Global declines in species diversity have profound implications for ecosystem functioning. The role of diversity in ecosystem functioning can be related to species richness, species identity or a combination. Species diversity can enhance ecosystem function and may lead to over-yielding of a given function relative to an additive estimation from single species’ performances. For ecosystems where resources are largely donor-controlled, such as sandy beaches, the effect of diversity of consumers on ecosystem function is not well known but may be similar to that found in other ecosystems. Characterized by low primary production, sandy beach food webs rely on marine subsidies, (e.g. drift macrophytes). Intertidal consumers process and convert subsidies into food energy, which supports higher level consumers, and enhances remineralization of nutrients that can be exported back to coastal waters. We experimentally investigated the effects of intertidal consumer species diversity (n=6) on consumption of giant kelp (Macrocystis pyrifera) subsidies.
Kelp consumption was largely driven by species identity, particularly with respect to body size. Large species of talitrid amphipods (Megalorchestia sp.) consumed significantly more kelp than did smaller talitrid species, an isopod and a beetle. However, scaling consumption values by consumer body size indicated an intertidal tenebrionid beetle had the greatest consumption per mg dry body weight. Overall, kelp consumption was generally additive under different combinations of species diversity (n=57) and primarily controlled by species identity (body size). Loss of these important intertidal consumers can have negative consequences for the processing of marine detrital inputs and the provisioning of food for higher level consumers, such as shorebirds. Conservation efforts that focus on maintaining species diversity are crucial and our results indicate the importance of individual species in maintaining ecosystem function.