Coral identity and structural complexity drive habitat associations and demographic processes for an increasingly important Caribbean herbivore

ABSTRACT: Habitat complexity plays a vital role in shaping ecological communities, but many coral reef ecosystems are shifting to alternative states with altered community compositions and reduced complexity. Sea urchins are common inhabitants of reefs, and their importance for controlling the distribution and abundance of algae in marine ecosystems is well understood. Less understood is the role of habitat complexity and species identity of foundational species in dictating the abundance of reef herbivores. We explored how the structural complexity and identity of 3 corals commonly observed on contemporary Caribbean reefs mediate the abundance, behavior, and demographic characteristics of an increasingly important herbivore, the sea urchin Echinometra viridis. Tethered urchins survived better on the more structurally complex coral Agaricia tenuifolia and hydrocoral Millepora alcicornis than on less complex branching Porites species. However, natural densities of urchins on these corals did not follow the same pattern, suggesting that coral identity, independent of complexity, also contributes to habitat associations. In habitat choice experiments, urchins preferred the structurally complex coral A. tenuifolia only when waterborne cues of predators were introduced. Despite minimal differences in the standing stock of algae associated with the different corals, urchins inhabiting Porites colonies had a marginally higher reproductive condition than those collected from the other corals, suggesting a fitness trade-off to inhabiting the riskier coral. Understanding the drivers of herbivore habitat associations is vital for predicting the persistence of coral-dominated reefs due to feedbacks between changing coral reef communities (both species identity and habitat complexity) and shifts to algal dominance.