The law of ecological succession
Ecological succession, a fundamental concept in ecology, refers to more-or-less predictable and orderly changes in the composition or structure of an ecological community. Succession may be initiated either by formation of new, unoccupied habitat (e.g., a lava flow or a severe landslide) or by some form of disturbance (e.g. fire, severe windthrow, logging) of an existing community. In general, communities in early succession will be dominated by fast-growing, well-dispersed species. As succession proceeds, these species will tend to be replaced by more competitive species. The law is used in urban ecology denoting the replacement of one dominant group by another, following the invasion of the territory of the later by the former.
The concept of ecological succession can be traced back to the work by Frederic Clements (1916). According to Clements, an ecological community advanced from its embryonic state of development gradually progressed through a series of stages to reach maturity – such that the climax state was the most stable and supported the greatest diversity of species. Thus, as community matured it progressively established its environment. According to Clement, succession is a process involving several phases:
1. Nudation: Succession begins with the development of a bare site, called Nudation (disturbance).
2. Migration: It refers to arrival of propagules.
3. Ecesis: It involves establishment and initial growth of vegetation.
4. Competition: As vegetation became well established, grew, and spread, various species began to compete for space, light and nutrients. This phase is called competition.
5. Reaction: During this phase autogenic changes affect the habitat resulting in replacement of one plant community by another.
6. Stabilization: Reaction phase leads to development of a climax community.
Ecological succession was formerly seen as having a stable end-stage called the climax,sometimes referred to as the 'potential vegetation' of a site, shaped primarily by the local climate. This idea has been largely abandoned by modern ecologists in favor of nonequilibrium ideas of how ecosystems function. Most natural ecosystems experience disturbance at a rate that makes a "climax" community unattainable. Climate change often occurs at a rate and frequency sufficient to prevent arrival at a climax state. Additions to available species pools through range expansions and introductions can also continually reshape communities.
A contrasting view, Gleason's ideas, first published in the early 20th century, were more consistent with the formal concept by Henry C.Cowles (1899). The Gleasonian framework is more complex, with three items: invoking interactions between the physical environment, population-level interactions between species, and disturbance regimes, in determining the composition and spatial distribution of species. It differs most fundamentally from the Clementsian view in suggesting a much greater role of chance factors and in denying the existence of coherent, sharply bounded community types.
Date: 2015-12-24; view: 499