Herbivorous sea urchins (Echinometra mathaei) support resilience on overfished and sedimented tropical reefs.

Human impacts are dramatically changing ecological communities, motivating research on resilience. Tropical reefs are increasingly undergoing transitions to short algal turf, a successional community that mediates either recovery to coral by allowing recruitment or transitions to longer turf/macroalgae. Intense herbivory limits turf height; subsequently, overfishing erodes resilience of the desirable coral-dominated reef state. Increased sedimentation also erodes resilience through smothering and herbivory suppression. In spite of this critical role, most herbivory studies on tropical reefs focus on fishes, and the contribution of urchins remains under-studied. To test how different herbivory and sedimentation scenarios impact turf resilience, we experimentally simulated, in situ, four future overfishing scenarios derived from patterns of fish and urchin loss in other reef systems and two future sedimentation regimes. We found urchins were critical to short turf resilience, maintaining this state even with reduced fish herbivory and increased sediment. Further, urchins cleared sediment, facilitating fish herbivory. This study articulates the likelihood of increased reliance on urchins on impacted reefs in the Anthropocene.

TESTING THE CONCEPTUAL AND OPERATIONAL UNDERPINNINGS OF HERBIVORY ASSAYS: DOES VARIATION IN PREDICTABILITY OF RESOURCES, ASSAY DESIGN, AND DEPLOYMENT METHOD AFFECT OUTCOMES?

Herbivory assays are a valuable tool used by ecologists to understand many of the patterns and processes affecting herbivory, a widely recognized driving force in marine communities. However, methods vary substantially among studies in both design and operation, and the effect of these differences has yet to be evaluated. We assessed the effects of several key components of assay design on estimates of herbivory to offer four recommendations. First, we found assays out-planted on sequential days in both predictable and random locations within a 60m2 site experienced temporal increases in herbivory by an increasingly diverse assemblage of fishes. Thus, we strongly advise against placing herbivory assays in the same site over a series of days. Second, we found while the amount of biomass consumed in assays was density dependent, the percent loss was not. Thus, we recommend researchers report percent consumption because this metric is robust to differences in biomass offered and will facilitate comparisons across studies. Third, we found associational effects, where proximity of species of differing palatabilities impacted estimates of herbivory rate on one or both species, but these impacts were not consistent across species or sites. Thus, we recommend the effect of association be directly tested for multi specie herbivory assays. Fourth, we found no effect of attachment method on estimates of herbivory rate and recommend researchers continue to use the attachment method in which they are most confident. We hope our experimental results prove useful in the future when designing, conducting, and interpreting herbivory assays.