In ecology, a community is an assemblage of populations of different species that occur together in a particular area and interact directly or indirectly. A community therefore represents the biological component of an ecological system at a level above individual populations. A woodland community might contain trees, shrubs, grasses, birds, mammals, insects, fungi, bacteria and other organisms, while an aquatic community could include algae, aquatic plants, fish, invertebrates and microorganisms.
The exact boundary of a community depends on the ecological question and the organisms being studied. Community ecology focuses on species composition, abundance, diversity, interactions, spatial patterns, succession, stability and the processes that determine which species coexist. A community is not simply a list of species because interactions among those species are central to its ecological meaning.
Species within communities interact through several major types of relationships. Competition occurs when individuals of different species require the same limited resource. Predation involves one organism consuming another, while herbivory involves animals feeding on plants or algae. Parasitism benefits one organism while harming its host. Mutualism involves interactions in which both participating species obtain benefits, although the strength and nature of such relationships can depend on environmental conditions.
Commensalism describes an interaction in which one species benefits while measurable effects on the other are limited or negligible. Species can also interact indirectly through shared predators, competitors, prey or environmental modification. These interactions influence population sizes and distributions and can create complex networks of positive and negative effects throughout a community.
Community composition refers to which species are present and their relative abundances. Species richness is the number of species present, whereas species evenness describes how evenly individuals are distributed among those species. Diversity indices combine richness and evenness into quantitative measures, although different indices capture different aspects of community structure. Dominant species may account for a large proportion of biomass or abundance, while rare species can nevertheless have important ecological functions.
Keystone species are species whose effects on community structure are disproportionately large relative to their abundance or biomass. Foundation species can strongly shape physical habitat and create conditions used by many other organisms. These concepts demonstrate that numerical abundance alone does not determine ecological importance.
Communities change through time. Ecological succession describes directional changes in community composition following environmental change or disturbance, although succession does not necessarily proceed along one universal predictable sequence. Primary succession can occur where soil is initially absent or extremely limited, while secondary succession occurs where an existing community has been disturbed but soil or biological legacies remain. Pioneer organisms can modify environmental conditions, facilitating subsequent colonisation. Disturbance can remove some species while creating opportunities for others. Fire, flooding, storms, grazing, disease and human land use can all influence community trajectories. Succession is affected by climate, dispersal, species interactions, soil development, disturbance frequency and historical conditions, so communities may follow different pathways under different circumstances.
Community structure is influenced by environmental filtering, species interactions, dispersal and historical processes. Environmental filtering favours species possessing traits compatible with local conditions. Competition can limit coexistence when species use highly overlapping resources, while predation can prevent dominant competitors from excluding other species. Facilitation can allow species to survive conditions that would otherwise be unsuitable. Dispersal determines which species can reach a location, and historical events can leave lasting effects on composition. These mechanisms interact, making it difficult to attribute community structure to a single factor. Ecologists often compare communities across environmental gradients or manipulate particular interactions to determine their relative contributions. Community ecology therefore combines observational, experimental and theoretical approaches to explain patterns of biodiversity.
Food webs are a central part of community structure because species are connected through feeding relationships. Plants and algae provide organic matter to herbivores, herbivores support predators, and decomposers process dead organic material from organisms at many trophic positions. A change in one population can therefore affect many others. Removal of a predator may allow prey populations to increase, potentially altering vegetation and other resources. Such indirect effects can propagate through multiple trophic levels.
The strength and direction of these effects depend on interaction networks, species traits, environmental conditions and alternative food sources. Community ecology consequently examines not only pairwise interactions but also complex networks in which species can simultaneously compete, prey upon one another, facilitate each other and share resources.
Community diversity has practical importance for ecosystem functioning and conservation. Diverse communities can contain species with complementary resource use, different environmental tolerances and overlapping ecological functions. Functional redundancy occurs when multiple species contribute similar ecological functions, potentially providing some resilience if one species declines, although redundancy is rarely complete. Biodiversity can influence productivity, decomposition, nutrient cycling, pollination and resistance to some disturbances. However, the relationship between diversity and ecosystem functioning depends on species identity, functional traits, environmental context and spatial scale. Conservation therefore aims not only to preserve species counts but also to maintain interactions, genetic diversity, functional roles and ecological processes that sustain communities over time.
A community differs from an ecosystem because a community includes interacting populations of organisms, whereas an ecosystem includes those biological communities together with the abiotic environment and associated energy and matter flows. It differs from a population because a population concerns one species, while a community includes multiple species. A community also differs from a biome, which is a much broader regional ecological category associated with climate and characteristic vegetation.
Community boundaries can be defined operationally according to habitat, geographic area, taxonomic group or ecological process, so there is no single universally correct community size. This flexibility is useful because researchers can study communities at scales ranging from microbial assemblages on a leaf surface to entire forest or reef communities.
