density-independent factor

density-independent factor, in ecology, any force that affects the size of a population of living things regardless of the density of the population (the number of individuals per unit area). Density-independent factors often arise from physical and chemical (rather than biological) phenomena.

Such factors stemming from weather and climate—as well as flooding, wildfires, landslides, and other disasters—affect a population of living things whether individuals are clustered close together or spaced far apart. For example, for most organisms that breathe oxygen, oxygen availability is a density-independent factor; if oxygen concentrations decline or breathable oxygen is suddenly made unavailable, such as when oxygen-using plants are covered by rising floodwaters, those organisms perish and populations of the various affected plant species decline.

The dynamics of most populations of living things are influenced by a combination of density-independent factors and density-dependent factors (that is, those factors that emerge when the concentrations of individuals in a population rise above a certain level). The relative importance of these factors varies among species and populations.