Application of diet theory reveals context-dependent foraging preferences in an herbivorous coral reef fish
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Dietary preferences of grazers can drive spatial variability in top-down control of autotroph communities, because diet composition may depend on the relative availability of autotroph species. On Caribbean coral reefs, parrotfish grazing is important in limiting macroalgae, but parrotfish dietary preferences are poorly understood. We applied diet-switching analysis to quantify the foraging preferences of the redband parrotfish (Sparisoma aurofrenatum). At 12 Caribbean reefs, we observed 293 redband parrotfish in 5-min feeding bouts and quantified relative benthic algal cover using quadrats. The primary diet items were macroalgal turfs, Halimeda spp., and foliose macroalgae (primarily Dictyota spp. and Lobophora spp.). When each resource was evaluated independently, there were only weak relationships between resource cover and foraging effort (number of bites taken). Electivity for each resource also showed no pattern, varying from positive (preference for the resource) to negative (avoidance) across sites. However, a diet-switching analysis consisting of pairwise comparisons of relative cover and relative foraging effort revealed clearer patterns: parrotfish (a) preferred Halimeda and macroalgal turfs equally, and those two resources were highly substitutable; (b) preferred Halimeda to foliose macroalgae, but those two resources were complementary; and (c) also preferred turf to foliose macroalgae, and those resources were also complementary. Thus parrotfish grazing rates depend on relative, not absolute, abundance of macroalgal types, due to differences in substitutability among resources. Application of similar analyses may help predict potential changes in foraging effort of benthic grazers over spatial gradients that could inform expectations for reef recovery following the protection of herbivore populations.
KeywordsDiet choice Diet switching Resource complementarity Foraging theory Sparisoma aurofrenatum
We thank W. Freshwater for advice on algal biology and helpful comments on the manuscript, and T.-L. Loh, S. McMurray, L. Deignan, I. Conti-Jerpe, M. Marty, A. Dingeldein, and C. Marino for assistance in the field. We also acknowledge the staff at UNCW and NURC for logistical support. Constructive suggestions from two anonymous reviewers improved the manuscript.
Author contribution statement
JH and JRP conceived and designed the study. JH collected the data. JH and JWW analyzed the data. JH, JWW, and JRP wrote the manuscript.
Compliance with ethical standards
This study was funded by the National Science Foundation (OCE-0550468, 1029515).
Conflict of interest
The authors declare that they have no conflict of interest.
All applicable institutional and national guidelines for the care and use of animals were followed.
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