densityindependent

Density-independent describes processes that affect a population's size regardless of its current abundance. In ecology, density-independent factors influence survival and reproduction without regard to how crowded the population is. These factors are often external to the organisms themselves and can produce abrupt changes in population size.

Examples of density-independent factors are primarily abiotic, such as climate and weather. Severe winters, extreme heat, droughts, floods, storms, fires, and volcanic eruptions can reduce or eliminate individuals independent of density. In some ecosystems, regular environmental cycles can also have density-independent effects if they impose periodic stress on all individuals equally.

In population models, when growth is density-independent, the per capita growth rate is constant and does not depend on population size. The resulting dynamics are typically exponential growth or decline described by dN/dt = rN. In practice, many populations experience a mix of density-independent and density-dependent factors; density-independent effects can trigger large fluctuations especially during adverse environmental conditions.

This concept is contrasted with density-dependent factors, such as competition for resources, predation, disease, and territorial behaviors, which cause per capita growth to change with population density. Understanding density-independent factors helps explain sudden crashes or booms not predicted by density-dependence alone. See also density dependence.