The Ecological and Evolutionary Implications of Recruitment for Competitively Structured Communities

The role of recruitment from nonbreeding juvenile or subadult to the breeding adult population in affecting community dynamics is not well understood. Yet, the recruitment process may be extremely important, especially for space-limited communities. Here, we are specifically concerned with how the interspecific interactions involved in recruitment of nonbreeding floaters to breeding adults will influence the ecological stability of the populations' densities and the longer-term persistence stability of the competing species. The conclusions we draw are only applicable to communities for which there are ecologically stable equilibria. We formulate and compare three deterministic models that differ only in the way nonbreeding floaters and breeding adults of each species influence recruitment of floaters into the breeding population. The simplest of these models is the Density Independent Recruitment (DIR) model, in which floaters are recruited into the breeding subpopulations of each species at a constant rate. In the Adult Dependent Recruitment (ADR) model, recruitment of floaters is dependent on the density of adults of both species. When adult density is high (near carrying capacity) recruitment is reduced. Finally, in the Community Dependent Recruitment (CDR) model, all components (floaters and adults of both species) of the community influence recruitment. The CDR model incorporates the effect of adults in the same way as the ADR model. Additionally, recruitment of floaters also depends on the relative frequencies of floaters of each species in the total floater subpopulation. If the pool of available floaters is predominantly of one species, that species enjoys a higher rate of recruitment than its competitor. We used mathematical analyses to determine the time to return to equilibrium after a perturbation and to define the relative sizes of parameter sets that allow stable coexistence. Our findings suggest that if recruitment is dependent on the relative frequencies of competing floaters (CDR model), the community is more likely to exhibit density variation over ecological time relative to communities without such frequency dependence (DIR and ADR models). This frequency dependence also severely restricts the range of demographic parameters compatible with coexistence, thus making coexistence over evolutionary time less likely.