The effects of biogeography and human influence on herbivore community structure and the coral reef benthos

Background/Question/Methods

The community structure of consumer assemblages is recognized as an important driver of ecological processes. On coral reefs, herbivorous fishes are known to shape the reef benthos through their grazing activity and this functional process can have strong implications for the overall coral reef community.  However, both biogeographic and anthropogenic factors demonstrably influence the community structure of herbivorous fish assemblages, suggesting that these variables may also indirectly drive systematic changes in the benthic community of coral reefs. Interestingly, however, the processes underpinning herbivore community structure on coral reefs and its consequences for the benthic community have never been evaluated simultaneously. Here, we use data from 63 sites throughout the South Pacific to investigate the response of herbivore communities to both the geographic distance to the coral reef biodiversity hotspot and to human population size, and relate these responses to benthic community structure.

Results/Conclusions

Functional richness of herbivore communities decreased with increasing geographical distance from the hotspot while human population density had no discernable effect on functional richness. In contrast, human population density significantly reduced the overall biomass of herbivorous fishes. Herbivore community structure and biomass differed in their apparent effect on the benthos. Increasing herbivore biomass was significantly correlated with a decrease in the prevalence of turf- and macroalgae, while increasing functional richness within herbivore assemblages increased both algal and live coral prevalence. These findings suggest that biogeographical factors rather than human population density are the primary drivers of systematic changes in the community structure of herbivores and that the latter mediates both coral and algal prevalence as well as the diversity of the benthic community. However, overall variation among sites had by far the greatest effect on the observed patterns, accounting for an average of 83.9% of the cumulative variation explained by fixed and random effects across models. These two key results suggest that the effects of herbivore communities on the benthos are multifaceted and call for caution when assessing the response of herbivore communities and the coral reef benthos to increasing anthropogenic pressure.