What do You Mean by Ecological Succession?

Ecological succession broadly means an orderly and progressive development and evolution of ecosystems in the course of time, or, in other words, how communities change with the progress of time. The development and evolu­tion of communities is a fairly directional and predictable phenomenon and results from alteration of the environment by the communities. With the changed environment, species structure and community composition also change to an extent limited by the environment.

Immature or early successional stages in ecosystem development com­monly have a much higher net productivity than late or mature successional stages. The mature ecosystem, otherwise also called the climax, is, however, highly stable and harbours maximum biomass (Odum, 1969).

The entire intervening sequence of evolving communities found in a given area is called a sere, and each of the changes is a serai stage. Each serai stage represents a temporary (evolving) community with its own characteristics.

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It may remain for a short period or for several years. Net productivity tends to be negligible in a climax community. (However, some ecologists have doubts as to whether any such thing as a climax really exists.

It is only a question of relative stability of a climax as compared to that of a seral stage; we do not know for sure that succession ends with the achievement of the climax. Even the seemingly most stable natural communities may undergo some change with time).

However, though their net productivity is very low, fully developed ecosystems are in other respects quite valuable for man’s welfare; they have: (a) a high rate of CO₂/O₂ exchange, (b) nearly closed nutrient cycles, (c) selection for quality rather than quantity of organisms, and (d) considerable ecological efficiency in terms of biomass supported per unit of energy flow.

Recent researches on the theory of ecosystem development, functioning and evolution, along with studies of species diversity have provided the beginnings of a model against which the coexistence between man and nature can be more meaningfully determined (UNESCO, 1973).

During the course of an ecosystem development, not only the species diversity and stratification increases but the diversity of organic compounds also increases. Further, some species may change the environment or habitat in such a way that it becomes unsuitable for themselves but more suitable for other species.

In general, ecosystem development is a highly complex process involving interactions among environment factors, nutrient cycles, and spe­cies. Succession involves progressive changes not only in species structure and habitat but also in energy flow. A gradual replacement of one species by another occurs until the whole community becomes replaced by somewhat more complex community. With the progress of succession, net community production (P) gradually decreases whereas community respiration (R) shows a corresponding increase, until finally equilibrium between P and R is attained.

One of the oldest approaches to succession studies involves investigation at the level of phytocoenoses. A common and reliable method is the study of permanent plots, where both vegetation science and succession science make use of typifications: vegetation units or associations and phases (Bomkamm, 1981). Both types are specified floristically.

For associations, use is made of characteristic species and of the spatial distribution; for the phases, use is mainly made of dominant species and of their distribution in time.

Some new conceptual frameworks for understanding vegetational succession have been proposed by Peet (1981). These include basing successional theory on com­parative physiology, life history characteristics, population dynamics and biogeochemical phenomena. These various methods can be effectively inte­grated by simulation modeling techniques.

One of the most effective approaches to predicting change is the use of the functional models such as stand models which incorporate the processes of disturbance, mortality, dispersal, establishment and competition.

Many examples of succession in lakes, ponds, walls, rocks and other habitats have been described in textbooks. Being in general a rather slow process, it is often not possible for any one man, during the short span of his life, to study and observe natural succession right from the beginning.

Mostly, it is the secondary succession (which occurs from a stage or an area in which other organisms already were present) is studied. On the other hand, succes­sion that occurs on areas devoid of living organisms is called primary succession. Once in a while, nature furnishes an opportunity for man to study this process of primary succession from the very initial stage.

A few years ago a completely new mass of virgin land appeared as an island, called the Surtsey Island, in the ocean off Iceland, probably as a result of volcanic or other geological upheaval. This has provided an ideal opportunity to European, British and American scientists to follow the whole process of early succes­sion starting from completely new mass of land.

A similar earlier occasion was in 1883 when the island of Krakatoa was completely sterilized of all life after a volcanic eruption. In fact, some of the most significant advances in ecology today have been coming from studies of the invasion, colonization and ecosystem development in or on isolated habitats, e.g., islands. These isolated habitats can be islands in the usual sense, or they may be completely new land masses such as Surtsey, or they can be some other “islands'” as glass slides, polyurethane, sponges, or other artificial substrata in lakes, etc.

However, exceptions such as Surtsey apart, we are living in an era of secondary vegetation. The secondary vegetation may be studied either at the level of the species or the ecosystem. Our environment is subject to frequent disturbance as a consequence of biotic and human interference and, in the disturbed environment, secondary species seem to be more successful than primary species, possibly because the secondary species possess certain adaptive features.

Secondary species often have a genetically controlled or environmentally controlled short-span life cycle. The time span of growth period for a secondary species to reach the reproductive phase can vary from a few days to a few years and is dependent on the environmental as well as inherent species characteristics.

Secondary species frequently are prolific seed producers and possess efficient mechanisms for seed dispersal and/or dormancy. The precise nutritional requirements of most secondary species are not properly understood, but they do seem to have efficient devices for growing on poor-nutrient content soils. Secondary species commonly lack mycorrhiza but possess a dense root system; the latter property enables them to absorb nutrients from the soil efficiently. Such species are also commonly highly resistant to herbivores and are able to withstand and recover from such injuries as trampling.

In tropical areas, species growth seems to occur rapidly at the beginning (i.e., first few years) of succession and is followed by some stabilization (Farnworth and Golley, 1974). It seems that a quick growing species is highly efficient photosynthetically and this may be the reason why the early succes­sional stages are highly productive.

Many ecologists have studied secondary succession on abandoned fields and reported the general orderly sequence as grasses—grasses plus forbs (weeds, etc.)—shrubs-trees-climax forest, and the following general features have emerged: (a) simple, short-lived communities give way to complex, long-lived communities with larger plants; (b) species numbers tend to increase with time, i.e., species diversity increases; (c) food-webs become more highly complex; and (d) the later stages tend to be more stable, persist longer, and periodically replace themselves.

Complex secondary successions take place in many tropical forests though such secondary successions have been very little studied. The greatest amount of disturbance has been caused by man by way of clearing, cutting and burning, especially in Africa and Asia. Indeed, it can be stated that no primary forests now exist anywhere in India except in the Andaman Islands.

Clear-cutting, grazing, and fire are the three major disturbing factors in bamboo forests. In India, grazing either alone or in combination with fire is often so severe as to exterminate the bamboo over extensive areas. Many bamboos are markedly gregarious, and some species (e.g., Dendrocalamus strictus) sometimes form the entire secondary regrowth over abandoned clearings; they tend to persist for long periods as a biotic sub climax since fires fail to kill the underground rhizomes.

Sal is likewise greatly disturbed by fire and grazing in India where it is often impossible to determine whether a given sal forest is a true climatic climax or a pre-climax to a moister evergreen forest type. The general view about the moist deciduous type of sal forests in Madhya Pradesh, however, seems to be that they represent a true climatic climax.

In Uttar Pradesh, effects similar to those caused by fire, felling, and grazing in Madhya Pradesh, are produced by deterioration of climate or sinking of the water- table. In these and other forests, one important aspect that has been neglected is the role of seed predators in the successional pattern.

In the tropical forests, disturbances generally originate by fire, or by the creation of fairly large gaps in the upper canopy; however, small gaps are sometime considered to be a normal feature of natural regeneration of the primary forest. However, one encouraging fact is that in recent years the use of fire in forest management has declined significantly in India.

The nutrient condition, texture, and type of soil exert an important influence in early successional stages, especially in seedling establishment, and this explains the success of taungya system in South-east Asia where forest exists not only on poor (ferralsols) soils, but also on such soils as acrisols, cambisols, and nitosols, which have a conspicuous agricultural capability.

Fire affects forest regeneration through burning of seeds and plants as well as through its action on soil. Organs located more than a few cm under the soil surface are, however, hardly affected by fire. Species vary in respect of their fire tolerance, and fire is often a selective factor, tending towards a simplification of the floristic composition and a complication of the succes­sional pattern. In the past, annual burning coupled with disturbances caused by grazing and shifting cultivation has led to the perpetuation of the fire- tolerant teak (Tectona grandis) and sal (Shorea robusta) in Asia.

Repeated outbreaks of fire tend to deflect the succession and lead to a grassland tire-climax. In fact, many of the so-called savanna areas are not a climatic climax but mainly the result of repeated fires.

With regard to successional changes following disturbance, we know much less about the zoological than the botanical aspects involved. Recent work of Opler et al. (1976) indicates that in the initial stages, secondary succession is often very rapid and that, after disturbance, forests may soon acquire a fair resemblance to their predistuibance status.

Their general con­clusion was that early secondary successional stages may be fairly resistant to perturbation and tend to recover rapidly. However, they have cautioned against indiscriminate extension of this conclusion to mature communities.

Succession is also attended by certain other changes, e.g., (a) the plant production is higher during early successional stages than later; this is because of a low standing crop of biomass or higher ratio of production/biomass. During later stages, this ratio declines; (b) climax communities become balanced with the result that biomass tends to remain constant and the ratio declines still further; (c) in early successional stages, a higher proportion of energy flows through herbivores whereas during later stages, the higher proportion passes through decomposers; it is interesting to note that man’s typical food-producing ecosystems, characterized by a high yield/biomass ratio, short food chains and higher simplicity, are akin to the ‘early’ succes­sional types as described above; (d) during succession, open systems tend to change to more closed systems with respect to nutrients, and climax commu­nities generally retain and efficiently recycle their nutrients. A good example of open ecosystems, which are linked by a flow of energy and materials, is freshwater streams and rivers.

Some ecologists visualize succession as a rather discontinuous process consisting of a series of discrete stages but other think that the change is gradual and continuous (Smith, 1966). It is also possible that some of the intervening stages may be deleted, telescoped, or prolonged in duration. Quite frequently, a certain seral stage becomes arrested, and prevented from being replaced by the next stage by soil conditions (forming an edaphic sub-climax) or by some other temporary climatic condition such as drought.

Margalef (1968) postulated that the structural information content of a community, calculated as species diversity (not food-web diversity) may have an energy equivalent such that the more complex community needs less maintenance energy per individual in the face of perturbation. This means that community complexity could somehow be utilizable by the system for its own stabilization.

With a view to evaluating the concept of diversity-stability in terms of successional series, Witkowski (1973) has studied weevil populations in a successional series of meadows that had sprung up as a result of drainage of wetlands in a valley in Poland. He found that species diversity of both weevils and meadow plants increased progressively with the progress of succession, but that the stability of weevil population declined with succession.

In the early progress of American ecological studies, two main models were postulated to explain vegetation dynamics. F.E. Clements (1916) thought of vegetation change as an ontogenic process whereas H.A. Gleason (1917) believed that any change in the relative abundance of species in the plant cover of an area or in its floristic composition with time was a successional change.

He felt that vegetation is constantly adapting to an ever-changing, unique and new set of physicochemical and biotic conditions. Later workers have broadly preferred Gleason’s so-called reductionist model and some have modified it to the now popular ‘individualistic’ hypothesis (see Mcintosh, 1980).

Van der Valk (1981) has adopted the Gleasonian approach in his studies of succession in certain wetlands. According to Van dei Valk, three life history characters can characterize wetland species, viz., (1) life-span, (2) propagule longevity, and (3) propagule establishment requirements.

Succes­sion occurs whenever one or more new species is established in freshwater wetland, or when some species already present are exterpated. He emphasizes the propagule longevity aspect of wetland species and recognizes two basic types of species, viz., those having long-lived propagules present in the wetland’s seed bank that can become established under proper conditions, and species having short-lived propagules present in the wetland’s seed bank that can become established under proper conditions, and species having short-lived propagules.

The wetland environment seems to exert a sieve-like action permitting the establishment of only certain species at any particular time. A study of a wetland’s seed bank can furnish vital clues to the successional status of the wetland, at least qualitatively (Van der Valk, 1981).