Populations are affected by a variety of intrinsic and extrinsic factors. Intrinsic factors include intraspecific competition, cannibalism, territoriality, etc. Extrinsic factors include abiotic conditions and other species. Populations showing wide amplitude of fluctuation may have weak intrinsic ability to regulate population growth (e.g., through depressed natality in response to crowding). Rather, such populations may be regulated by available food supply, predation, or other extrinsic factors. These factors can influence population size in two primary ways. If the proportion of organisms affected by a factor is constant for any population density, or the effect of the factor does not depend on population density, the factor is considered to have a density-independent effect. Conversely, if the proportion of organisms affected varies with density, or the effect of the factor depends on population density, then the factor is considered to have a density-dependent effect (Begon and Mortimer 1981, Berryman 1981, L. Clark et al. 1967, Price 1997).
The distinction between density independence and density dependence is often confused for various reasons. First, many factors may act in both density-independent and density-dependent manners, depending on circumstances. For example, climatic factors or disturbances often are thought to affect populations in a density-independent manner because the same proportion of exposed individuals usually is affected at any population density. However, if shelter from unfavorable conditions is limited, the proportion of individuals exposed (and, therefore, the effect of the climatic factor or disturbance) may be related to population density. Furthermore, a particular factor may have a density-independent effect over one range of population densities and a density-dependent effect over another range of densities. A plant defense may have a density-independent effect until herbivore densities reach a level that triggers induced defenses. Generally, population size is modified by abiotic factors, such as climate and disturbance, but maintained near an equilibrium level by density-dependent biotic factors.
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