Do Ecologists Study Communities

Apart from ecologists who have not been consistent in using any definition of a community, there are many who have attempted to define community and, perhaps, have attempted to demonstrate that what they study actually fits the definition. Searching the ecological literature for studies of communities is, however, revealing about the nature of community ecology. It is surprising to discover that many studies are, in fact, only about components of a community. The most common type of study is actually about a group of species that are closely related taxono-mically, for examples, the warblers in a forest, or all the birds in a watershed, or all the beetles on a farm. Apart from being in different habitats, these three examples would usually involve increasing numbers of species. But, does this sort of study actually have anything to do with a community, where this has been defined in a useful or meaningful way.?

The assemblage of fish on a patch-reef, or birds in a patch of forest, could only represent a community if no other species were interacting with them. Each group is generally interactive with other animals and plants as sources of food (and for other things). Such groupings are sometimes and should more generally be described as 'taxocoenes', that is, species that are related taxonomi-cally and are found together in the same area.

Other ecologists study a 'guild', defined as an assemblage of species of similar or quite different taxonomic affinity that are found together and that use the same resources (for food, or shelter, etc.). For these, the term 'guild' is more informative than community (unless the definition of community is the same as guild, which makes the former redundant). An example would be to study, in some area, the set of birds, mammals, and ants that feed on seeds of the same species of plants.

So, most community ecologists do not actually study communities! There are, nevertheless, some cases where the unit of study is a community, where this means tightly coevolved species of different kinds of animals, living in close association, in the same arrangements at many times, in many places.

The classical examples are communities of hostparasite or host-pathogen relationships. The parasites are generally considered to persist because they adapt, immunologically, behaviorally, etc., to the host, despite the host's attempts to get rid of the parasites. Thus, nematodes and tapeworms are well adapted to their host's morphology, behavior, physiology, biochemistry, etc. If they were not so tightly integrated with the host, they might be likely to kill the host or to prevent it from reproducing. Both host and parasite would go extinct. Alternatively, the host would adapt to be able to rid itself of infestation and the parasite would not persist. Close coadaptation is therefore essential for the various species involved in a host-parasite community.

It is thus the case that some specialist communities do exist, can be objectively defined, and can be demonstrated to fit the definition. These are then the cases of communities which deserve the term as a description that identifies their usefulness as units of study. Caution is, however, still required. Not all cases of host-parasite (and similar) assemblages are so tightly structured. Many parasites have a complex life cycle, involving several quite different types of hosts (snails and sheep for some cestodes; invertebrates, birds, and fish for others). Under these circumstances, understanding the ecology of the assemblage of host and parasites requires understanding the direct and indirect trophic and competitive interactions between the hosts and other species that interact with them, but which are not involved with the parasites. This immediately creates all the problems of definition that were involved for any other apparent community.

See also: Ecosystems; Emergent Properties; Gaia Hypothesis; Succession.

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