Functional changes in reef systems in warmer seas: Asymmetrical effects of altered grazing by a widespread crustacean mesograzer

Highlights: • Warmer seas may alter species interactions and ecological functions (e.g. grazing). • We measured grazing response in the lab and modelled changes under future climates. • Grazing changes unimodally with temperature and may increase in Indo-Pacific. • Small decreases in tropics offset by big increases at higher latitudes in the future. • Similar responses in other grazers suggest possible broad effects as seas warm. Grazing is a pivotal function in many marine systems, conferring resilience to coral reefs by limiting algal overgrowth, but triggering phase shifts on temperate reefs. Thus, changes to consumption rates of grazing species in response to higher future temperatures may have broad ecological consequences. We measured how the consumption rates of a widespread mesograzer (the hermit crab Clibanarius virescens) responded to changing temperatures in the laboratory and applied these findings to model the spatial footprint on grazing animals throughout the Indo-Pacific region under climate change scenarios. We show that mean grazing capacity may increase in shallow coastal areas in the second half of the century. The effects are, however, asymmetrical, with tropical reefs predicted to experience slightly diminished grazing whilst reefs at higher latitudes will be grazed substantially more. Our findings suggest that assessments of the effects of climate change on reef ecosystems should consider how warming affects grazing performance when predicting wider ecological impacts.