Coral Reef Recovery on Guam (Micronesia) After Catastrophic Predation by Acanthaster Planci.

Abstract

At Tanguisson Reef, Guam, in 1968-1969 a population explosion of the coral-eating sea star Acanthaster planci devastated the coral community. In the wake of this predation, coral species richness, density, and cover were drastically reduced, and the species composition was altered. In two of three reef zones examined, < 1% coral cover remained. At the time of disturbance, some considered the magnitude of this and similar Acanthaster disturbances unprecedented, and predicted long recovery times because reefs were viewed as mature, stable communities. This major disturbance offered an opportunity to study the processes and rate of community recovery. Using the data from previous studies in 1970, 1971, and 1974, and my data from 1980-1981, I present a long-term analysis of coral community development. I have focused on coral recruitment, survivorship, and species settlement patterns in relationship to the remnant surviving population. The preference of Acanthaster planci for certain prey (e.g., Montipora and Acropora) shifted the species composition to one in which nonpreferred prey predominated (e.g., Porites and Leptastrea). The predominance of nonpreferred prey was short-lived, and by 1980, preferred prey were the primary colonists of two zones. The three biological zones (reef front, submarine terrace, and seaward slope) had their own characteristic species assemblages soon after the disturbance. Nonrandom recruitment or survival of juvenile corals (diameter ≤ 4 cm) established these zones. Although the percentage of juvenile corals fell throughout the survey, their highest density was reached after adult colonies were established. In 1980, of the 34 species examined, 79% showed a significant coefficient of association between adult and juvenile conspecifics. This association caused clumped species dispersion patterns to develop within zones. As the result of recruitment, species diversity rose from 84 species in 1970 to 154 species in 1981. As surviving corals and new recruits developed, the distribution of coral growth forms became more diversified. There was a drastic increase in coral cover (e.g., submarine terrace zone, 0.9% in 1970 to 65% in 1981). This increase in cover corresponded to an increase in size and number of the coral colonies In 12 yr, species richness, cover, and composition reached or exceeded measurements of comparable reefs before the disturbance. The recovery occurred faster than predicted by a stable equilibrium model because long-term successional changes were not required. Recovery was accelerated because Acanthaster did not destroy the structural integrity of reef framework. This rapid recovery from a natural disturbance demonstrates that some coral communities have a greater resilience than was once believed.