Herbivore species richness and feeding complementarity affect community structure and function on a coral reef

Abstract

Consumer effects on prey are well known for cascading through food webs and producing dramatic top-down effects on community structure and ecosystem function. Bottom-up effects of prey (primary producer) biodiversity are also well known. However, the role of consumer diversity in affecting community structure or ecosystem function is not well understood. Here, we show that herbivore species richness can be critical for maintaining the structure and function of coral reefs. In two experiments over 2 years, we constructed large cages enclosing single herbivore species, equal densities of mixed species of herbivores, or excluding herbivores and assessed effects on both seaweeds and corals. When compared with single-herbivore treatments, mixed-herbivore treatments lowered macroalgal abundance by 54-76%, enhanced cover of crustose coralline algae (preferred recruitment sites for corals) by 52-64%, increased coral cover by 22%, and prevented coral mortality. Complementary feeding by herbivorous fishes drove the herbivore richness effects, because macroalgae were unable to effectively deter fishes with different feeding strategies. Maintaining herbivore species richness appears critical for preserving coral reefs, because complementary feeding by diverse herbivores produces positive, but indirect, effects on corals, the foundation species for the ecosystem.

Fig. 1.

Year 1 abundance of algae (mean ± SE) at the end of the experiment. P values from two-factor ANOVA. * indicates that single- vs. mixed-herbivore treatments differ as determined via resampling statistics. R = redband parrotfish, S = ocean surgeonfish. n is in brackets below the x axis.

Fig. 2.

Year 2 algal abundance at the end of the experiment. Statistics and symbols as in Fig. 1. R = redband parrotfish, P = princess parrotfish.

Fig. 3.

Nonmetric multidimensional scaling showing similarity of macroalgal communities among the treatments from year 1 and 2. Lines delineate axis space occupied by redband- (filled triangles), princess- (open squares), and surgeonfish-only treatments (open triangles).

Fig. 4.

Treatment effects (year 1) on coral mortality (A), absolute change (cm²) (B), or proportional change (%) (C) in colony area per coral (mean ± SE). Statistics are via (A) t tests (*, P < 0.05; **, P < 0.01) of mortality against an expected of 0% (i.e., did significant mortality occur?), and (B and C) two-factor ANOVA. In B and C, * indicates a difference between single- and mixed-herbivore treatments as in Fig. 1. R = redband parrotfish, S = ocean surgeonfish.