Practice question

When the per-capita risk of death rises as population density increases, mortality supplies negative feedback. Crowding may increase infection, attract predators, intensify fighting, or deplete food, causing population growth to slow. This is density-dependent regulation because the strength of the demographic effect depends on how many individuals occupy the available habitat. 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.