by Mark Vellend
For almost as long as ecology has been a discipline, it has struggled to define what constitutes an ecological community. A sharp dichotomy emerged early on, contrasting the view that communities were tightly integrated entities consisting of interdependent species (the community-unit concept) vs. the view that species co-occur largely according to the individualistic response of each species to spatially variable environmental conditions (the individualistic concept). To a large degree, the latter view has dominated ecological thought since the mid-20th century, based to a considerable extent on empirical patterns of community composition along environmental gradients. However, it has been repeatedly pointed out that neither view (in its extreme form) can capture the reality of processes and patterns in real communities, in which species often show both some degree of interdependence and gradual change in composition based on environmental conditions. Despite the debate regarding the concept of an ecological community, the discipline of community ecology has thrived and remained a key pillar of the broader field of ecology, with intense debates over the importance of competition in driving community structure and the relative importance of processes occurring at different spatial and temporal scales, among others. Finally, while few contemporary theoretical ecologists treat communities as belonging to discrete types, the community-unit concept lives on in applied ecology, where the classification of communities (often described as “vegetation” or “ecosystem”) is commonplace in order to facilitate conservation management, prioritization, and policy.
Frederic Clements put forth the formal concept of a community as a coherent unit of study for ecologists. Clements considered the different species in a community as being tightly integrated and interdependent, much like the organs that make up a plant or animal, leading him to use the metaphor of the community as a “complex organism” (Clements 1916). Forbes 1887 articulated a similar view of the species living together in lakes. The community-unit view was challenged by Leonty Ramensky (see McIntosh 1983) and Henry Gleason (Gleason 1926), who argued that species respond individualistically to environmental factors, such that their distributions and abundances vary continuously from place to place, rather than forming discrete and internally integrated community types. Tansley 1935 is a sharp criticism of Clements’s views, especially his terminology. McIntosh 1985, Kingsland 1991, Kingsland 2005, and Kingsolver and Paine 1991 provide historical accounts of this debate and place it within the context of the development of ecology as a scientific discipline.