Density-dependent factors regulate population size

Limited amounts of factors such as food or habitat can influence population size in a density-dependent manner, which means that they cause birth rates, death rates, or dispersal rates to change as the density of the population changes (see Figure 11.7B).

When one or more density-dependent factors cause population size to increase when numbers are low and decrease when numbers are high, population regulation is said to occur. Ultimately, when the density of any species becomes high enough, density-dependent factors decrease population size because food, space, or other essential resources are in short supply. Note that “regulation” has a particular meaning here, referring to the effects of factors that tend to increase λ or r when the population size is small and decrease λ or r when the population size is large. Density-independent factors can have large effects on population size, but they do not regulate population size because they do not consistently increase population size when it is small and decrease population size when it is large. Thus, by definition, only density-dependent factors can regulate population size.