INTRODUCTION
All the living organisms of an ecosystem form a single biotic component, the community or biotic community (communities of the environment). All the organisms of a community live together, share same habitat, influence each other’s life directly or indirectly and have reached a survival level within a given radiant energy. Thus, a community is any assemblage of populations of living organisms in a prescribed area of habitat. A community is claimed to have one or more of the following attributes :
(1) Co-occurrence of species,
(2) Recurrence of groups of the same species, and
(3) Homeostasis or self-regulation.
Community is a larger unit than the population and it achieves many characteristics that are not found in its constituents, ie., the organisms and the populations. Communities may have a wide range of sizes, ranging from a small patch of land or water-body to extensive forests. Minor communities are greatly influenced by inputs from adjacent communities, while major communities are relatively independent and self-sufficient of their habitat. Communities differ from place to place and at the same place at different times.
CHARACTERISTICS OF A COMMUNITY
Like a population, a community has a series of characteristics such as follows:
- Species diversity. Various species of plants and animals live in a community and exhibit species richness or species diversity. The study of species diversity is an essential component of community study or animal communities, a study of age structure and growth pattern is important, while for plant communities, floristic, study of taxonomy, life forms such as herbs, shrubs, climbers, trees are important.
2. Growth form and structure. The type of the community is described by major categories of growth forms (e.g., trees, shrubs, herbs, mosses, etc.) These different growth forms determine the stratification, or vertical layering of the community.
3. Dominance. Among several species present in a community, a few exert a major controlling influence by virtue of their size, numbers, or activities. These are called as ecological dominants or dominant species.
4. Relative abundance. Different populations in a community exist in relative proportions andthis idea is called as relative abundance.
5. Trophic structure. Who eats whom ? The feeding relations of the species in the communitywill determine the flow of energy and materials from plants to herbivorous animals to carnivorousanimals.
CLASSIFICATION OF THE COMMUNITIES
Communities have been classified by different ecologists from different view points. In terms ofthe general growth, composition, shape, etc., of vegetation, and organisms associated with them,communities may be classified as forests, deserts, grasslands, tundra and so on.
Likewise, accordingto the amount of water in the habitat, communities may be divided as:
- hydrophytic in predominantlyaquatic habitats,
- mesophytic in moderately moist soils and xerophytic in arid or dry conditions.
- Communities growing in condition of abundant light are called heliophytic and those growing in shadeare called sciophytic.
Clements (1916) recognized the fact that plant communities are not always thesame at any place and he classified the communities on two parallel lines :
- One in the process of changewhich are called seral communities and the others are called stable or climax communities.Further, the global community is an enormous mass of life, comprising all the plants and animalsin the world.
The global community is further divided into: continental communities and oceaniccommunities. Since due to great variability in climatic factors, an exhaustive study in such vast areasis practically impossible, therefore, communities are often studied as biotic province.
COMPOSITION OF COMMUNITY
Composition of community comprises of following parameters:
- Size: communities may be large or small. Large ones are forests and deserts and small ones like ponds, lakes etc.
- Number of species: number of species in a community varies greatly. Like 1 specie of earthworm and 22 species of grass are found in India.
- Dominants: When any species show dominance over other than it is known as dominant species. In some communities only one type of species show dominance but in some more than one type of species show dominance.
- Ecological amplitude: The range of environmental conditions which a taxon can tolerate is called ecological amplitude.
CHARACTERS USED IN COMMUNITY STRUCTURE
QUANTITATIVE STRUCTURES OF PLANT COMMUNITIES
Quantitative characters of the community include such characters as frequency, density, abundance, cover and basal area, etc.
1. Frequency. Various species of the community are recorded by different phytosociological methods, by taking any sampling unit such as transect, bisect, quadrat, point centre or point quarters, etc. Frequency is the number of sampling units (as %) in which a particular species occurs.
2. Density. Density represents the numerical strength of a species in the community. The number of individuals of the species in any unit area is its density. It gives an idea of degree of competition.
3. Abundance. This is the number of individuals of any species per sampling unit of occurrence.
4. Cover and basal area. The above ground parts of plants (such as leaves, stems and inflorescence) cover a certain area-if this area is demarcated by vertical projections, the area of the ground covered by the plant canopy is called canopy cover, foliage cover or herbage cover.
QUALIFICATION CHARACTERISTICS OF PLANT COMMUNITIES
These involves parameters such as physiognomy, phenology, stratification, abundance, sociability or gregariousness, vitality and vigor, life form (growth form), etc. All these parameters can be described and may be grouped in point scales.
- Physiognomy: Physiognomy is the general appearance of vegetation as determined by the growth form of dominant species. Such a characteristic appearance is often expressed by single term. For example, observation of a plant community having trees and some shrubs as the dominants, clearly forced us to conclude that it is a forest. Similarly on the basis of appearance of a community, it may be a grassland, desert, etc.
- Phenology: The life history of a plant species involves seed germination, vegetative growth, flowering, fruit formation, seed maturation, leaf fall, seed dispersal and death. These events are different for different species and are recorded for the individual species. A study of the date and time of occurrence of these events is called phenology. In other words, phenology is the calendar of events in the life history of the plant.
- Stratification: Stratification of the community is the way in which plants of different species are arranged in different vertical layers in order to make full use of the available physical and physiological requirements.
- Abundance: plants are not found uniformly distributed in an area. They are found in smaller patches or groups, differing in number at each place. Abundance is divided into five groups: 1. Very rare, 2. Rare, 3. Common, 4. Frequent and 5. Very much frequent.
- Vitality: it is capacity of normal growth and reproduction, which are important for the survival of the species. In plants vitally is defined by stem height, root length, leaf area, fruits and seeds.