Climax Community
The term "climax community," or "climax," is used by ecologists to describe a stable ecosystem that occurs at the end of a successional sequence. (Succession is the process of community recovery that occurs after an event of disturbance.) The climax is a theoretical ecosystem that is in an equilibrium with the environmental conditions occurring in the region. Terrestrial plant ecologists are particularly interested in the idea of climax ecosystems, but the theory is also relevant to animal and microbial communities, and to all environments.
Frederic Clements, an American ecologist, was influential in the initial development of the concept of climax ecosystems. Clements' most influential paper on this subject, published in 1916, suggested that there was only one true climax ecosystem for any given climatic region. He referred to this as the "climatic climax," and theorised that it was the eventual end-point of all successional sequences. Clements believed this was the case regardless of whether the succession had started after fire, timber harvesting, or other kinds of disturbances, or from a pond or lake gradually filling in with sediment and organic debris, and even regardless of soil type. In eastern North America, for example, all areas climatically suitable to supporting forest would eventually become dominated by climax stands of old-growth sugar maple (Acer saccharum), beech (Fagus grandifolia), eastern hemlock (Tsuga canadensis), and other tolerant species (this means that they can reproduce themselves under deeply shaded, highly competitive conditions, and therefore eventually dominate stands).
The so-called "monoclimax" theory of Clements was criticised as being too simple by other ecologists. The British ecologist A.G. Tansley proposed a more realistic, "polyclimax" theory that accommodated the well-known influences of local soil type, to pography, and disturbance history on community development during succession. Therefore, on drier sites in eastern North America, climax stands might be dominated by red oak (Quercus rubra) and other trees that are tolerant of droughty conditions, and wet sites by silver maple (Acer saccharinum) and other species tolerant of flooding. Climax communities of sugar maple, beech, and eastern hemlock would only occur on intermediate, well-drained, moist sites (these are known as mesic conditions).
In the early 1950s, the American ecologist R.H. Whittaker suggested that landscapes actually support gradually varying climax communities, which are associated with continuous gradients of environmental change (as occurs, for example, with increasing altitude up a mountain). Therefore, according to Whittaker, ecological communities vary continuously, even in the climax (or old-growth) condition. Therefore, climax communities cannot be objectively divided into only one or several discrete types. This view of continuously varying community change is the one that most ecologists hold today.
In a practical sense, it is not possible for ecologists to identify the occurrence of an absolute climax ecosystem. However, the climax condition is suggested when there are relatively slow changes in the structure and function of old-growth ecosystems, compared with earlier, more dynamic stages of succession. Moreover, the old-growth ecosystem would be dominated by large, old individuals of the most competition-tolerant species occurring in the region (assuming they are tolerant of the local sites conditions). However, all ecosystems change over time, even in the climax (or old-growth) condition ( therefore, the climax state cannot be regarded as being static in community composition. For example, even in old-growth forest, microsuccession is always occurring within stands, usually associated with the deaths of individual trees or small groups of trees.
Moreover, it is common over many large regions that the frequency of stand-level disturbance events is rather short. For example, catastrophic wildfires, windstorms, or insect epidemics might occur every few decades or so. Under such conditions, an old-growth or climax community does not have an opportunity to develop before another disturbance initiates another succession.
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