Recruitment-limitation in open populations of Diadema antillarum: an evaluation

Empirical evidence from studies of the sea urchin Diadema antillarum suggests that this organism widely disperses its offspring, that both recruitment and mortality rates are independent of local densities, and that local food availability does not regulate local population sizes. These attributes would indicate that local populations are generally open and recruitment-limited. Given that current populations have been devastated by a 1983-1984 mass mortality event which spread throughout the range of this species, we examine current population trends and evaluate the prospects for population recovery under the assumptions of recruitment-limitation and density-independent rates of recruitment and mortality. Specifically, we evaluate the dynamics of five, local populations at Lameshur Bay, St. John, U.S.V.I. in order to 1) determine current rates of recruitment and mortality, 2) predict population densities based on the above assumptions, 3) compare predicted densities against observed 1984-1988 densities, and 4) predict future population densities based on current trends. We estimate current recruitment rates at 0.02-0.11 individuals/m²/yr and per capita mortality rates at 0.27-0.47 deaths/yr. Over the period 1985-1988, predicted densities based on these annual rates did not differ significantly from actual observed densities. Therefore, the assumptions that recruitment and mortality rates are density-independent and that local populations are recruitment-limited are sufficient, at present, to adequately predict current population trends. These trends indicate no recovery towards pre-mass mortality densities. The above description of the dynamics of open, recruitment-limited populations may be appropriate for a wide variety of organisms. We note the prevalence of animals with extensive larval dispersal capabilities. Populations located near the limits of their distribution, in freshwater streams and ponds, mountain tops, or other similarly isolated populations may also be subject to recruitment-limitation. Remote, recruitment-limited populations are likely to be more susceptible to local extinction than less remote populations. Dispersal distances and the scale of the processes controlling recruitment and mortality are important determinants of the degree of openness of local populations.