In ecology, a community is all the populations of different species living in the same area. It includes plants, animals, fungi, and microorganisms that interact with each other. A pond community might include algae, water lilies, frogs, dragonfly larvae, fish, and bacteria. A forest community includes trees, shrubs, birds, insects, and soil organisms. The boundaries of a community are defined by the researcher's question. A fallen log can be studied as its own community of decomposers.

Community ecology examines how species interact. Competition, predation, mutualism, and parasitism shape which species persist and how many individuals each population holds. The composition of a community changes over time through succession and in response to disturbance. A fire, flood, or human development can shift a community from forest to grassland or from diverse to simplified. Invasive species often disrupt communities by outcompeting native species for resources.

Key concepts

• Species richness: the number of species present.
• Species evenness: how evenly individuals are distributed.
• Trophic structure: feeding levels from producers to predators.
• Keystone species: species with outsized influence on the community.

Communities are not fixed. They are dynamic assemblages that shift with climate, disturbance, and the arrival or loss of species.