Mission 7 — Herbivore Resistance to Seaweed Defenses and the Effects on Reef Community Structure.

Mission 7 — Herbivore Resistance to Seaweed Defenses and the Effects on Reef Community Structure.

Principal Investigator: Dr. Mark Hay, Georgia Institute of Technology
Training: November 1-5
Mission: November 8-17

Herbivory plays a crucial role in structuring coral reef communities and in selecting for algal traits that deter feeding by herbivores. Over the past 20 years, understanding of the ecology and evolution of seaweed defenses has grown dramatically. In contrast, little is known about how herbivore traits affect resistance to algal defenses, and almost nothing is known about how among-herbivore variance in susceptibility to defenses affects coral reef community structure. Recent studies in the Pacific have noted dramatic among-species variance in the susceptibility of herbivorous fishes to chemical and mineral defenses, but the consequences of this for seaweed/herbivore/coral interactions on natural reefs remains unknown.

We propose using marine chemical ecology and manipulative field ecology to investigate how seaweed defenses interact with herbivore variance in susceptibility to defenses to affect reef community structure and the interactions between seaweeds and coral. In preliminary field investigations where we monitored free ranging fish feeding on algal transplants and enclosed fishes on the reef to determine the effects of their feeding, it appeared that surgeonfish selectively grazed chemically-noxious red seaweeds and prevented their recruitment, but enhanced the growth of calcified species. In contrast, parrotfish suppressed calcified species, but certain chemically defended red seaweeds flourished in their presence. These studies were short-term and conducted on only shallow reefs, but it appeared that neither herbivore species alone could prevent certain macrophytes from becoming abundant, and possibly overgrowing corals. From these limited data, we speculate that certain herbivorous fishes will be much more important than others in removing macrophytes and that there may be a specific mixture of species that will be critical for maintaining low algal and high coral cover on tropical reefs. Thus, the particular biodiversity of herbivores may be as important as the density or mass of herbivores in determining the structure of reef communities. We know little of the species-specific effects of reef herbivores, how effects of particular species sum to produce an overall effect, or whether a particular species mix of reef herbivores is critical, or of minimal importance, for reef function.

If most reef herbivores are ecologically redundant of each other, then one species may be an adequate substitute for another. However, if ecologically important herbivores differ considerably in their response to seaweed defenses, then particular herbivores, or mixes of herbivores, could be crucial in maintaining ecosystem function by preventing macrophytes from overgrowing corals. If this is the case, then particular species of herbivores may serve as biological fulcrums that resource managers should consider in efforts to leverage reefs back to desirable states of ecosystem function. Manipulation of certain echinoderms has shown this to be a viable strategy in some areas, but this approach has not been used with reef fishes. This hypothesis will be evaluated by building large enclosures on hard grounds near the Aquarius in the Florida Keys, enclosing specific fishes, and mixes of fishes, in these to determine (1) their long-term effects on community structure, (2) how small mobile species (that can move through the cage mesh (juvenile fishes, amphipods, crabs, etc.) respond to these community changes, and (3) how algal chemical and mineral defenses generate the mechanisms driving these changes, possibly affecting seaweed overgrowth of corals. Year 1 efforts focused on differences between the most common parrotfish and most common surgeonfish, on the mechanisms producing these differences, and on the cascading effects of those differences for community structure.In addition to improving our basic understanding of the ecology and evolution of interactions affecting coral reefs, this investigation may offer additional options for coral reef management and restoration.

Year 2 efforts will parallel those of year 1 but will focus on differences among common parrotfishes (and among common surgeonfishes, if time allows), on the mechanisms producing these differences, and on the cascading effects of those differences for community structure. There will also be more extensive follow-up investigations on how seaweed traits and herbivore physiologies interact to produce the differential effects noted in the first year’s data.