Ecological units: definitions and application.

Concepts of ecological units, such as population, community, and ecosystem, are at the basis of ecological theory and research and have increasingly become the focus of conservation strategies. Concepts of these units still suffer from inconsistencies and confusions over terminology. The different concepts are treated here together as a common "conceptual cluster," with similar ecological functions (roles) and similar problems in their definition and use. An analysis of the multitude of existing terms and concepts that have been developed for these units reveals that they differ with respect to at least four basic criteria: (i) the questions as to whether they are defined statistically or via a network of interactions; (ii) if their boundaries are drawn by topographical or process-related criteria; (iii) how high the required internal relationships are; and (iv) if they are perceived as "real" entities or abstractions by an observer The various definitions cannot be easily sorted into just a few types, but they can be characterized by several independent criteria. I argue that serious problems arise if the different possibilities of defining ecological units are not recognized and if the concepts are perceived as self-evident. The different concepts of ecological units should be defined and used in a philosophically informed manner I propose a dual approach to the use of ecological units. Generic meanings of the main concepts (especially population, community, and ecosystem) should be retained only as heuristically useful perspectives, while specific and "operational" definitions of the concepts as units should be developed, depending on specific purposes of their use. Some thoughts on the basic requirements for such definitions and the domains of their uses are briefly explained.