Abstract

Ideally, experimental studies in ecology examine the response of genetically similar organisms to different environments or of genetically different organisms to similar environments, where environment is taken to include both biotic and abiotic components. Since environmental differences in nature may rapidly lead to subtle genetic differences among populations (e.g. 72, 177, 178), it is sometimes perilous to compare the response of a species among differing natural environments by observation alone. Likewise, simply observing or measuring the response of clearly different organisms in a single natural environment may fail to reveal the extent of their interaction. This means that experimental intervention or manipulation is usually required to establish controls for any rigorous test of hypotheses concerning biotic interactions or the differential response of organisms to environments. The concept of the ecological niche is a useful device for describing concisely the patterns and limits of response of organisms and for abstracting certain generalities from the vast array of particulars comprising the literature of ecology. As Elton put it fifty years ago: "The importance of studying niches is partly that it enables us to see how very different . . . communities may resemble each other in the essentials of organization" (56). In its broadest sense the title of this review could include most of experimental ecology. Besides the obvious impossibility of reviewing the literature of such a broad area, we believe that the niche concept is most useful in a much more restricted domain. It is almost entirely at the focus of communities (or at least interspecific relationships) that the concept of the niche has figured in ecological theory (31, 109, 116, 121, 123, 159--161, 190, 194) and in the large number of empirical studies of competition and coexistence not based on experiment. [T. W. Schoener (165) recently reviewed studies concerned with three or more species; see also (31).] Conse-