Tropical fish diversity enhances coral reef functioning across multiple scales

Abstract

There is now a general consensus that biodiversity positively affects ecosystem functioning. This consensus, however, stems largely from small-scale experiments, raising the question of whether diversity effects operate at multiple spatial scales and flow on to affect ecosystem structure in nature. Here, we quantified rates of fish herbivory on algal turf communities across multiple coral reefs spanning >1000 km of coastline in the Dominican Republic. We show that mass-standardized herbivory rates are best predicted by herbivore biomass and herbivore species richness both within (α-diversity) and across sites in the region (β-diversity). Using species-diversity models, we demonstrate that many common grazer species are necessary to maximize the process of herbivory. Last, we link higher herbivory rates to reduced algal turf height and enhanced juvenile coral recruitment throughout the ecosystem. Our results suggest that, in addition to high herbivore biomass, conserving biodiversity at multiple scales is important for sustaining coral reef function.

Fig. 1. Herbivore biomass, local α-diversity, and between-community β-diversity significantly predict mass-standardized herbivory rates.

Plotted values are the partial effects, which, having accounted for the influence of all other predictors (Z) in the linear mixed-effects model, thus reflect the statistically independent effect of herbivore (A) biomass, (B) α-diversity, and (C) β-diversity on the response (mass-standardized bite rate). Fitted lines are linear regressions ± 95% confidence intervals. Points in (C) are scaled by local herbivore richness so that larger points reflect sites with more species. The full model results are found in table S1.

Fig. 2. Contributions to total bite rate by each grazer species at each reef.

Values are averaged across all cameras at the 10 reef sites (primary x axis). Reef sites are in order of increasing herbivore species richness (secondary x axis).

Fig. 3. Higher herbivore bite rates were associated with more finely cropped turfs, which would otherwise reduce the recruitment of corals to the reef.

Plotted values in (A) are the partial effects of bite rate, which, having accounted for the influence of all other predictors (Z) in the multiple regression model, thus reflect the statistically independent effect of mass-standardized bite rate (kg bites per m² per hour) on algal turf canopy height (mm) (R² = 0.80). (B) Bivariate correlation between algal turf canopy height and the number of juvenile corals per m² (R² = 0.48). Fitted lines are linear regressions ± 95% confidence intervals.