Consumers mediate natural variation between prey richness and resource use in a benthic marine community

Space is the limiting resource for sessile organisms on marine rocky substrata, and the availability of space is decreased by recruitment and growth but increased through senescence, physical disturbance and consumption. In the present study, we examined whether consumers mediate variation in the relationship between prey richness and resource (space) use in subtidal epifaunal communities. First, we used surveys to identify relationships between prey richness, consumer richness, consumer identity and consumer abundance with available space. As predicted, available space was inversely correlated with sessile prey richness and positively correlated with consumer richness. However, a model selection approach identified the abundance of sea urchins and chitons specifically as the best predictors of available space, suggesting that the proportion of available space is a reasonable indicator of recent disturbance. Next, we manipulated urchin density in the field to test the hypothesis that urchins control the structure of this community by grazing sessile taxa and facilitating smaller consumers. Diet analyses and structural equation models together indicate that urchins generate available space directly by consuming macroscopic sessile prey, and indirectly by facilitating chitons, which maintain patches of space free of microscopic algae and recruits of larger sessile taxa. The significant interaction between prey richness and experimental urchin density on available space suggests that prey richness may buffer the impacts of urchin grazing. More generally, we highlight the need to study the effects of species richness on the structure of communities in the context of relevant ecological processes.