To give order to the study of communities, some system of classification is needed, even though the communities of a region often cannot be placed in discrete units. The most widely used classification systems are based on physiognomy, habitat, floristic characteristics like species composition and dominance and community dynamics.
1. Physiognomic Classifications:
Physiognomy, or general appearance, is a highly useful method of naming and delineating communities, particularly in surveying large areas, and as basis for further subdivision of major types into their component communities. Because animal distribution is mainly depended on the structure of vegetation and not on the species composition, therefore, classification by physiognomy willrelate both the animals and the vegetation of an area.
Communities so classified are usually named after the dominant form of life, usually plant, such as the coniferous or deciduous forest, sagebrush, short-grass prairie, and tundra. A few arc named after animals, such as the barnacle—blue mussel (Balanvs—Mytilus) community of the tidal zone. One may grade into the other; so even here the classification may be based on arbitrary, although specific, criteria. Some of the earlier attempts of physiognomic classification by different plant ecologists can be summarized as follows:
Warming (1909) recognised autotrophs and heterotrophs. The aquatic autotrophs, called hydatophytes, were grouped into seven classes, while, the aerial autotrophs were divided into atmophytes, ombrophytes, (plants growing in areas of abundant rainfall), psyebrophytes (plants of cold soils), halophytes (plants of soils having high salt contents), psammophytes (plants of sandy soils), agrophytcs (plants having natural habitat in crop-fields), oxy- lophytes (plants of acidic soils), chasmophytes (plants growing in rock crevices), etc. Raunkiaer (1934) classified plants on the basis of adaptations of life forms to their climates.
He selected about 1000 plant species from all over the world and assigned them to their life forms. Certain plant ecologists have applied Raunkiaer’s classification method to classify Indian communities Iversen (1936) based his classification on the water requirements of plants.
He recognised the following class’s terriphytes (land plants); telmatophytes (aerial plants with aerenchyma), amphiphytes (marsh plants) and limnophytcs (water plants). The physiognomic classification also includes symbolic classification in which different characteristics of a species, such as life form, plant size, leaf shape and size, function, leaf texture, etc., are represented by symbols. Dansereau (1957) has used graphic symbols, while Kuchler (1967) has used several alphabets.
2. Habitat Classification:
In areas where the habitat is well defined, physiographic is used to classify and name communities. Examples of such are sand dunes, cliffs, tidal mud flats, lakes, ponds, and streams.
3. Floristic Classification:
The floristic classification requires detailed study of the individual community and also involves many concepts like frequency, dominance, constancy, presence and fidelity. The Scandinavian ecologist Du Rietz (l921) classified communities into called sociation, which was defined as a recurring plant community of essentially homogeneous species composition with at least certain dominant species in each layer.
Another floristic unit consociation was proposed for the community if only the upper stratum of a multi-layered community is dominated by one species. Thus, the consociation can be defined as a class composed of individual concrete associations whose upper strata are dominated by the same species.
European ecologists group communities into classes, orders, alliances, and associations. Association is defined as a community of definite floristic composition, uniform physiognomy and occurring in uniform habitat conditions. The associations are recognized on the basis of common dominant species in several stands and they are used in naming the association, for example, Anogeissus- Boswellia association, Shorea-Terminalia association, or Balanus- Mytilus association. The floristic system of community classification Works much better with plants alone than with animals or with both.
The floristic classification system is modified when the stands are treated as a continuum. The community complex of a major physiognomy is subdivided by species composition and correlated with an environmental gradient arbitrarily divided into five segments: wet, wet mesic, mesic, dry mesic, and dry (Curtis, 1959). For example, the deciduous forest in Wisconsin (USA) has been divided into southern and northern hardwoods and northern forest. These are further divided in a moisture gradient.
The southern hardwoods include the dry southern hardwoods with bur or black or white oak as the dominants; the dry mesic with red oak or basswood; the mesic with sugar maple and beech; the wet mesic with silver maple, elm, and ash; and the wet woods with willow or cottonwood. Such a system recognises the influences of habitat on community composition. Detailed studies on animal distribution also reveal similar influences of animal composition.
4. Dynamic System:
Clements (1928) modified the floristic system by laying great emphasis on the community dynamics. He called the climax vegetation in a macroclimatic region as “formation” (whenever the vegetation reaches highest development becoming more or less stable for more-or less definite period, under the existing climate, it is called a plant formation).
Each formation was divided into a few associations which were considered units of climax communities in which a few species were dominant. An association was named after the two or three dominant species. The association was further subdivided in a hierarchical series as follows:
Unit with a single dominant species. The developmental or communities of a climax consociation are known as conscious.
An association developed under different microclimatic conditions within the same general climate. It contains two or more dominant species and is characterized by specific precipitation, evaporation and temperature. The seral communities are accordingly known as facies.
This is a localised variant of an association, differing from it in the composition of some main subdominant and chief secondary sepcies. Seral communities are accordingly called locies.
A community with one or more sub-dominants. The seral communities are called Society.
5. Biome System:
Usually animal and plant communities are studied separately, which unfortunately obscures the wholeness of the community and limits our understanding of its function. To escape this dilemma in part, the distribution of animals can be related to the life form of plants and types of vegetation.
These results in a more inclusive classification, which embraces several plant communities but includes all animal life associated with them; this classification is called a biome. The biome is a broad ecological unit characterized by uniformity and distinctive life forms of the climax species, plant or animal.
It is subdivided into smaller units distinguished by uniformity and distinctness in the species composition of the climax and its successor stages. Thus the life forms of plants are emphasized, rather than the taxonomic composition.