Environmental factors can affect population growth differently depending on how their effects change with population density.

A density-dependent factor has a stronger per-capita effect when population density is higher. Common examples include competition for food or nesting sites, transmission of contagious disease, accumulation of waste and some forms of predation.

For example, if a fixed food supply supports 1,000 herbivores comfortably but 3,000 herbivores must compete intensely for it, mortality and reduced reproduction can increase with density. The result counteracts further population growth as crowding increases.

A density-independent factor affects populations without requiring crowding to intensify its effect. A severe freeze, hurricane, wildfire or toxic spill can kill organisms whether the population was initially sparse or dense.

The distinction concerns how effect size depends on density, not whether the factor is biological or physical. A disease can be density dependent; a physical disturbance can be density independent; and real events can combine both kinds of effects.

Density dependence provides a mechanistic interpretation of the logistic model's declining per-capita growth rate. But real populations need not approach carrying capacity smoothly: time delays, seasonal environments, predators, disease and disturbances can produce oscillations or abrupt changes.

Thus population regulation is best understood by connecting demographic rates to mechanisms rather than assuming that every population is automatically held at a fixed equilibrium.