Despite the widespread use of the term 'community', there has been remarkably little detailed ecological discussion of what it means. This remains a strange omission, given that communities are supposedly the actual units of study for many ecologists. There seem to be two, very different ways of viewing a community. One is that species exist in integrated communities that have persistent features through time (e.g., composition of species or structure of food webs) and are repeated in different places. Thus, the ecological community of species is organized, structured, and integrated. The species in the community are interdependently interactive and, often, it is presumed that the interactions are, at least in part, responsible for maintaining the entity.
The alternative view is much less about the structure of a community. It holds that communities are simply a human invention. The term community is used to describe the collection of organisms that are found in the same place at the same time. They may (or may not) exhibit interdependencies. They may or may not interact. They coexist because they have similar physiological responses to physical components of the environment and/or they have similar needs for resources of food and shelter. Alternatively, some of them may be present because they need others as prey (or as some other resource).
There is no doubt that these two views are very different and lead to very different forms of ecological study. If the former view is correct, the community is, indeed, a valid and necessary object of study. It will only be possible to understand the ecology of the component species, if their interactions and interdependencies are understood. There could be no argument that the populations (and, in fact, the individuals in the populations) that make up the community are not the only valid units to investigate and explain. The community, itself, has ecological features and properties.
In contrast, if the latter, more descriptive and less structured view is correct, then the community per se has no structured existence and is simply a collection of entities - the populations of the species present. These populations and the interactions among them are the important units to study.
Throughout the history of ecology, there have been periods when one or other view has been (or has been considered to be) dominant. There does not ever seem to be a time when one definition looks likely to prevail over the other. As a result, there is no particularly successful or acceptable definition of community that will satisfy all - or even most - ecologists. The most extreme views are a very tightly coevolved and integrated community versus a random collection of species that happen to co-occur. Neither view is particularly likely to be sufficient or even correct.
The reality is probably somewhere in between. Some components of a community may, in fact, be highly interdependent and coevolved. Others are not. At the same time, coevolved traits can almost certainly be caused or maintained even in the loosest co-occurring set of species. For example, it is not a necessary condition that coevolved features of inferior competitors for food must be caused by consistent interaction with the same dominantly competitive species. Such features could well result from consistent competition for resources by a range of different and changing superior competitors. The intensity of and responses to competition can be ongoing and progressive, even though the superior competitors change from one generation to the next (or faster) for inferior competitors. Competition can be extensive, but caused by different species in varying mixes from one part of a habitat to another.
To avoid some (but sadly not all) of the confusion associated with the term community, the two ecological approaches will here be referred to using the term 'community' for tightly knit, consistent sets of species. 'Assemblage' will be used for the more loosely associated set of co-occurring species, where the whole set of species is not a repeatable, identifiable set.
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