Species differences drive spatial scaling of foraging patterns in herbivorous reef fishes

Abstract

Herbivory is a core ecosystem function that is delivered heterogeneously across space. Disentangling the drivers of foraging patterns is key to understanding the functional impact of herbivores. Because intrinsic drivers of foraging like metabolism, nutritional requirements, and movement costs scale allometrically, foraging movement patterns in terrestrial herbivores have been shown to also scale positively with body size. However, spatial patterns of herbivory can also be explained by orthogonal factors such as trophic position, competition, and functional groupings. Here, we investigate body size and species traits as drivers of the spatial scaling of foraging patterns in herbivorous coral reef fishes. We quantified foraging patterns of 119 individuals from nine common herbivorous species using focal individual surveys. Body size, species identity, feeding substrata, social grouping, and functional group were tested as predictors of three foraging metrics: foraging area, inter-foray distance, and tortuosity. Our resulting model revealed that species identity overshadowed body size as a predictor in models for all foraging metrics. While foraging area was explained best by species only, the resulting tortuosity and mean inter-foray distance models included a small effect of body size that explained within-species variation. We do not find strong support for size-scaling of foraging patterns in our study species. These findings indicate that foraging allometry based on Optimal Foraging Theory cannot be generally applied to reef fish assemblages due to a diversity of foraging strategies, such as spatial partitioning and territoriality. Our work reveals the importance of behavioural ecology and taxonomic diversity in understanding herbivory, especially given the functional differences across species. With coral reefs under threat across the world, this is an important step to disentangling the spatial delivery of a core ecosystem function.