Manipulating herbivore diversity to restore coral reefs


Principal Investigator: Dr. Mark Hay, Georgia Institute of Technology


Herbivory plays a crucial role in structuring coral reefs and in selecting for algal traits that deter herbivores. Recent studies note dramatic among-species variance in the susceptibility of herbivorous fishes to seaweed chemical and structural defenses; these differences can translate into dramatic direct effects of herbivore diversity on seaweeds, indirect effects on corals, and changes in the structure and function of coral reefs. I propose continuing use of the Aquarius to determine experimentally how herbivore diversity may be managed to conserve and restore reefs.

In previous investigations, we demonstrated that herbivore diversity affected the function and structure of coral reefs. The mix of two specific species of herbivorous fishes decreased macrophytes by 54-76%, increased corallines by 23-117%, increased coral growth by 22%, and prevented coral mortality in mixed-herbivore compared to either single-herbivore treatment. When we enclosed redband parrotfish vs. ocean surgeonfish in large cages affixed to coral reefs, the surgeonfish prevented recruitment by several chemically noxious seaweeds, but enhanced the growth of calcified algae, while the parrotfish suppressed calcified algae, but some chemically defended seaweeds flourished. Repeating this with two species of parrotfish demonstrated that two species suppressed seaweeds more than single species, but the differences were not as dramatic and princess parrotfish produced effects more like ocean surgeonfish than redband parrotfish (suggesting that functional groups based on taxonomy or jaw morphology will be inadequate to predict community level effects). I now propose finishing this contrast by assessing different species of surgeon fishes (year 1) and by contrasting mixed species of fishes versus differing densities of Diadema (year 2). Present data suggest that specific mixtures of herbivores are critical for suppressing macrophytes and enhancing coral cover. Thus, the particular biodiversity of herbivores may be as important as the density or mass of herbivores in determining the structure, function, and health of reef communities. We know too little of the species-specific effects of reef herbivores, how effects of multiple species sum to produce an overall effect, or which particular mix of reef herbivores is critical for suppressing aggressive seaweeds and maintaining reef function. Much evidence shows that removing herbivores leads to reef decline, but our last NURC grant appears to be the first experimental demonstration that replacing specific mixes of herbivores can reverse this trend and facilitate the survival and growth of reef corals.

This proposal seeks to acquire the information needed to optimize this approach to coral reef restoration. If most reef herbivores are ecologically redundant, then one species may substitute for another. However, if ecologically important herbivores differ considerably in their response to seaweed defenses, then particular mixes of herbivores may be crucial in maintaining ecosystem function and may need focused management and protection.

This hypothesis will be evaluated by building large enclosures on reefs near the Aquarius in the Florida Keys, enclosing specific fishes, mixes of fishes, and mixes of fishes versus differing densities of grazing urchins to determine (1) their long-term effects on community structure, (2) which seaweeds are most damaging to corals and which herbivores best control these species, (3) how small mobile species and recruiting juvenile fishes that can move through the cage mesh (amphipods, crabs, etc.) respond to these community changes, and (4) how algal chemical and mineral defenses generate the mechanisms driving these changes. These findings, when coupled to our earlier studies, should provide new tools for restoring Caribbean reefs