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Population Ecology

Author(s): Evan Siemann, PhD

Density-dependent and Density-independent Effects on Populations

Density-independent factors are events and influences that affect the growth of a population, independent of the population's size. Often, these are environmental factors, such as extreme cold, drought, tornados, or volcanic eruptions. For many organisms, harsh abiotic conditions keep populations far from carrying capacity much of the time. Such species often show little density-dependent regulation.

Density-dependent effects of specialist herbivores and diseases are thought to be important in promoting the high diversity of tropical rain forests. Seedlings growing in the vicinity of their parents experience higher losses than those growing distant from conspecifics. In general, plants with a diverse range of other plants growing in their vicinity experience lower herbivory and disease than those growing in a monoculture. If plants are sown at high densities, the number of plants surviving decreases as they grow in stature (self-thinning).

Animals also show density-dependent regulation. As density increases, mortality decreases from limited food, higher disease frequencies, and other factors. Animals in crowded populations are less likely to breed, and their success in food-limited, crowded conditions is lower than in less crowded populations.