Populations that are efficient enough to withstand competition and that have a sufficiently broad niche to insure their survival can be found in the community at any point in the succession, where as those without these qualities either become locally extinct or never enter the community in the first place. The progressive alteration of the community comes about because the array of niches in the ecosystem changes with the maturation of the system.
As a general rule, the environment that presents itself to a pioneer community is a harsh place. Pioneer species must be generalists with wide niches, able to withstand fluctuations of biotic factors unmitigated by intracomrnunity forces. Consequently, early stages of succession are characterized by relatively few species, biomass, and dependence on a biotic source of nutrients.
The ratio between gross primary production and biomass is high and of community production is greater than respiration, resulting in increase of biomass. This ratio is a measure of the amount of photosynthetic energy fixed by the community that is needed to support a given level of biomass, and is thus a measure of the inefficiency of energy transfer.
Its high level during pioneer stages in succession indicates either that it takes a great deal of energy to support a given amount of biomass, or that generalist pioneer populations are not very efficient in their disposition of energy throughout the community.
Diversity of the pioneer community is low, so that food webs poorly developed; food chains are short, linear and largely grazing and interspecific interactions are minimal. Ecosystems in easily succession stages tend to be fairly open with respect to nutrients, that is, the organic phases of nutrient cycles are poorly developed, and nutrient movement into or out of the system may proceed readily. Thus, the pioneer community is not a highly organised and stable entity.
If there were no alteration in the characteristics of the ecosystem, the pioneer community would last indefinitely. For example, Tilly (1968) found that the community of a cold spring supported largely by detritus was young or immature yet was a stable ecosystem in the sense that it was not subject to rapid change.
It was maintained in that condition by restricted space, a heavy flow of water and a low but constant temperature. In such physically controlled environments succession may never occur. However, in most biologically regulated ecosystems, as succession progresses, the ecosystem becomes less harsh, either because the environment is moderated by the organisms themselves or because materials that act to moderate the environment are added from outside.
The former involves phenomena such as the weathering of rocks or stabilization of shifting sand by plants or the addition of organic material to the soil; the latter might involve the importation of material such as nutrients or organic detritus from an adjacent ecosystem.
The mature stages in succession are characterized by a greater diversity of species, great physical stratification, complex food webs, high biomass and a nutrient source largely organic in nature, high net production and a low ratio between gross production and biomass and by gross community production that about equals respiration.
In these advanced stages of succession energy is channelled down many diverse pathways and shared by many units. Food chains are complex and largely detrial. Inorganic nutrients accumulate in soil and vegetation.
The soil, too reflects the progressive development of the ecosytem, exhibiting increasing depth, increasing organic content, and increasing differentiation of horizons as the mature soils of the final community are approached. Finally mature systems or climaxes are more stable and their population of long-lived species resistent to external disturbances.
Connell and Orias (1964) have proposed a useful model to show the mechanisms of community change during succession (Fig. 143). Succession proceeds through a classic positive feedback circuit, as changes in the total environment which make it more habitable lead to the successful introduction of increasingly specialized populations which further stabilize the ecosystem and allow the establishment of even more specialized forms.
At the same time, generalist species which were present during the earlier stages of the succession are forced out, as they become competitively inferior to the more specialized species of the later stages.
The climax is reached when the ecosystem reaches its maximum stability and has been modified as far as possible. Additional species at this point tend to stabilize it further, and are likely to be overspecialized forms which cannot remain in the community.
Thus, the negative feedback loop through which superfluous populations are lost to the community becomes more important than the positive feedback loop that drives the progression, and succession change ceases.
Recently, certain serious anomalies have been recorded which contradict these t-ends of succession. Diversity of species does not necessarily increase with advancing succession changes.
For example, in some old fields, the very early stages may have greater diversity than the later stages (Tramer, 1975). Some of the later stages may be dominated by plants with strong allopathic chemical or other method of effective interference that reduces species diversity (Bazzaz, 1975).
The net productivity of the community does not necessarily increase through succession. A recent study suggests that the average net productivity decreases as relative dominance declines and diversity increases. Further, vegetation types do not necessarily follow one another sequentially. Several stages may be skipped, telescoped or extended. River bottom farmland, for example, may grow up quickly in yellow poplar and other forest trees, missing earlier successional stages completely.