Supply-side ecology works both ways: The link between benthic adults, fecundity, and larval recruits

"Supply-side" ecology recognizes the potential role that recruitment plays in the local population dynamics of open systems. Apart from the applied fi sheries literature, the converse link between adults and the production of cohorts of recruits has received much less attention. We used a hierarchica l 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 th e length of the Great Barrier Reef, Australia. Our goal was to quantify pat terns of recruitment at multiple scales, and to explore the underlying mech anisms. Specifically, we predicted that large-scale patterns of recruitment could be driven by changes in the abundance of adults and/or their fecundi ty, 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 amo ng sectors and was not correlated with recruitment at the sector or reef sc ale. In contrast, fecundity levels (the proportion of colonies on each reef that contained ripe eggs) varied from 15% to 100%, depending on sector, ye ar, and species. Spatial and temporal variation in the fecundity of each of three common Acropora species explained most of the variation (72%) in rec ruitment by acroporids, indicating that the production of larvae is a major determinant of levels of recruitment at large scales. Once fecundity was a ccounted for, none of the other variables we examined (sector, reef area, a bundance of adults, or year) contributed significantly to variation in recr uitment. 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 w ith the hypothesis that enhanced fertilization success and/or predator sati ation occurs during mass-spawning events. Furthermore, it implies that smal l, sublethal changes in fecundity of corals could result in major reduction s in recruitment.