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

Karlson, Ronald; Levitan, Don

Compared actual population dynamics (population density, mortality and recruitment) of the sea urchin Diadema antillarum with that anticipated by a model. We have compared a simple differential equation model describing the dynamics of an open, recruitment-limited population against observed changes in populations of Diadema antillarum. In this model, recruitment rate is assumed to depend only on extrinsic factors (i.e., there is no density feedback) and per capita mortality rate is assumed to be density-independent. Under these assumptions, population size stabilizes when the number of individuals recruiting into the population is balanced by the number dying. Equilibrium population size is globally stable. Over a wide range of parameter values, population size is more sensitive to mortality rate than to recruitment rate. Under the conditions of the model, shifts in recruitment rate (e.g., as a result of large scale mass mortality events) result in new equilibrial population sizes. Such a shift in recruitment rates occurred in populations of the sea urchin Diadema antillarum as a consequence of a Caribbean-wide mass mortality. Population densities and subsequent recruitment rates decreased two orders of magnitude following this event. We have independently estimated post-mass mortality (1984-1988) recruitment and mortality rates at Lameshur Bay, St. John, U.S.V.I. and have generated a set of predicted population densities using the open population model. Our results suggest that the open population model describes the dynamics of these populations of D. antillarum and that current population densities may be at or near new (i.e., post-mass mortality) equilibria. We estimate current recruitment rates at 0.017 - 0.534 recruits/m2/yr and mortality rates at 0.27 - 0.47 per capita deaths/yr. 1986 - 1988 densities and recruitment rates have not changed significantly. Full recovery to pre-mass mortality densities will require a large increase in recruitment rates and/or much lower mortality rates. The open population model may represent an appropriate description of the dynamics of a wide variety of natural populations. We note the prevalence of marine, benthic organisms 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.

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

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