A community is made up of all of the plants, fungi, protozoa,bacteria and animals which are found together in an area. Communities are neither discrete nor fixed, but are rather associations within which there is more interaction and exchange of materials among members than there is between members and other organisms living outside of the community. Communities are recognizable by the pattern they show, for example, within a stream, on a mountain slope, a patch of soil, or a seashore. Such patterns are visible because of the structure of the community, where "structure" includes such things as species composition, the diversity of plant and animal species,production of engergy, life-forms of common species, and the patterns of distribution of all of these within the community. Some of this pattern is generated by variation in the physical environment; that is by variation in factors such as nutrients, soil pH, soil moisture, and many others. A considerable portion of the pattern we see in natural communities results from biological interactions among members of the community. One such pattern-generating interaction is herbivory.

Evolutionary effects of herbivory are much more long-term than ecological effects and must be inferred from ecological knowledge and an often limited fossil record. Evolution occurs in response to selection pressures exerted by ecological interactions at the community level. For example, chemical, structural and other defenses against herbivory have probably evolved in response to the strong selection pressure exerted by the feeding of herbivorous animals. Competition within a community, both between individuals within a species, and among different species, is also a major selective force directing evolution. The presence of one or more herbivorous animals in a community can even alter the competitive hierarchy among plants in the community, and can thus indirectly affect the selection pressures acting on species which are not even their food. Some seed and fruit eaters have evolved along with their food plants while, on the other hand, many fruits and seeds have evolved structures and chemicals which make them difficult or dangerous to eat.

One of the more obvious ways in which herbivores can influence natural communities on an ecological scale is by determining which species make up a particular community. This is usually a positive benefit to the community as a whole but, at one extreme, large populations of one or more species of herbivore can lead to severe habitat destruction, and decreases in productivity and diversity. In terrestrial ecosystems, such overgrazing can lead to soil erosion, and even desertification. In some marine systems, such heavy grazing may be the normal state of the community.