Community in Ecology
Community, If you look around yourself you will notice that population of plants and animals seldom occur by themselves. The reason for this is quite obvious.
In order to survive individuals of any one species depend on individuals of different species with which they actively interact in several ways. A population of squirrels would require fruits and nuts for food and trees for shelter.
Even plants cannot exist by themselves; for example, they require animals for seed dispersal, pollination and soil microorganism to facilitate nutrient supply to them through decomposition.
In nature ‘an aggregation of population of different species in an area, living together with mutual tolerance and beneficial interactions amongst themselves and their environment, form a biotic community.
Communities in most instances are named after the dominate plant form species. Grassland, for example, is dominated by grasses, though it may contain herbs, shrubs, and trees, along with associated animals of different species.
The definition and description of the community so far must have made you aware that the size of a community is not fixed or rigid; communities may be large or small.
Types of Community:
On the basis of size and degree of relative independence communities may be divided into two types:
i)Major Community :
These are large-sized, well organized and relatively independent. They depend only on the sun’s energy from outside and are independent of the inputs and outputs from adjacent communities.
A tropical ever green forest in the North-East of India is a good example of a major community.
ii) Minor Community:
These are dependent on neighboring communities and are often called societies. They are secondary aggregation within a major community and are not therefore completely independent units as far as energy and nutrient dynamics are concerned. A cow dung pad would be a good example of such a community.
Growth-form and Structure
In a community the member of species and size of their populations vary greatly. A community may have one or several species.
The environment factors determine the characteristic of the community as well as the pattern of organization of the members in the community.
The characteristic pattern of the community is termed structure which is reflected in the roles played by various populations, their range, the type of area they inhabit, the diversity of species in the community.
And the spectrum of interactions between them. As a result the structure of community is as follows:
- I) Dominance: In each community, a few over topping species are present in greater bulk. By their greater number or biomass the dominant species modify the habitat characteristic and influence the growth of other species in the community.
In most communities only a single species, being particularly conspicuous, is dominate and in such case the community is named after the dominate species, as for example spruce forest community.
In some communities, however, there may be more than one dominant species, as in oak-fir forest in the west Himalayas.
- II) Species diversity: An important attribute of a community is its species diversity. The diversity is calculated both by the number of species and the relative abundance of each species.
The greater the number of species and more even their distribution the greater is the species diversity.
It is important to realize that species diversity and dominance are interrelated.
Communities with one or a few dominant species are characterized by low species diversity whereas communities where no single species is truly dominant and individuals are equally distributed among all species are characterized by high species diversity.
Diversity is also related to the stability of the community. A stable community is one which is able to return to its original condition after being distributed in some way.
Communities with high species diversity are comparatively more stable because many alternative pathways exist in such communities to enable the individuals to obtain the required energy and nutrients.
To put it differently, the presence of a large number of species would mean that if one species disappears or declines, its function and place can be assumed at least in part by another.
It is now however, increasingly realized that in some situation greater diversity does not necessarily result in greater stability. Stability is more dependent on the number of well adapted species than on the total number of species present.
Communities created by man such as lawns or agricultural fields are very unstable and require great deal of constant manipulation and maintenance.
III) Mutual interrelationship among individuals of a community: Mutual interrelationship includes all the direct and indirect effects that organism have upon each other.
The three relationships which we shall discuss are (a) competition, (b) stratification, (c) dependence.
- Competition: Demand for a common resource by different organism results in competition. Competition between individuals of different species is called interspecific; when it occurs between individuals of the same species it is called intraspecific.
- Stratification: Different organisms in a community develop a characteristic pattern of stratification to minimize competition and conflict among the members of the community.
- Plants and animals of each layer differ in size, behavior and adaptation from those of other layers.
The tall growing trees form the over story and modify the light and moisture conditions for the shorter trees growing under them.
These in turn determine the conditions for the ground vegetation. In moist tropical rain forests up to five distinct strata can be formed.
Competition in the community is not limited within a species. Different species in the community complete with each other for nutrients, space, light and other resources.
Plants and animals of each layer differ in size, behavior form and adaption from those of other layers. Stratification is a practical strategy to minimize interspecific competition i.e. competition between different species.
For example, if several different species develop simultaneously and have similar demands, then they will all survive in approximately equal numbers and occupy the same position and layer in the community.
Those species which do not have overlapping requirements will affect each other much less and hence will occupy different functional positions or sub layers in the community.
For example, consider a tall growing tree which outgrows a potentially short one under the same conditions. The former occupies a higher level.
The tall trees with widespread canopy dominate the area and influence light and moisture conditions. As a result the tall trees control the characteristic in a forest community.
Only those species of organisms can survive which can withstand the environmental conditions created by the dominate species.
The community characteristics changes if the dominate species is eliminate due to some reason. This is because the position of dominance would be assumed by other species.
The non-dominant species present in the lesser strata probably offer little direct competition to the dominant species. Indirectly, however, they may offer serious competition to the dominant species in matters of regeneration.
For a dominant species to be successful it should be able to compete with other species during its early phase of growth. Competition for survival is most obvious between seedlings of different species since all depend on the same restricted environment.
- c) Dependence: In a community there are some species which are wholly dependent on the dominant member for survival. Bryophytes, thallophytic and a few vascular small plants are examples of such organisms.
These dependence organisms require special conditions such as shade and moisture provided by the dominant species. The dependent species will die if the dominate species are eliminated.
Animals in a community are usually dependent on plants. Large mobile animals like deer are not necessarily limited to a single community.
However, several less mobile species are definitely restricted to a single community. For example, certain species of insects and birds have specific association with a particular vegetation type.
In addition to the mutual interactions among organisms of a community, they also actively interact with their environment. In a community only those plants and animals survive which are adapted to a given environment.
The climate determines the type of environment, and hence the type of organisms in a community. For example, it is the variation in the climate which determines whether a given area become a desert of a forest.
The environment of an area in turn would determine the types of organisms which could survive there.
- IV) Trophic structure: Organisms in a community are closely interrelated with each other through feeding relationships.
Another aspect which is quite obvious in a community is that in areas of extreme climatic conditions both species diversity or the numbers of species are greatly reduced.
This is because only a few species are able to adapt to the difficult environment.
Communities which extend over a considerable area generally have also locally diverse conditions of soil or topography. Thus in a community local habitats may be supporting markedly different species which are very different from the general community composition.
Members of a community share the same habitat and its resources. A community represents only the living organisms occupying a given area.
When both the living and non-living components are considered as an integrated unit we would be dealing with an ecosystem, a concept which will be considered in the next subsection.
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