THE NICHE CONCEPT REVISITED - MECHANISTIC MODELS AND COMMUNITY CONTEXT

The niche concept is a central organizing aspect of modern ecology. Al though its history has often been reviewed, the structure of the conce pt and its connection to advances in ecological theory has received le ss recent attention. I review the niche concept using ''mechanistic'' models of community theory to identify two distinct components. One de scribes the environmental requirements of organisms and the other desc ribes the per capita impact of organisms on the environment. I argue t hat these correspond to significant differences between Grinnell's and Elton's concepts distinct from the previously discussed ''habitat'' v s. ''functional'' dichotomy. I illustrate the distinction between the requirement and impact components of the niche using models of resourc e competition and of keystone predators, and I discuss ''Gause's axiom '' and conventional ''niche theory'' in the context of these two disti nct niche components. I suggest that the niche concept be elucidated b y explicit reference to these two distinct components; the ''impact'' niche (corresponding to Elton's concept) describing instantaneous per- capita effects of species on the environment, and the ''requirement'' niche describing the response of species to the environment (correspon ding to Hutchinson's definition). This approach connects conventional niche theory with ''mechanistic'' individual-based ecological models a nd can help provide a more modern context for the niche concept.