We know the biomass of macroalgae on coral reefs is largely controlled by herbivory and that one of the most important groups of grazers are parrotfish. A new study published in PNAS (Berkepile and Hay 2008) indicates that the richness and composition of grazer species is also important. In a nutshell, different fish consume different seaweeds because of their differing chemical defenses. Similar work in other benthic marine systems has found that consumer species richness can be an important determinant of ecosystem functioning, yet this is the first such study on a coral reef.
Our study shows that in addition to having enough herbivores, coral ecosystems also need the right mix of species to overcome the different defensive tactics of the seaweeds. explained Mark Hay, the Harry and Linda Teasley Professor of Biology at the Georgia Institute of Technology.
Despite different species of parrotfish in the Caribbean having different feeding behaviors, bioerosion rates, and preferred diets, parrotfishes are often considered as a unified functional group when inferring their effects on community structure. However, we found that redband and princess parrotfish had considerably different effects on communities, suggesting that grouping all parrotfishes may blur important distinctions among species.
Despite their different feeding morphologies, ocean surgeonfish and princess parrotfish generated similar macroalgal communities dominated by upright brown macroalgae (e.g., L. variegata and Sargassum spp.). In contrast, despite their more similar jaw morphology, the communities generated by redband and princess parrotfish differed considerably in the abundance of upright macroalgae. Similar to the work of Bellwood et al., these results show that fishes with different feeding morphologies can have similar effects on community structure, suggesting that relying primarily on jaw functional morphology to construct functional groups or infer a species’ impact may be unreliable.
Working out of the underwater Aquarius laboratory off Key Largo Florida, Hay and co-author Deron Burkpile – who is now at Florida International University in North Miami – constructed 32 cages on the reef. Each cage was about two meters square and one meter tall and was sealed so that larger fish could neither enter nor leave.
The number and type of fish placed into each four-square-meter cage varied. Some cages had two fish that were able to eat hard, calcified plants; some had two fish able to eat soft, but chemically-defended plants; some had one of both types, and some had no fish at all
For the cages in which we mixed the two species of herbivores, the fish were able to remove much more of the upright seaweeds, and the corals in those areas increased in cover by more than 20 percent during ten months, Hay said.
The data we are seeing in Fiji [from similar experiments] suggests that diversity may be even more important there than it was in the Caribbean. There are a lot of different species doing a lot of very different things. These consumers are very important, and in areas where they are over-fished, the reefs are crashing.
Berkepile, D.E. and M.E. Hay. 2008. Herbivore species richness and feeding complementarity affect community structure and function on a coral reef. PNAS 105: 16201–16206