Picky eater or generalist feeder? Diet diversity and functional homogenization in herbivorous reef fishes

Abstract

Herbivorous fishes are critical to the function and resilience of coral reefs, but their diets and functional roles can show great variability, even among closely related taxa. I assessed the diet specialization of herbivorous reef fishes at multiple scales ranging from an examination of diet variation on an individual scale to a Pacific wide analysis of functional homogenization in herbivore assemblages. Using a molecular metabarcoding approach, I first identified the algal diets of two common Hawaiian surgeonfishes, Acanthurus triostegus and A. nigrofuscus, which consume large amounts of turf algae. Because turf algae are difficult to identify visually, especially in the field, our understanding of the true diet breadth of these species has been limited. A. nigrofuscus exhibited greater diet diversity, more variability between individuals, and less variation between sites than A. triostegus. I then used this same metabarcoding approach to examine the diets of eight other herbivores and conducted a systematic literature review of studies examining herbivorous reef fish diets. I combined these data to generate a standardized index of diet diversity for herbivorous reef fishes, which indicated that parrotfishes and detritivorous surgeonfishes have more limited diets, but there is also a range of specialization within each functional group and taxonomic family. I combined the index of diet diversity with fish abundance and trait data to examine functional homogenization in herbivore assemblages throughout the Pacific. Using statistical models, I examined the direct and indirect effects of a suite of ecological, biophysical, and anthropogenic drivers on herbivores. Differences in herbivore assemblage composition between islands, regions, and human population densities were apparent, but local and indirect effects drive variability in the relationship. This work will assist in reevaluating the functional role of herbivores to guide effective management by examining herbivore specialization at scales ranging from an individual to an assemblage.