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. Bite rates
increased with increasing group sizes 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.