Succession

As noted previously, succession generally begins after disturbance creates a situation of great resource availability that can be exploited by organisms, but under conditions of little competition. The classical explanation of the ecological mechanism of community change during succession is the so-called facilitation model. This theory suggests that the recently disturbed situation is first exploited by certain pioneer species that are most capable of reaching and establishing on the site. These initial species modify the site, making it more suitable for invasion by other species, for example, by carrying out the earliest stages of soil development. Once established, the later-successional species eliminate the pioneers through competition. This ecological dynamic proceeds through a progression of stages in which earlier species are eliminated by later species, until the climax stage is reached, and there is no longer any net change in the community.

Another proposed mechanism of succession is the tolerance model. This concept suggests that all species in the succession are capable of establishing on a newly disturbed site, although with varying successes in terms of the rapid attainment of a large population size and biomass. In contrast with predictions of the facilitation model, the early occupants of the site do not change environmental conditions in ways that favor the subsequent invasion of later-successional species. Rather, with increasing time, the various species sort themselves out through their differing tolerances of the successionally increasing intensity of biological stresses associated with competition. In the tolerance model, competition-intolerant species are relatively successful early in succession when site conditions are characterized by a free availability of resources. However, these species are eliminated later on because they are not as competitive as later species, which eventually develop a climax community.

A third suggested mechanism of succession is the inhibition model. As with the tolerance model, both earlyand later-successional species can establish populations soon after disturbance. However, some early species make the site less suitable for the development of other species. For example, some plants are known to secrete toxic biochemicals into soil (these are called allelochemicals), which inhibit the establishment and growth of other species. Eventually, however, the inhibitory species die, and this creates opportunities that later-successional species can exploit. These gradual changes eventually culminate in development of the climax community.

All three of these models, facilitation, tolerance, and inhibition, can be supported by selected evidence from the many ecological studies that have been made of succession (especially plant succession). Although these models differ significantly in their predictions about the organizing principles of successional dynamics, it appears that none of them are correct all of the time. Facilitation seems to be most appropriate in explaining changes in many primary successions, but less so for secondary successions, when early post-disturbance site conditions are suitable for the vigorous growth of most species. The relatively vigorous development of ecological communities during secondary succession means that competition rapidly becomes an organizing force in the community, so there is an intensification of interactions by which organisms interfere with and inhibit each other. Aspects of these interactions are more readily explained by the tolerance and inhibition models. Overall, it appears that successions are idiosyncratic—the importance of the several, potential mechanisms of succession varies depending on environmental conditions, the particular species that are interacting, and the influence of haphazard events, such as which species arrived first, and in what numbers.