Fear effects and group size interact to shape herbivory on coral reefs

1. Fear of predators (‘fear effects’) are an important determinant of
foraging decisions by consumers across a range of ecosystems. Group size
is one of the main behavioural mechanisms for mitigating fear effects but
also provides foraging benefits to group members. Within coral reef
ecosystems, fear effects have been shown to influence the feeding rates of
herbivorous fishes (i.e. browsers), a key functional group that prevent
macroalgal overgrowth. Yet, how fear effects and group size interact to
shape macroalgal removal on coral reefs remain unclear. 2. Here, we
conducted field-based experiments using models of a common piscivorous
fish, the leopard coral grouper (Plectropomus leopardus), and a series of
macroalgal (Sargassum ilicifolium) assays positioned at increasing
distances from the models (1, 2, 3 and 4 m) on two Singaporean coral reefs
to investigate how acute fear effects shape the intensity of herbivory,
and whether these effects were influenced by variation in the group size
of herbivorous fishes feeding on the assays. 3. We found acute fear
effects strongly influenced the foraging behaviour of herbivorous fishes
over small spatial scales. Rates of Sargassum biomass removal, feeding
rates and the total number of individual feeding events were all lower
near the predator model. These effects dissipated rapidly with increasing
distance from the predator model, and were undetectable at a distance of 4
m. We also found generally larger group sizes of herbivorous fishes
further from the predator model presumably reflecting decreased risk.
Further, the number of individual bites-event-1 increased significantly
with increasing group size for two common browsing fishes, Siganus
virgatus and Siganus javus. 4. Our findings highlight that acute fear
effects influence the distribution and intensity of herbivory over small
spatial scales. Fear effects also interacted with herbivore group size
resulting in changes in the number of individual feeding events and bite
rates that collectively shape the realised ecosystem function of
macroalgal removal on coral reefs. Group size is an important
context-dependent factor that should be considered when examining fear
effects on coral reefs.