Practice question

If density simultaneously lowers the per-capita birth rate and raises the per-capita death rate, both responses oppose further population increase. Their combined effect narrows the difference between births and deaths until net growth can reach zero. This is density-dependent regulation, the demographic feedback underlying a stable equilibrium such as carrying capacity in the logistic model. Demographic mechanisms are linked by the balance dN/dt = births + immigration - deaths - emigration. Density-dependent changes in any of these terms can alter net growth. To infer regulation, ecologists compare per-capita rates across densities and distinguish causal feedback from coincidental correlations produced by weather, age structure, or habitat quality. This causal chain is what makes the keyed content ecologically meaningful rather than merely definitional. This interpretation connects individual-level processes with measurable changes in survival, reproduction, recruitment, or abundance across the population. Field evidence should therefore be compared with the model assumptions before extending the conclusion to every species, habitat, or time period.