Influence of habitat condition and competition on foraging behaviour of parrotfishes

Abstract

Metrics of foraging by herbivorous reef fishes have been shown to vary across space and time, and among species. However, little work has explicitly assessed how fish use space within their foraging ranges, or characterised relative foraging mobility in response to habitat condition. This knowledge is fundamental to understanding the functional impact of reef herbivores, and how spatially explicit functional roles may be modified by future reef degradation. In this study, we assessed the influence of among-site variation in habitat condition, competition and potential predation risk on the short-term foraging range of 2 species of parrotfish, Scarus niger and S. frenatus, on mid-shelf reefs of the Great Barrier Reef, Australia. Foraging ranges were evaluated using 3 metrics: (1) inter-foray distance and the (2) area and (3) shape of the short-term foraging range. The primary predictor of these metrics of foraging behaviour was coral cover. Inter-foray distance decreased with increasing coral cover for both S. niger and S. frenatus, and foraging ranges became more circular with increasing coral cover. Competitor abundance was a secondary driver of foraging behaviour, whereas potential predation had no detectable effect. This research provides a fine-scale understanding of how habitat condition and competition among herbivores shapes the spatial scales at which herbivores interact with their environment in the short term, and at which they perform functions essential for coral reef resilience. Critically, the study suggests that predicted changes in coral cover are likely to alter the way reef herbivores forage, and will shape the extent to which they can compensate for declining habitat condition through changes in their feeding behaviour.