Where Are All of the Herbivores?
By Emily Kelly
In the community of coral reef managers and conservation practitioners, there is general consensus that herbivores are a critical component of the reef community. The herbivorous fishes that eat seaweed help to control the growth of these fleshy algae that compete with corals for space on the bottom. When too many herbivorous fishes are removed from a reef, the algae can grow with fewer constraints. In the worst case, this unrestricted algal growth can result in a reef losing most of its corals and becoming something of an algal landscape.
Here on Flint, there are very few herbivores. During any particular dive, we see only a couple species of herbivorous fish, with the only common herbivore being the Achilles tang. This is a black fish with a teardrop of orange at the base of its tail which is an attractive fish, but strangely enough a fish that stands pretty much as the sole member of the group of herbivores. Given that there is no fishing on Flint, the question emerges – where are all of the herbivores? Common paradigms would expect an unfished island to have lots of herbivores, including not only surgeonfish (like the Achilles tang) but also the parrotfishes.
Part of the answer, we believe, lies in the natural conditions here on Flint. With few nutrients in the water, there is little opportunity for algae to grow. Without algal productivity, there is simply not very much food for herbivorous fish to eat. No food, no herbivores…easy enough.
The application of these observations, however, is a bit of a challenge. One could imagine that if an unfished island like Flint naturally has very low herbivore density, then there is no clear expectation that inhabited and fished islands should not also have low herbivore density. A skeptic could then claim that a heavily fished island with a depauperate herbivore assemblage could be the ‘baseline’ case and that unregulated fishing may not substantively change the composition of the herbivore guild. Well, an important distinction lies in comparing the herbivore population to the density of herbivore food, namely to the density of algae. Here on Flint, there are few herbivores and little seaweed. In heavily fished islands, there are few herbivores and a lot of seaweed. With the removal of herbivores, the algal ‘fish food’ thrives. On this island, the low density of herbivores does not allow algae to bloom because of natural conditions.
Coral reefs differ from place to place, and the local oceanographic conditions seem to matter quite a bit. The southern Line Islands are a fascinating natural laboratory for us to build our understanding of the potential of coral reef ecosystems and to apply this understanding to improve the management of reefs worldwide. This latter step of worldwide application, however, is a bit beyond the scope of our efforts today and tomorrow. At this point, we will keep our heads underwater and see what else we can learn during this unique research opportunity.