Density Independent Mortality

Density-independent mortality is defined as a population-limiting process that is often caused by environmental stochasticity and not influenced by the density of the population. The most dramatic form of density-independent mortality occurs from natural catastrophes such as hurricanes, floods, or wild fires. Less dramatic forms of environmental stochasticity occur from seasonal variation in weather patterns, habitat quality, and fluctuating generalist predators. An example of density-independent mortality on a population was observed in the initial decline of the heath hen (Tympanuchus cupido cupido). This medium-sized chicken-like bird was once fairly common from New England to Virginia, USA, but declined with European settlement and was isolated to Martha's Vineyard, an island off Massachusetts, by 1876. In 1916, approximately 800 birds still existed. However, between 1916 and 1917 a fire ravaged the island (natural catastrophe) and a northern goshawk (Accipiter gentiles) invasion that greatly increased predation (environmental stochasticity), caused the population to decline to approximately 100 birds. The population eventual went extinct in 1932.

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