Essay on Major Synthetic Characters of Community !
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These include the following parameters:
1. Presence and Constance:
The presence expresses the extent of occurrence of the individuals of a particular species in the community. The species on the basis of its percentage frequency may belong to any of the following five presence classes that were first proposed by Braun Blanquet:
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I. Rare: present in 1 to 20% of the sampling units.
II. Seldom present: present in 21-40% of the sampling units.
III. Often present: present in 41-60%
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IV. Mostly present: present in 61-80%
V. Constantly present: present in 81-100%
A species with high frequency is considered constant. The term constancy is used when equal sample areas are taken in each stand, and presence is used when the area of the sample varies from stand to stand.
2. Fidelity:
Fidelity or “faithfulness” is the degree with which a species is restricted in distribution to one kind of community. Such species are sometimes known as indicators. The species have been grouped into the following fidelity classes which were first formulated by Braun Blanquet:
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1. Strangers:
Rarely occurring species which either have arrived from other communities or are relicts from earlier stages of succession.
2. Indifferent (or ubiquitous):
Species which occur in any community without exhibiting any preference to any particular kind of community.
3. Preferential:
Species which occur in several kinds of communities but are predominant in one.
4. Selective:
Species occurring frequently in one kind of community and sometimes also in others sometimes preferential and selective species are considered together as characteristic.
5. Exclusive or true:
Species which are completely restricted to one kind of community.
3. Importance value:
To determine dominance, ecologists have used several approaches like relative abundance, relative dominance and ratio of basal area. Often all the three measurements are combined to arrive at an importance value for each species. This index is based on the fact that most species do not normally reach a high level of importance in the community, but those that do serve as an index, or guiding species.
Once importance values have been obtained for species within a stand, the stands can then be grouped by their leading dominants according to importance values. Such techniques are useful in the study and classification of communities on some environmental gradient.