| This dissertation examined factors shaping resilience of coral reef communities to shifts to macro-algal domination by investigating the interactions between two major functional components, herbivorous fishes and benthic algae. Two research approaches were taken, top-down trophodynamics and fish population dynamics. The impact of herbivorous fishes on the benthic algal community was determined by measuring grazing intensity and algal production rates on a cross-shelf gradient in the upper Florida Keys, where herbivorous fishes are not fished but coral cover has declined drastically. Algal production rates were similar between inshore and offshore reefs, but spatial variation in biomass and feeding rates of herbivorous fishes resulted in strong spatial differences in the proportion of algal production consumed between offshore (near 100%) and inshore (< 50%) reefs. Secondly, density, size structure, biomass, and grazing rates of an important herbivore, the stoplight parrotfish (Sparisoma viride), were compared among reefs in the upper Florida Keys, Bonaire and the Bahamas. Spatial differences in algal consumption rates were found to be a function of both differences in biomass and bite rates. These trophodynamic studies indicate that herbivorous fishes are capable of consuming a majority of the algal production even on low coral cover reefs, but that feeding rates can be modified by local reef conditions.; The second research tactic involved two studies of S. viride populations in the upper Florida Keys: an analysis of patterns of recruitment and an age-based analysis of population demographics. On Florida's low coral cover reefs, S. viride recruited preferentially to areas of high macroalgal cover, resulting in stronger recruitment on reefs with higher macroalgal cover. The age-based analysis revealed significant cross-shelf differences in the growth trajectory, maximum size, and longevity of S. viride. This second set of studies suggests that S. viride is resilient to loss of coral cover on reefs at early life history stages, but that populations exhibit high demographic plasticity as a result of local environmental characteristics. Overall, this dissertation demonstrates that herbivorous fishes can exert a strong control on benthic algae, but that benthic characteristics can in turn influence feeding rates, recruitment, growth, and mortality rates. |
