Ecological communities: conceptual problems and definitions

Abstract Three major problems with respect to ecological communities are tackled. The first is the problem of ambiguity, i.e. the fact that the term “community” is being used for various kinds of objects at different levels of organisation. We argue that this problem can be resolved by restricting use of the term “community” to sets of co-occurring organisms belonging to a single taxonomic phylum or class (plants, birds, insects, etc.) and by using the term “biocoenosis” for sets of organisms belonging to multiple phyla or classes and comprising the biotic components of ecosystems. We also argue that interaction between organisms is neither a necessary nor a sufficient condition for community membership, or, in other words, that communities may consist of both interacting and non-interacting organisms. The second and third problems are the boundary problem and the problem of heterogeneity, i.e. the fact that communities, as currently defined, most often do not have discrete boundaries and are quite heterogeneous with respect to species composition. We argue that both problems result from the fact that communities are seen as groups of co-occurring populations of species, whereas actually populations of different species rarely if ever co-occur in exactly the same area. These problems can be resolved by redefining communities as particular sets of individuals occurring in the intersection of the areas occupied by different populations of species. In the final section we defend our definition against some potential objections, viz. (1) that it would lead to an excessive number of small communities, especially in species-rich situations, and (2) that, because population boundaries may change all the time, such communities would be very unstable.