The series of main events involved in ecological succession of the autotrophic type can be broadly summarized as follows:
1. Species Structure:
Species composition changes at first rapidly and then more gradually. The number of species of autotrophs increases in primary and sometime early in secondary succession but it may decrease in older stages. The number of heterotrophic species continues to increase until fairly late in the sere. Species diversity increases initially, then becomes constant, and finally may or may not decline in older stages.
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2. Organic Structure:
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Both nonliving organic matter and total biomass increase with the progress of succession from early stages to climax. Pigment diversity also shows an increase at least in the sere. Chlorophylls increase during early phase of primary succession but little or no increase occurs during secondary succession.
3. Community Metabolism (Energy Flow):
Food-webs become more complex and both gross and net primary production increases during early phases of primary succession. Community respiration increases and net community production declines. The total information in the community increases; this means that the number of possible interactions between species, individuals, and materials increased during succession.
It is known that any qualitative change in the composition of biotic communities is generally attended by a change in the number of species (see Ricklefs, 1973). There are various ways in which increased numbers of species may be accommodated within a single community. It illustrates how resource utilization along a continuum or gradient can be changed in such a way as to accommodate more species.
Any increase in productivity of the habitat, with the resource variety remaining constant, often goes hand in hand with the ability of individuals to utilize a particular section of the resources spectrum; in this process the resource utilization curves of each species become raised, as compared to (represents the distribution of several species along a single resource continuum).
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On the other hand, if the resource variety increases but the productivity (within a narrow segment) remains constant, then additional species could be rather readily accommodated within the resource continuum provided, of course, that the total productivity of the entire habitat is proportionately enhanced to account for the newly added species.
If there is no such overall increase in the community productivity then any addition of new species would be attended by a reduction in the productivity of the preexisting species either through greater resource overlap or competition, or by specialization to narrower segments of the resource spectrum.
No individual can possibly maintain its biological activity equally well under different kinds of environments. Even within a habitable environment an individual is adapted to only a small range of conditions. Any change in a suitable environment elicits equilibrium-seeking (homoeostatic) responses from the inhabitants. It illustrates the relationship between a gradient of environmental conditions and biological activity.
The highest biological activity occurs at some optimum point along the gradient and it progressively declines on either side of the optimum. The level of biological activity required to maintain an individual organism is generally lower than that required to maintain a whole population. An individual can maintain itself more or less indefinitely within the range (b-b) of the environmental gradient. It is unable to live indefinitely within the range (b-c).
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It is generally agreed that both stabilizing and nonstabilizing influences are capable of affecting the growth rates of natural populations. Some of these effects e.g., those of stabilizing factors such as death from starvation or predators, depend on the population density.
In dense populations, competition for food, water, space and other resources and for opportunity of escape from predators is often very acute. Some ecologists (see Ricklefs, 1973) are of the opinion that the size of a stable population may be maintained in equilibrium by the balancing effect of certain influences that oppose population growth.
In a properly regulated population at low density, the birth rate commonly exceeds death rate. On the other hand, when population density is high, the death rate is higher than the birth rate. The birth and death rates are equal at some intermediate density K at which density the population can often maintain itself fairly indefinitely.
4. Herbivores and Plant Succession:
Does herbivore feeding alter the rate of addition or deletion of plant species from communities during secondary succession? Vertebrate herbivores in particular tend to produce a notable impact on plant secondary succession, though invertebrates also have some effect. Cattle and livestock arrest grasslands at a sub climax stage, preventing their progression to forests.
Watt (1981) made a critical study of the development of plant communities in rabbit exclosures on sandy soils with a view to following vegetation changes under release from grazing. During over 35 years following fencing, over 20 new plant species were added to the original 11 on a 6 x 6 m plot, and Festuca ovina, Hieracium pilosella and Thymus drucei became dominants.
These new colonizers were known to be palatable and sensitive to rabbit grazing. Watt stated, “protection from grazing leads to the development of a richer flora, a more continuous cover of vegetation, the provision of greater protection from forest-heaving and erosion, and is likely to lead to the stabilization of the soil surface and the development of a soil profile” (Watt, 1981).
Lepidopteran insects by defoliating dominant trees affect succession in forests by altering the conditions experienced by the suppressed trees of the understorey. Adams (1975) has reviewed the impact of sheep and cattle grazing on the growth and death of trees in forest succession.