Depensation

Depensation is a term used in population dynamics to describe a reduction in a population’s per-capita fitness or recruitment as population density declines. It is the low-density side of the broader Allee effect, and is often contrasted with compensatory density dependence, where individuals experience relatively higher fitness or recruitment as density increases from very low levels.

Mechanisms contributing to depensation include mate limitation (fewer potential mates at low density), breakdown of social structures or cooperative behaviors required for foraging or defense, reduced pollination or cooperative breeding, and genetic problems associated with small populations. Habitat fragmentation and reduced gene flow can exacerbate these effects by lowering effective population size and connectivity.

Consequences of depensation can be significant for conservation and management. Because growth may slow or reverse at low densities, populations can exhibit threshold dynamics: if numbers fall below a critical level, negative growth can persist even under favorable environmental conditions, increasing the risk of extinction. Conversely, recovery often requires interventions to push the population above the threshold, such as habitat restoration, supplementation, or translocations.

In practice, depensation is incorporated into population models through Allee terms that produce reduced growth at low densities and may create multiple equilibria. It is relevant to wildlife and fisheries management, conservation planning, and ecological theory, highlighting the importance of maintaining populations above critical densities to ensure viability.