life11e_ch54

Factors limiting population growth can be density-dependent or density-independent

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As you just saw, the density of a population can have large effects on population growth and ultimately the size of a population through time. Density-dependent factors can include:

Limiting resources. As a population increases, it depletes its resources as a result of intraspecific competition.
Predators may be attracted to areas with high densities of their prey, allowing them to capture a larger proportion of individuals and causing the death rate of the prey population to rise.
Pathogens may spread more easily in dense populations than in populations with fewer individuals, resulting in a rise in the death rate.

It is important to note that density does not always cause a decline in population growth. Some populations grow better, albeit up to some limit, when population densities are higher rather than lower; this is known as the Allee effect (named after the ecologist W. C. Allee, who first described it). Sometimes individuals survive better in a group than on their own, as you saw with the predator avoidance strategies used by wood-pigeons and ground squirrels living within groups (see Figure 52.21).

Moreover, not all factors that change population size act in a density-dependent manner. A period of extreme cold, or an exceptionally strong hurricane, may kill a large number of individuals in a population regardless of the population’s density; such an event is density-independent and may be a significant factor in depressing growth and keeping populations from reaching their carrying capacity.