A shifting budget of grazing and production: Functional responses of a coral reef community to herbivore protection
Herbivore grazing pressure on coral reefs is considered a major driver in the maintenance of coral dominance over algae in competition for space. While the general importance of herbivores on coral reef community structure and health has been widely studied, the individual roles of different herbivores within a reef community context is less understood. Particularly, while a strong negative relationship between herbivore biomass and algal biomass on reefs has been demonstrated many times, the functional mechanism behind this relationship has not. Here, we calculate a budget for herbivore consumption and algal growth on a Hawaiian coral reef. Daily algal production on the reef is determined through analysis of benthic community composition, standing stock of algal biomass, and growth rates of algal species. Production is compared to daily consumption on the reef, determined by combining abundance, biomass, and size class of herbivores, and the consumption rates of herbivores on different species of algae. These data were collected through a series of field and lab experiments as well as semiannual surveys at Kahekili Herbivore Fisheries Management Area (HFMA), established in 2009 to prohibit take of herbivorous fish and urchins on this declining reef.
Algal production at Kahekili HFMA currently exceeds the grazing capacity of the herbivore community on the reef. Calculated annual budgets, however, reveal that this gap in production and consumption is shrinking with increasing herbivore biomass. This trend is driven largely by increasing parrotfish populations. Herbivorous fish and urchin grazing rates show clear preferences for different species of macroalgae among herbivores, though the majority of grazing occurs on turf algae, the most dominant benthic constituent. Fish and urchins graze blooming algal species an order of magnitude faster than non-blooming algae. They also preferentially graze nutrient-enriched algae, which are abundant on the reef as a result of runoff from land-based sources. These preferences suggest that a continued increase in herbivore biomass due to the HFMA designation could be effective in reducing algal abundance, especially for blooming and nutrient-enriched algae. Examining the changes in the herbivore grazing and algal growth budget elucidates the role of different herbivores in influencing overall grazing on the reef. It also allows for a closer examination of what future increases in herbivore populations mean for the recovery of this declining coral reef and the success of the HFMA could provide a model for coral reef management.