A community comprises the populations of different species that naturally occur and interact in a particular environment. Some communities are relatively small in scale and may have well-defined boundaries. Some examples are: species found in or around a desert spring, the collection of species associated with ripening figs in a tropical forest, those clustered around a hydrothermal vent on the ocean floor, those in the spray zone of a waterfall, or under warm stones in the alpine zone on a mountaintop. Other communities are larger, more complex, and may be less clearly defined, such as old-growth forests of the northwest coast of North America, lowland fen communities of the British 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 those species living in the canopy, those on the forest floor, and those moving between these two habitats, as well as the functional interrelationships between all of these. Similarly, some biologists working on ecosystem management might distinguish between 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 that area. The introduced species form part of the larger, modified community of the area, but might not be considered as part of the regions original and distinctive community.

Communities are frequently classified by their overall appearance, or physiognomy. For example, coral reef communities are classified according to the appearance of the reefs where they are located, i.e., fringing reef communities, barrier reef communities, and atoll communities. Similarly, different stream communities may be classified by the physical characteristics of that part of the stream where the community is located, such as riffle zone communities and pool communities. However, one of the easiest, and hence most frequent methods of community classification is based on the dominant types of species present for example, intertidal mussel bed communities, Ponderosa pine forest communities of the Pacific northwest region of the U.S., or Mediterranean scrubland communities. Multivariate statistics provide more complex methods for diagnosing communities, for example, by arranging species on coordinate axes (e.g., x-y axes) that represent gradients in environmental factors such as temperature or humidity. 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 what these 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) [link] notes that, generally speaking, organisms can persist and evolve in places where there are sufficient environmental resources for the organisms to channel energy into growth and reproduction rather than simply the metabolic requirements for survival. In other words, organisms are less likely to thrive in a harsh environment with low energy resources. One way of measuring community diversity is to examine the energy flow through food webs that unite the species within the community; the extent of community diversity can be measured by the number of links in the food web. However, in practice, it can be very difficult to quantify the functional interactions between the species within a community. It is easier to measure the genetic diversity of the populations in the community, and to count the numbers of species present, and use these measures of genetic diversity and species richness as proxies for describing the functional diversity of the community. The evolutionary or taxonomic diversity of the species present is another way of measuring the diversity of a community, for application to conservation biology.