0.13 Community diversity

A community comprises the populations of different species that naturally occur andinteract in a particular environment. Some communities are relatively small in scale and may have well-definedboundaries. Some examples are: species found in or around a desert spring, the collection of species associated withripening figs in a tropical forest, those clustered around a hydrothermal vent on the ocean floor, those in the spray zone ofa waterfall, or under warm stones in the alpine zone on a mountaintop. Other communities are larger, more complex, and maybe less clearly defined, such as old-growth forests of the northwest coast of North America, lowland fen communities of theBritish Isles, or the community of freshwater species of Lake Baikal.

Sometimes biologists apply the term "community" to 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 thosespecies living in the canopy, those on the forest floor, and those moving between these two habitats, as well as the functionalinterrelationships between all of these. Similarly, some biologists working on ecosystem management might distinguishbetween the community of species that are endemic to an area ( e.g. species that are endemic to an island) as well as those "exotic" species that have been introduced to thatarea. The introduced species form part of the larger, modified community of the area, but might not be considered as part of theregions original and distinctive community.

Communities are frequently classified by their overall appearance, or physiognomy . For example, coral reef communities are classified according to the appearanceof the reefs where they are located, i.e. , fringing reef communities, barrier reef communities, and atollcommunities. Similarly, different stream communities may be classified by the physical characteristics of that part of thestream where the community is located, such as riffle zone communities and pool communities. However, one of the easiest, andhence most frequent methods of community classification is based on the dominant types of species present for example, intertidalmussel bed communities, Ponderosa pine forest communities of the Pacific northwest region of the U.S., or Mediterranean scrublandcommunities. Multivariate statistics provide more complex methods for diagnosing communities, for example, by arranging species oncoordinate axes ( e.g. , x-y axes) that represent gradients in environmental factors such as temperature orhumidity. For more information, see the module on " Natural communities in space and time."

The factors that determine the diversity of a community are extremely complex. There are many theories on whatthese factors are and how they determine community and ecosystem diversity. Environmental factors, such as temperature,precipitation, sunlight, and the availability of inorganic and organic nutrients are very important in shaping communities and ecosystems . Hunter (2002: 81) notes that, generally speaking, organisms can persist and evolve in places where there are sufficientenvironmental resources for the organisms to channel energy into growth and reproduction rather than simply the metabolicrequirements for survival. In other words, organisms are less likely to thrive in a harsh environment with low energyresources. One way of measuring community diversity is to examine the energy flow through food webs that unite the species withinthe community; the extent of community diversity can be measured by the number of links in the food web. However, in practice, itcan be very difficult to quantify the functional interactions between the species within a community. It is easier to measurethe genetic diversity of the populations in the community, and to count the numbers of species present, and use these measures ofgenetic diversity and species richness as proxies for describing the functional diversity of the community. The evolutionary ortaxonomic diversity of the species present is another way of measuring the diversity of a community, for application toconservation biology.