Grazing fishes in the Caribbean, like the many species of parrotfish pictured here, can help corals recover by keeping macroalgae, a main source of food for the fish, under control.
Perhaps one of the most integral contributors to any ecosystem anywhere is the coral, reef building, habitat-providing colonial organisms facing big challenges from bleaching and disease, pressures which have been linked to climate change.
Two major events led to declines in coral populations in the Caribbean, the worldwide bleaching events of 1998 and Hurricane Frances, which swept through the Bahamas in 2004. The damage sustained by colonies in the wake of a hurricane can be debilitating; often macroalgae (colloquially known as seaweeds, marine algae observable without a microscope), are given the opportunity to dominate, especially in an environment where populations of herbivorous fishes (particularly the parrotfishes) are reduced by fishing, limiting their ability to keep the algae at bay through grazing. This is further compounded by the massive die-off of the sea urchin Diadema antillarum in the 1980's, another major regulator of algae in these waters.
A new study from PLoS ONE takes a look at this recovery process and the impact that macroalgae has on coral growth within marine reserves, where herbivorous fishes were protected, and outside their boundaries, where the fish were not. Ten sites were evaluated over a two-and-a-half year period in and around the Exuma Cays Land and Sea Park (ECLSP), which was established as a no-take marine reserve in 1986.
Macroalgae outcompetes corals in two major ways: overgrowth (eliminating the light source) and sprawl (assuming real estate that could be used by the coral for recruitment). While the researchers were unable to determine the relative contributions of these factors, on a population scale, the results were significant. For the two brooding species of coral, Porites astreoides and Agaricia agaricites, somatic growth rates were higher in the reserves. Outside of the reserves, however, there was little recruitment (a measure of the incorporation of young into the population) and the densities of larger colonies were either in decline or plateauing in Porites. In these species, the macroalgae are acting as a sort of bottleneck to larger colonies, and more so in Agaricia, which showed no net growth or recruitment outside of the reserves.
Determining recovery factors for the reef-building coral Montastraea annularis was a bit more difficult:
Recruitment occurs rarely in this species and the increased densities found in the smaller size classes outside the reserve were almost entirely attributable to fission of established colonies rather than recruitment.
Inside the reserve, M. annularis saw recovery in all sizes, but outside researchers found that only the larger size class were in decline, but this species of coral is also very sensitive to disease, which can further "obscure" competitive factors.
When the researchers started the study in 2004, the coral cover in the area - both reserve and non-reserve - was 7 percent. By 2007, mean proportional coral cover had increased 19 percent, good news in a torrent of bad for corals around the world. No net recovery was seen outside the ECLSP, highlighting just how important these reserves play in a large system where the biodiversity of herbivorous organisms is significant;y pressured by commercial fishing and disease. While the reserves obviously cannot protect these ecosystems from every danger (particularly climate change), they can provide a direct service in keeping the system intact and giving researchers the opportunity to study and implement further, more far-reaching solutions without the immediate danger of losing their test subjects.
Mumby, P., & Harborne, A. (2010). Marine Reserves Enhance the Recovery of Corals on Caribbean Reefs PLoS ONE, 5 (1) DOI: 10.1371/journal.pone.0008657