Density-dependence and territorial competitors can modulate parrotfish social foraging and herbivory on coral reefs

Social context can modify the behaviour of animals, influencing how they interact with their environment - potentially cascading to effects on ecosystem functions. However, typical proxies for functions, such as biomass or densities, may not completely capture this behavioural variation. We examined how grouping behaviour of an abundant parrotfish species, Chlorurus sordidus, influences the critical function of herbivory on coral reefs in the Lakshadweep Archipelago and evaluated the role of density-dependence and territorial aggression from competitors in driving group formation in the entire herbivorous fish assemblage. Feeding rates increased with group size and decreased with aggressive encounters, with parrotfish in larger groups consuming about 80% more algae per capita than solitary individuals. Group foragers also benefitted marginally from reduced territorial aggression from competing herbivores. The propensities of herbivores to form groups, and their group sizes were positively density dependent, and, to a lesser extent, were driven by access to resources defended by territorial competitors. Our study highlights the importance of incorporating behavioural variation in assessments and predictive frameworks of ecosystem functioning. As our results indicate, for social animals, key ecosystem functions are particularly sensitive to animal densities and interspecific interactions, especially when individual function varies with social context.