The significance of geographical variation in species interactions

Geographical variation in species interactions serves as a lens through which many prominent but unresolved ecological and evolutionary questions can be examined profitably. It can also direct our attention to questions that have been less visible but whose answers nonetheless have profound implications. Although many naturalists are aware that the strength and sometimes even the nature of a particular interaction may vary from one location to another, the sources of the variation have not often been examined. In some cases, differences in the environment from one location to another cause it; the interesting questions for these cases revolve around how abiotic and biotic factors combine to produce different interactions in different locations. In other cases, the variation seems to be caused by genetic distinctions between the populations of the interacting species; these cases might represent alternative evolutionary outcomes of species interactions. If many examples of variable species interactions prove to be evolutionary products, evolutionary biologists and ecologists alike will be forced to reexamine current notions of genetic and ecological interchangeability among members of a species and, perhaps, to wonder about what maintains the cohesion of species.