ECOLOGICAL SUCCESSION & EQUILIBRIUM

 ECOLOGICAL SUCCESSION & EQUILIBRIUM

Ecological succession is the term that expresses how a community changes over time in regards to the composition and dominant species found in that community. Equilibrium in ecology refers to a state that occurs such that a small disturbance or change is counter balanced by another change so that the community is restored to its original state. Thus, as a community goes through multiple changes through each stage of succession, it is not in equilibrium. New species enter the community and take the place of others and the community does not return to the way it was formerly.

When a community eventually reaches the last stage of succession, what is typically referred to as a climax community, the composition of the community is stable. Thus, any small disturbances will be counterbalanced. The system is at a relative equilibrium.

Meaning of Climax Community: Climax community is the stable end product of successional sequence or sere. It is a community that has reached a steady state of species composition, structure and energy flow, under a particular set of environmental conditions. Steady state indicates the dynamic nature of the climax.

Also the end of successional change does not mean that community development has come to an end. As has been stated above, climax community is always in a state of flux and its structure undergoes changes due to birth, death and growth processes. However, these changes are less dramatic than the community transformations observed during succession.

The characteristics of a climax community are:

1.  The climax community is able to tolerate its own reaction.

2.  It tends to be mesic (medium moisture content) rather than xeric (dry) or hydric (wet).

3.  The climax community is more highly organised.

4.  The climax community with its more complex organisation has large number of species and more niches.

5.  The organisms of earlier successional stages tend to be smaller, shorter lived with a higher biotic potential (r-selected). In contrast, the species of climax community tend to be relatively large, long lived and with a low biotic potential (K-selected).

6.  In climax community, energy is at a steady state (net primary production is zero), whereas, in immature stage of succession, gross primary production tends to be greater than community respiration, signifying accumulation of energy.

7.  Immature ecosystems are temporary while in climax community the stability is high.

8.  Climax communities show less broader changes and are more resistant to invasions than immature ecosystems.

Nature of Climax Community:

A. Mono-climax and poly-climax:

According to Clements (1916) succession resulted in a single true climax community, determined primarily by the climate of the region. This view of his is called the mono-climax theory of succession, which elaborates that the many different vegetation communities found in a region are successional stages of the true climax community.

These different vegetation communities were often called subclimax, preclimax or post-climax communities. This theory further stresses that, given sufficient time, the difference in local conditions of soil moisture, temperature, nutrient availability, hydrology and so on (that give rise to different vegetation types) would be over comed and a homogeneous true climax would develop.

Many observations seem to conflict this hypothesis as it is evident that even under primeval conditions it was difficult to find large areas of uniform vegetation. Rather, it is appropriate to recognise several different communities as climax.

Poly-climax theory of succession; stresses that many different types of vegetation form the climax community, depending on local conditions. The climax community should be in harmony with the whole environment and not just climate. However, the hypothesis of poly-climax is also basically terminological.

B. Climax pattern theory:

More recently a third hypothesis was proposed by Robert H. Whittaker (1953) known as climax pattern theory, which rejects the classification approach. It recognises a regional pattern of open climax communities whose composition at any particular locality depends on the specific environmental conditions present at that time.  The climax pattern concept, in a sense, views only one big community that changes according to soil, slope and other habitat factors. This approach is considered to be more useful and closer to reality to describe such pattern of variation.

Factors Determining the Nature of Climax Community:

Many factors such as soil nutrients, moisture, slope, exposure etc. determine the nature of the climax community. Fire is another important feature of many climax communities. Fire-resisting species are favoured while other species that would have dominated are excluded. Fire triggers the release of seeds in some pine species. After the fire has receded the pine seedlings grow rapidly in the absence of competitors. Grazing pressure is another factor that determines the nature of climax community. Intense grazing may turn grassland into shrub-land. Shrubs and cacti may establish themselves as they are unsuitable for forage. The grazing of many herbivores may suppress many species of plants and favouring competitors that are less desirable as food.