Practice question

The kelp-perch example links density-dependent mortality to a finite number of refuges. As fish density rises, safe crevices or shelter sites become limiting, leaving more individuals exposed to predators. Mortality therefore increases disproportionately with abundance. The important mechanism is not simply predation, but the interaction between predation risk and competition for protective space. Density dependence is identified by a change in a per-capita demographic rate as abundance changes. Negative density dependence restrains growth and can regulate abundance; positive density dependence can make sparse populations vulnerable. A factor can affect population size without regulating it if its impact does not create a restoring response to density. A useful check is to ask what happens at the biological extremes and whether the proposed mechanism still makes sense. Interpreting the example at the appropriate population scale keeps the causal mechanism distinct from a simple correlation or an absolute rule. Ecological predictions remain conditional on the stated environment, because changing resources, mortality, or interactions can alter the observed demographic pattern.