UC San Diego

Herbivores facilitate the existence of coral reef ecosystems by limiting the abundance of fleshy algae. Though fishes often dominate in deeper fore-reefs, echinoids (sea urchins) often dominate herbivore communities in shallow fringing- and back-reefs. Given their strong effects on benthic dynamics in the Caribbean Sea and Indian Ocean, echinoids may drive many key biological processes on shallow coral reefs across the globe. Though described as generalists, echinoid species appear to exert unique effects on coral reef benthic communities. This dissertation examined how variation in the structure of echinoid communities drives benthic community dynamics on Hawaiian reefs. The four research projects (data chapters) used a combination of mensurative and manipulative approaches in the lab and field to address this topic and are organized into two themes: (1) spatial variation in consumer (echinoid) effects in relation to environmental conditions and (2) functional diversity among echinoid species and communities, and their consequent effects on benthic dynamics.

In sum, this dissertation demonstrates that (1) herbivory by echinoids varies greatly among sites and can be a dominant structuring force on coral reefs in Maui, HI, (2) the in situ effects of echinoids on coral recruitment appears to be driven by a variety of factors (e.g., echinoid community structure and sediment exposure), (3) echinoid species vary greatly (> 10-fold) in their metabolic demands, grazing rates, and diet preferences, and (4) functional diversity among species leads to significant and large differences in the effects of unique echinoid communities on benthic dynamics and coral growth. These results emphasize the importance and complexity of echinoid community demographics and consequent effects in coral reef ecosystems. Though echinoids are able and willing to consume and survive on a variety of different food items, their unique preferences and grazing behaviors can result in diverse ecological functions and effects on coral reefs. These results suggest that models of coral reef ecosystem dynamics and conservation/management efforts (particularly for shallow reefs where echinoids often dominate) likely need to account for echinoid community structure and function in order to maximize their accuracy and long-term effectiveness.