Density dependence
Density dependence means that an individual's birth, death, survival or growth rate depends on population density.
Negative density dependence
As density increases, competition for food, space or other resources may reduce population performance. Logistic growth contains negative density dependence through
\[1-\frac{N}{K}.\]The per-capita growth rate is
\[\frac{1}{N}\frac{dN}{dt}=r\left(1-\frac{N}{K}\right),\]which decreases as \(N\) increases.
Positive density dependence
At low densities, population performance may instead improve as density increases. This is associated with Allee effects.
Why density dependence matters
Density dependence creates feedback between population size and demographic rates. This feedback can stabilise populations, create thresholds or alter the response to harvesting and environmental change.
Key idea. Density dependence is a mechanism, not a single equation. Different models represent different biological ways in which population density changes demographic rates.