I experimentally manipulated trophic composition and drought in mesocosms mimicking a single bromeliad leaf well and measured changes in community composition, decomposition, and water quality. Climate directly governed community composition at the scale of the bromeliad, as drought sensitivity strongly predicted habitat size sensitivity. At the scale of the leaf well, drought altered community composition and ecosystem properties indirectly by reducing top-down control from the top predator. Moreover, indirect effects of drought cascaded through the food-web to affect ecosystem functioning (decomposition) and state (water quality). These findings suggest that in complex habitats, such as bromeliads, direct (physiological) effects of climate may sufficiently explain community composition. However, in isolated habitats, such as a single leaf well in which dispersal is hindered, indirect effects of climate, via altered trophic interactions, may emerge and cascade through the ecosystem. The relative importance of direct and indirect effects of climate may thus depend on habitat scale.Science, Faculty ofZoology, Department ofGraduat