Practice question

A strong Allee effect creates a critical abundance or density at which population growth changes sign. Above the threshold, births can exceed deaths and recovery is possible; below it, mate limitation, inbreeding, or failure of cooperation makes growth negative and drives further decline. The threshold is therefore an unstable equilibrium with direct importance for conservation and biological invasions. 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. The decisive distinction is therefore between a descriptive label and the demographic mechanism that generates it. 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.