No plant or animal lives as isolated individual. Plants and animals generally prefer to live in groups or colonies.
Different plants and animals living in a habitat constitute a biotic community. When only assemblage of plants in a habitat is considered, it is plant community.
Similarly, assemblage of animals in a habitat is called animal community. In any biological organization plants and animals are very closely related and interdependent and at a particular place plants and animals share the same set of conditions and same environment.
In view of these facts, modem biologists prefer use of biotic community to plant community or animal community. The study of the relationships of plants and animals making up a natural community is termed as community ecology or synecology. The basic unit of vegetation is called plant community or a plant association. The communities are not the random mixtures of species. The species living together in groups exhibit various degrees of adjustment among themselves and with their physical habitats as well.
Each community consists of a set of many different species which persist year after year. In a community, each plant species is represented by innumerable individuals. A group of individuals of the same species is commonly known as population.
Thus a population is a part of community and populations of different species may be intermingled in a community. Costing defines community as “an aggregation of living organisms having mutual relationship among themselves and to the environment.” In modem books of ecology, plant community is defined as uniform flonstic composition. Earlier Gleason (1939) in his individualistic concept of plant association suggested that an association is a complex of slight irregularities, all of which blend into an entirety of apparent homogeneity. It is uniform either in space or in time and possesses definite limits of area only to a reasonable extent.
Pondeyar (1960, 1961) has discussed this subject and suggested that an association is a temporary phenomenon which is continuous in space and with slight irregularities, all of which blend into an entirety of apparent abstract homogeneity.
According to him the association is not a final vegetation but an assemblage of plants of any status. Misra and Pun (1954) are of the opinion that Clements’ original concept which defined “community as any unit of vegetation whether developmental or climax in status”, should be accepted as such and they used the term association for climatic climax. According to them, the climatic climax community is in, complete equilibrium not only with climate but also with the whole complex of environment.
The following points characterize the community:
(1) Species diversity:
The biotic community is a natural assemblage of a large number of plant and animal species in an area. Actually it is a part of a larger whole ecosystem in which living and non-living components interact and bring about circulation, transformation and accumulation of energy and matter. Various species of plants and animals living in a community exhibit species diversity. Of the total number of species in a biotic community a few species are abundant while a large percentage is rare.
The common abundant species are called dominants whereas rare species determine species diversity. The ratios between the number of species, their biomass and productivity provide valuable information about community and are called species diversity indices.
Species diversity is high in natural community and low in managed ecosystem such as croplands. Because in any particular habitat there is no considerable variation in environmental conditions, the plants growing together in a community show unique uniformity in their behaviour. Vegetation therefore, is reflection of a climate and, in general, widely separated areas having similar climate have similar aspects of landscape.
Some community areas have limits but more often the community boundaries are hard to define. A clearly distinguished area or a type of area with uniform habitat conditions and supporting characteristic type of vegetation is termed “biotope”.
Each species of community has got definite range of tolerance towards the physical and biological environmental conditions of the habitat. The range of environment a species can tolerate is called its ecological amplitude. The nature of community of a particular habitat is determined by the species contents, ecological amplitudes of the species and physical and biotic influences prevailing in the locale of community.
Species occurring in the particular habitat do not live in complete isolation as pure cultures, but they coexist in mutual adjustment. The coexisting populations are interrelated and they show some sorts of interaction. The relationships, between coexisting species may be obligatory in one direction or in both. The trees in a forest community can live just as good as without shrubs and herbs which grow under them. This relationship is obligatory in one direction only. The relationship between plants and animals which pollinate them is obligatory in both directions.
The nature of interaction between two coexisting species may fall into one of the following types:
In this, one species lives at the expense of another.
In this, two or more coexisting populations benefit from the relationship but none suffers.
In this, two populations may compete for same resources of the habitat.
In this, two populations may be quite independent and neither population affects the other.
All the members of a community have ability to live under the conditions of the habitat and they are interdependent upon one another to some extent. It is called dependency. Thallophytes, mosses, ferns and many shade loving herbs that are found on the forest floor are dependent on the forest trees because trees provide shadow and moist conditions. If the trees of forest are removed, the ground vegetation may disappear.
Similarly, the fungi and saprophytes found in the forest depend upon the roots of plants and on the rich humus and some fungi form mycorrhizal associations with the plant roots. Some sort of relationship also exists between plants and the insects and other animals which pollinate them.
(4) Species dominance:
Not all the species of a community are found in abundance. Only a few species are found in abundance, either in number or in biomass (living weight) while the majority are rare. The common species which are abundant and contain maximum biomass are considered to be dominants. Dominant individuals influence the associated individuals.
In the forest, tallest trees, for example, influence the understory plants and ground vegetation not only by decreasing the intensity of light reaching the forest floor and increasing the moisture content of air but also by changing the soil structure and its chemical composition. The dominance in the community may be the result of co-action between two or more species. Different communities are generally recognized and named on the basis of dominant species occurring in them.
In a plant community, the plants, which have some sort of relationship among themselves, may be trees, shrubs, herbs, mosses, lichens and thallophytes. These plants form, more or less, distinct strata or layers or storeys on vertical as well as in horizontal planes. This is characteristically known as stratification. The individuals of different layers represent different ”life forms “.
Each layer of community may sometimes include individuals of different morphological classes, as for example, the top layer or canopy of forest may be formed by tallest trees and lianes (woody climbers). In order to overcome this objection, plants belonging to different morphological classes are put in ‘sinusiae’ (singular—sinusia), as for example, trees are put in sinusia of trees, epiphytes are put in sinusia of epiphytes, and so on.
In grassland, there are essentially three strata:
(1) The root and rhizome layer,
(2) The ground layer, and
(3) Herbaceous layer.
In forest vegetation, the stratification reaches its greatest complexity. As shown in Fig. 6.1, five vertical subdivisions (units of homogeneous life forms and ecological relations) may be present:
(1) Subterranean zone,
(2) Forest floor,
(3) Ground vegetation nearly extending up to a metre or so,
(4) Understory tree and shrub layer extending to the height of one to five metres and
(5) Tree layer or top storey extending to the extent of 5 to 15 metres in most of the forests, but sometimes it may extend up to 25-30 m in the coniferous forests and to about 40 or 50 m in rain forests.
In Sequoia forest, the upper canopy may surpass 100 m. The different strata of plant community are determined by light penetration; the maximum light is available to plants of top storey and the minimum light reaches the ground vegetation. It is evident from Fig. 6.2.
The factors causing stratification of biotic communities are as follows:
1. Specific tolerance and adaptations.
2. Chemical reactions between the lay-products and physical stratification.
3. Changes in species structure with time. New organisms take up temporary or permanent position as a direct response to the presence of initial or preexisting population in the area.
Interacting populations of community are characterized by continuous death and replacement and usually by immigration and emigration (one way movement from home range to other habitat) of their individuals. In this way, composition and shape of community remains in changing state.
The changes in the community go on taking place until a complete balance IS established between community and environment. This is called succession. At complete equilibrium state a stable community is established which is called climax community. In the climax community very little or no change in the shape can be anticipated over a long period of time.
A mixture of species which live in a habitat and are held together by common ecological tolerances form a community. As indicated earlier, some communities have sharply defined limits and they are characterized by a, more or less, definite species contents, as for example, community of a pond or a small island or old field. Thus, they show discontinuous vegetational patterns.
There are, however, several instances of community types which do not have well defined limits and one community generally merges with another community. The species occurring in a community are not bound together into group of associates; and they respond independently to the physical and biotic factors of the environment.
Thus with the change in the environment the composition of community also tends to change. The sequence of communities showing a gradual change in composition is called continuum (Curtis 1959). Community in such a gradient can be recognized as a discrete point with uniform floristic composition in the continuum.
As regards the composition of community, there are two opposing philosophies:
(i) Organismic view,
(ii) Individualistic view.
The organismic view was advanced by Clements (1916) and is still supported by his school of ecology. According to this view, the community is a sort of “super organism”, the highest stage in the organisation of living world, rising from cell to tissue, organs, organ-systems, species, population and to community. The community is regarded as super organism because it grows adjusts under some circumstances, reproduces itself, and functionally represents higher level of integration than individual plants and animals that make it up. The community acts as unit in the succession.
The individualistic concept was first advanced by H.A. Gleason (1926) and further supported by Whittaker (1951, 1952, and 1956), Curtis (1958), McIntosh (1959) and several other ecologists. This approach emphasizes that no community necessarily reaches any prescribed composition or steady state. In this, species has been recognized as essential unit because it is only the species, and not the community, which is directly involved in inter-relationships and distribution.
The species directly respond to the environmental conditions independently. They are not bound together in associates. The plants and animals exhibit a gradient or continuum from one extreme of environmental conditions to the other. In this approach of community composition the community is regarded as a continuous variable.
In nature, there is no sharp line to indicate the end of one community and the beginning of another community. When two communities are adjacent, there is a zone of transition between them where the species of the two communities intermingle. The transition or marginal zone between two major communities presenting a situation of special ecological interest is called an eco-tone or tension zone.
The transition zone between grassland and forest or the intermediate zone between any two major land or aquatic communities are the examples of eco-tone. Strictly speaking, a transition zone is an eco-tone only if tension exists between the bordering communities. The eco-tone is colonized by the species that are commonly found in the communities on both the sides, as well as by some versatile species of plants and animals. As a rule, the eco-tone contains more species and often denser populations than the bordering communities. This is known as ‘principle of edges’ (Fig. 6.3). This edge effect is chiefly due to wider range of suitable environmental conditions.