Density Dependence and Independence

Density dependence occurs when the population growth rate, or constituent gain rates (e.g. birth and immigration) or loss rates (death and emigration), vary causally with population size or density (N). When these parameters do not vary with N, they are density-independent. Direct density dependence, where the population growth rate or gain rates vary as a negative function of N, or the loss rates vary as a positive function of N, is necessary but not always sufficient for population regulation. The opposite patterns, inverse density dependence or the Allee effect, may push endangered populations towards extinction. Direct density dependence is caused by competition, and at times, predation. It is detected observationally by analysis of abundance time-series; however, experimental field manipulations provide the most rigorous analytical methods for both detecting and understanding underlying mechanisms. Future directions include expanded and more detailed mechanistic field data integrated with more sophisticated population models. Key concepts: Direct density dependence occurs when the population growth rate varies as a causative inverse function of population size or density. Inverse density dependence – the Allee effect – may occur in a small population, driving it extinct. Direct density dependence is necessary but not always sufficient for a population to be regulated and may occur simultaneously with density independence. The ecological mechanisms of density dependence are competition, and in some circumstances, predation (including parasitism and disease). Density dependence is most commonly tested by examining the relationship between the population growth rate (or individual demographic rates) and population density, either observationally or experimentally. Understanding density dependence and regulation within the context of a metapopulation – a population of local populations linked by occasional dispersal – faces major empirical challenges. Recommended future research on density dependence includes detailed, mechanistic field observations and experiments, as well as modelling of complex dynamics in complex systems, such as metapopulations. Keywords: population ecology; demographic rates; Allee effect; population regulation; metapopulation

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