Communities include all the different species living in a given area. The variety of these species is called species richness. Many organisms have developed defences against predation and herbivory, including mechanical defences, warning colouration, and mimicry, as a result of evolution and the interaction with other members of the community.

Two species cannot exist in the same habitat competing directly for the same resources. Species may form symbiotic relationships such as commensalism or mutualism. Community structure is described by its foundation and keystone species. Communities respond to environmental disturbances by succession (the predictable appearance of different types of plant species) until a stable community structure is established.