Observations over a 30‐yr period revealed a considerable degree of natural variation in the abundance of corals on Heron Island, Great Barrier Reef, Queensland, Australia. Cover ranged from <0.1% to >80%, with a similar large range in colony density, at several temporal and spatial scales. Much of this variation was due to the type, intensity, and spatial scale of disturbances that occurred. Coral assemblages usually recovered from acute disturbances, both on Heron Island and on other Indo‐Pacific reefs. In contrast, corals did not recover from chronic disturbances of either natural or human origins, or from gradual declines. Recovery was slower after acute disturbances that altered the physical environment than after disturbances that simply killed or damaged corals. The space and time scales of declines and recoveries in abundance were much smaller on the wave‐exposed side of the reef than on the side protected from storms. Recruitment rates were reduced by preemption of space by corals or macroalgae, and by storms that altered the substratum. Thus, the dynamics of abundance in this coral community can be largely understood through the variation in types and scales of disturbances that occurred, and the processes that took place where disturbances were rare.

No abstract

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“Supply‐side” ecology recognizes the potential role that recruitment plays in the local population dynamics of open systems. Apart from the applied fisheries literature, the converse link between adults and the production of cohorts of recruits has received much less attention. We used a hierarchical sampling design to investigate the relationships between adult abundance, fecundity, and rates of larval recruitment by acroporid corals on 33 reefs in five sectors (250–400 km apart) stretching from north to south along the length of the Great Barrier Reef, Australia. Our goal was to quantify patterns of recruitment at multiple scales, and to explore the underlying mechanisms. Specifically, we predicted that large‐scale patterns of recruitment could be driven by changes in the abundance of adults and/or their fecundity, i.e., that corals exhibit a stock–recruitment relationship. The amount of recruitment by acroporids in each of two breeding seasons varied by more than 35‐fold among the five sectors. Adult density varied only twofold among sectors and was not correlated with recruitment at the sector or reef scale. In contrast, fecundity levels (the proportion of colonies on each reef that contained ripe eggs) varied from 15% to 100%, depending on sector, year, and species. Spatial and temporal variation in the fecundity of each of three common Acropora species explained most of the variation (72%) in recruitment by acroporids, indicating that the production of larvae is a major determinant of levels of recruitment at large scales. Once fecundity was accounted for, none of the other variables we examined (sector, reef area, abundance of adults, or year) contributed significantly to variation in recruitment. The relationship between fecundity and recruitment was nonlinear, i.e., rates of recruitment increased disproportionately when and where the proportion of gravid colonies approached 100%. This pattern is consistent with the hypothesis that enhanced fertilization success and/or predator satiation occurs during mass‐spawning events. Furthermore, it implies that small, sublethal changes in fecundity of corals could result in major reductions in recruitment.

Population decline, local extinction, and recovery are profoundly influenced by variation in demography and life‐history traits. In open populations, changes in patterns of recruitment may also have a major influence on the size of local populations, particularly for short‐lived organisms. We examine here the demographic processes underlying a slow decline of corals on Jamaican reefs, where coral cover has decreased by fourfold over a 16‐yr period. We divided the study into three approximately equal intervals (1977–1982, 1982–1987, and 1987–1993) and constructed size‐based transition matrices for each of three abundant species of corals (Montastrea annularis, Agaricia agaricites, and Leptoseris cucullata) that differ substantially in life history: Montastrea is slower‐growing, longer‐lived, and has lower rates of recruitment than the other two species. Rates of survival, population growth (λ), and recruitment declined substantially over time for all species and the stable size structures became increasingly dominated by small colonies. Elasticity and life table response analysis showed that changes in the persistence of large colonies had the biggest impact on population growth in all species. Simulations indicated that the levels of larval recruitment required to maintain populations at 1977 levels increased sharply over time, even as the actual recruitment rate declined. Recruitment failure was much more important to A. agaricites and L. cucullata than to M. annularis, which could survive long periods with minimal larval input. Recovery of these populations will require an increase in both survival and recruitment. The likelihood of the latter will depend on the scale of larval dispersal, and on the impact of large‐scale mortality of adults on stock‐recruitment relationships. Differences in connectivity and life histories of corals will determine future patterns of recovery or further decline.