A community comprises the populations and species that naturally occur and inter act in a particular environment to effect a transfer of energy between members of the community. Although some communities, such as a desert spring community, have well-defined boundaries, others are larger, more complex, and less defined, such as mature forest communities. Biologists are selective when applying the term community, sometimes using it for a subset of organisms within a larger community. For example, some biologists may refer to the community of species specialized for living and feeding entirely in the forest canopy, whereas other biologists may refer to this as part of a larger forest community. This larger forest community includes those species living in the canopy, those on the forest floor, and those moving between those two habitats, and the functional interrelationships between all of them.
The diversity of a community depends on the natural resources available to support its populations and species. Therefore the most effective way of measuring community diversity is to examine the energy cycles/food webs that unite the populations and species within their community. The extent of community diversity is then expressed by the number of links in the food web. However, in practice, it can be very difficult to quantify the functional interactions between organisms, populations, and species that share a habitat. It is easier to measure and quantify the diversity of the organisms themselves and use that as an indication of functional diversity of the system. The quickest way to evaluate community diversity is to count the number of populations and species present. The evolutionary or tax-onomic diversity of the species present is another way of measuring the diversity of a community.
Communities are most easily classified by their overall appearance, or physiognomy. In some cases this is based on a diagnostic, physical feature of the community's habitat, such as the riffle zone community of a stream. However, in most instances the classification is based on the dominant types of species present—for example, a fringing reef community, or a Mediterranean scrubland community. Multivariate statistics provide more complex methods for diagnosing communities, by arranging species on coordinate axes that represent gradients in environmental factors such as temperature or humidity.
Christen Raunkiaer, a Danish botanist, developed a classification of plants that provides a useful measure of community diversity. Raunkiaer's five main life forms are shown in Table 3, with one additional life form (epiphytes) not originally included in his classification. The number of species, for any community, that fall into the different categories
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