Ecological Strategies and Population Parameters

著者: T. R. E. Southwood, Robert M. May, M. P. Hassell, Gordon Conway - The American Naturalist 1974 被引用: 273

Habitat is the template against which evolutionary pressures fashion the ecological strategy of a species; the instability-stability habitat spectrum gives rise to the r-K-selection continuum. Habitat stability for any animal is conveniently expressed by τ/H (τ = generation time and H = the length of time the habitat remains suitable for food harvesting). In animals whose habitats have a value of τ/H approaching unity, one generation will not affect the resources available to the next. The strategy for these habitats can allow overshooting of the equilibrium. In animals with permanent habitats (τ/H very small), overshooting, with the consequent overexploitation of resources, will be selected against. The logistic equation cannot represent a situation that involves overshooting. A more realistic approach arises from the consideration of the difference equation (2) which includes a time delay between density-dependence acting and the subsequent population change. Its three basic parameters are the equilibrium population (N*), the finite growth rate of the population (λ), and the density-dependent moderator (b), which is related to the return time (Tr = 1/b) near equilibrium. Characteristics of successful r- and K-strategists are investigated using the model and conclusions summarized above. While we accept that many vertebrates may have arisen as a result of K-selection (in comparatively stable geological periods), many groups within these taxa will have their population parameters modified toward those characteristics for the type of habitat they occupy (table 1). Really successful K-strategists become precisely adapted to a very permanent (in generation terms) habitat type, they become larger in size, and, because of their extreme K-type population parameters, they lose their plasticity for selection. When their habitats change owing to major environmental variations in geological time, these species become extinct. This, we suggest, is the ecological explanation of Cope's rule.