Climax (ecological)

Referring to a community of plants and animals that is relatively stable in its species composition and biomass , ecological climax is the apparent termination of directional succession—the replacement of one community by another. That the termination is only apparent means that the climax may be altered by periodic disturbances such as drought or stochastic disturbances such as volcanic eruptions. It may also change extremely slowly owing to the gradual immigration and emigration—at widely differing rates—of individual species, for instance following the retreat of ice sheets during the postglacial period. Often the climax is a shifting mosaic of different stages of succession in a more or less steady state overall, as in many climax communities that are subject to frequent fires. Species that occur in climax communities are mostly good competitors and tolerant of the effects (e.g., shade, root competition ) of the species around them, in contrast to the opportunistic colonists of early successional communities. The latter are often particularly adapted for wide dispersal and abundant reproduction, leading to success in newly opened habitats where competition is not severe.

In a climax community, productivity is in approximate balance with decomposition . Biogeochemical cycling of inorganic nutrients is also in balance, so that the stock of nitrogen , phosphorus , calcium, etc., is in a more or less steady state.

Frederic E. Clements was the person largely responsible in the early twentieth century for developing the theory of the climax community. Clements regarded climate as the predominant determining factor, though he did recognize that other factors—for instance, fire—could prevent the establishment of the theoretical "climatic climax." Later ecologists placed more stress on interactions among several determining factors, including climate, soil parent material, topography , fire, and the flora and fauna able to colonize a given site.

[Eville Gorham Ph.D. ]

RESOURCES

BOOKS

Hagen, J. B. An Entangled Bank: The Origins of Ecosystem Ecology. New Brunswick, NJ: Rutgers University Press, 1992.