Variation in the presence and cause of density-dependent mortality in three species of reef fishes

Determining the mechanisms by which natural populations are regulated is a key issue in ecology. Identifying the biological causes of density dependen ce has, however, proved difficult in many systems. In this study we tested whether adults of three species of reef fish (all gobies) suffered density- dependent mortality, and whether the density-dependent component of mortali ty was caused by predation. We used field experiments to test for density d ependence in each prey species, manipulating the presence of predators and prey density in a factorial design. Prey were stocked on replicate patches of reef constructed of natural materials, with each reef receiving a differ ent density of gobies. Predatory fishes were excluded from half of the reef s using a combination of removals and exclusion cages. Survival of the firs t species, Lythrypnus dalli, was high and density independent on reefs free of predators, but declined rapidly with increasing density on reefs to whi ch predators had access. Density dependence in L. dalli was thus a result o f mortality inflicted by predatory fishes. In the second species, Coryphopt erus nicholsii, predators caused a large reduction in the survival in one e xperiment but had a negligible effect in a second experiment. More importan tly, though, survival of C. nicholsii was always independent of its density , regardless of predator presence. In the final species. Coryphopterus glau cofraenum, two separate experiments showed that natural changes in adult ab undance (experiment 1) and survival of stocked adults (experiment 2) were d ensity-dependent regardless of predator presence. Both experiments thus ind icated that the density-dependent component of loss in C. glaucofraenum was caused, at least in part, by an interaction other than predation. The pres ence, intensity, and biological cause of density-dependent survival were th us strikingly different for each of these three fishes, despite the fact th at they are taxonomically closely related and ecologically similar. These f indings suggest a need for further studies aimed at predicting under what c ircumstances different mechanisms of population regulation will operate.