Recruitment Limitation, Mortality, and Population Regulation in Open Systems: A Case Study

In most populations of mammals and birds offspring live with (and initally may depend upon) their parents. Consequently, population size is strongly influenced by local fecundity. In contrast, most species of marine invertebrates have a dispersive larval stage, so that local fecundity has little or no effect on the establishment of new individuals. Similarly, many insects, amphibians, and fish have multiple life stages that live apart. Population regulation in such open systems is fundamentally different from more closed populations. I investigated the role of recruits in determining and regulating local population size of a sessile colonial invertebrate, Cellepora pumicosa (Bryozoa, Ascophora). Recruitment into a field population of C. pumicosa over an 18—mo period was less than post—recruitment mortality, causing a decline from 230 to 64 colonies (6.1 to 2.3% cover). The mortality rate was density independent. Colony growth rate declined among larger individuals, which usually regressed in size (shrank) before they died. The mechanics of recruitment limitation was further investigated using a simple demographic model. The model was designed to mimic an open population in which recruits were added to the local adult population at a rate independent of local fecundity or density. After they recruited, colonies could grow, shrink, or die at rates determined in the field. There were no density—dependent terms in the model, yet the population attained an equilibrium size and structure that was determined by a balance between rates of recruitment and size—specific mortality (i.e., storage) of recruited individuals. The equilibrium occurred because the per capita number of recruits declined as more and more individuals became established. Modelling based on the measured demography of C. pumicosa demonstrates that the observed levels of recruitment and mortality will limit the population size to very low levels, although relatively small increases in numbers of recruits would yield a substantially large population. In comparison, post—recruitment mortality would have to decrease greatly and remain density independent to achieve the same result. In contrast to the result for short—lived animals such as C. pumicosa, the population size of longer lived organisms is more sensitive to changes in mortality than recruitment, even when mortality is density independent and populations are recruitment regulated.