Selective consumption of macroalgal species by herbivorous fishes suggests reduced functional complementarity on a fringing reef in Moorea, French Polynesia

Abstract

Worldwide, many coral reefs are at risk of shifting to degraded algal-dominated states, due to compromised ecological conditions. Functional diversity of herbivorous fishes maintains coral reef health and promotes reef resilience to disturbances. Given previous evidence, it appears the functional roles of herbivorous fishes differ across geographical locations, indicating a need for further assessment of macroalgal consumption by herbivorous fishes. We assessed functional diversity by examining foraging behavior of herbivorous fish species on macroalgae on a fringing reef in Moorea, French Polynesia. We video-recorded choice experiments containing seven common macroalgae and used Strauss' linear resource selection index to determine macroalgal selectivities. We used cluster analysis to identify any distinct groups within herbivorous fish species, given the macroalgal species they targeted, and fitted generalized linear mixed-effects models to identify factors that best predicted the number of bites taken on macroalgae. Seven species from 3 fish families/tribes took a total of 956 bites. Fish species differed in their selectivity with some species (Naso lituratus, N. unicornis, Calatomus carolinus) strongly preferring one or two macroalgal species, while other fish species (Acanthurus nigrofuscus, Ctenochaetus striatus, Chlorurus sordidus, Balistapus undulatus) were less selective. This resulted in fish species forming two clusters. Only 3 of 7 macroalgae were preferred by any fish species, with two fish species both preferring the same two macroalgae. The limited differences in fish species' preferences for different macroalgae suggests limited functional complementarity. Two models (macroalgal species identity+fish functional group, macroalgal species identity+fish species) best predicted the number of bites taken on macroalgae compared to models incorporating only a single explanatory factor or fish family. In the context of this Moorean fringing reef, there is greater functional redundancy than complementarity of herbivorous fishes consuming macroalgae, and the fishes grouped together according to their relative selectivity. We observed fish species that are not classified as browsers consuming macroalgae, suggesting diets of herbivorous fishes may be broader than previously thought. Finally, we observed macroalgal selectivities and consumption that differed from previous studies for the same fish species. Our results contribute to the understanding of functional diversity of herbivorous fishes across coral reefs, and also highlight the need for additional research to further elucidate the role of context and functional diversity of herbivorous fishes consuming macroalgae on coral reefs.

Keywords: functional complementarity; functional redundancy; herbivorous fishes; macroalgae; selectivity.

Figure 1.

Percent change in biomass (mean±SE) for each macroalgal species presented in choice assays (n=11).

Figure 2.

Bites per fish visit (mean±SE, column 1) and Strauss selectivity index values (±95% CI, column 2) for fish species taking bites on macroalgal choice experiments. Macroalgae are presented in order of increasing complexity and defenses against herbivory, according to Steneck and Dethier (1994). Fish species are color coded according to their functional group based on Green and Bellwood (2009): green = browser, orange = grazer/detritivore, pink = scraper/small excavator, and purple = unclassified. Note: panels F and M are on different scales than the rest. Sample sizes are the following: Naso lituratus n = 73, Naso unicornis n = 11, Calotomus carolinus n = 56, Acanthurus nigrofuscus n = 17, Ctenochaetus striatus n = 9, Chlorurus sordidus n = 4, Balistapus undulatus n = 31.

Figure 3.

Clusters of fish species based upon their Strauss’ selectivity indices for macroalgae presented in choice experiments. Black versus gray lines indicate which fish species group together into each cluster. Fish species names are color coded according to their functional group based on Green and Bellwood (2009): orange = grazer/detritivore, pink = scraper/small excavator, purple = unclassified, and green = browser. Fish drawings provided by Nury Molina.

Figure 4.

Number of individual fish of each fish species within different size classes. Total number of fish visits for each species is indicated below their name.