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

Andrew G Bauman,Jenny Fong,Glenn Dunshea,Ian Z W Chan,Andrew S Hoey,Peter A Todd

doi:10.1111/1365-2435.13867

Abstract

AbstractFear of predators (‘fear effects’) is 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 while also providing foraging benefits to group members. Within coral reef ecosystems, fear effects have been shown to influence the feeding rates of herbivorous fishes, a key functional group that prevents macroalgal overgrowth. Yet, how fear effects and group size interact to shape macroalgal removal on coral reefs remains unclear.Here, we conducted field‐based experiments using models of a common piscivorous fish, the leopard coral grouperPlectropomus leopardusand a series of macroalgalSargassum ilicifoliumassays positioned at increasing distances from the models (1, 2, 3 and 4 m) on two coral reefs in Singapore 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.We found acute fear effects strongly influenced the foraging behaviour of herbivorous fishes over small spatial scales. Rates ofSargassumbiomass 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. Furthermore, the number of individual bites/event increased significantly with increasing group size for two common browsing fishes,Siganus virgatusandSiganus javus.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 realized 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.A freePlain Language Summarycan be found within the Supporting Information of this article.

Highlights

Predation is a key process influencing species interactions and the flow of energy through food webs, and ecosystem structure and function (Lima & Dill, 1990; Sih et al, 1985)
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 coral reefs in Singapore 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
Fear effects have increasingly been shown to be important drivers of herbivorous fish foraging behaviour (Mitchell & Harborne, 2020), influencing the spatial and temporal distribution of herbivory on coral reefs (Madin et al, 2010; Rasher et al, 2017; Rizzari et al, 2014), yet how browser foraging behaviour is influenced by group size in the

Summary

| INTRODUCTION

Predation is a key process influencing species interactions and the flow of energy through food webs, and ecosystem structure and function (Lima & Dill, 1990; Sih et al, 1985). Previous studies of fear effects have associated changes in herbivorous fish foraging rates to spatial and temporal differences in predator presence and/or abundance (Hoey & Bellwood, 2011; Madin et al, 2010; Rasher et al, 2017), the availability of refugia (e.g. grazing halos; Madin et al, 2011) or the presence of static predator models (Bauman et al, 2019; Catano et al, 2016; Rizzari et al, 2014) Combined, these findings suggest that fear effects are important determinants of the spatial and temporal distribution of herbivory on coral reefs (Mitchell & Harborne, 2020). We hypothesized that macroalgal removal would increase with decreasing acute risk, and the formation of larger browser group size closer to the predator model to reduce acute predation risk

| MATERIALS AND METHODS

Findings

| DISCUSSION