Recruitment limitation, population regulation, and larval connectivity in reef fish metapopulations

Two central debates in marine ecology concern the role of connectivity patterns via larval dispersal in structuring marine metapopulations and the relative importance of larval supply vs. events occurring during or after settlement in determining adult abundance. Both issues were examined using age-structured models and simulations of reef fish population dynamics at the regional scale of closed metapopulations and the local scale of individual reefs. Local populations on individual reefs were assumed to be at once both partially open and partially closed. Two sets of models over both spatial scales are presented. One set examines density-independent dynamics. In the other set, mortality in the first year depends on the density of the settling cohort, and the density dependence is compensatory. The sensitivities of local population dynamics to the rates of self-recruitment and external larval supply were predicted. If external larval supply is regular and there is little self-recruitment, then a local population can appear to be regulated without conventional forms of density dependence. Elsewhere, this process has been termed “recruitment regulation.” However, with increased self-recruitment a local population will grow without bound in the absence of regulatory density dependence. Metapopulation persistence requires a sufficiently strong linkage between stock and recruitment in some local population. In such a local population, individuals must contribute sufficiently many offspring to replace themselves locally in subsequent generations. Such a local population could either be partially closed or lie in a region of the metapopulation that is strongly interconnected. Metapopulation regulation requires that density dependence acts to curb population growth in local source populations but does not require that it acts in local sink populations. The density dependence need not be so strong as to prevent subsequent cohort sizes from correlating well with varying recruitment levels. Recruitment regulation alone cannot regulate a metapopulation. Two versions of the recruitment limitation hypothesis were examined. The first version states that varying recruitment levels are good predictors of subsequent population size; the second version states that postrecruitment demographic rates are density independent. The present models, along with a growing body of empirical data, support the first hypothesis but provide less support for the second one. Population regulation and the first form of recruitment limitation are not antithetical processes