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WHAT CONSTITUTES ECOSYSTEM HEALTH? DAVIDJ. RAPPORT* The Use ofMetaphor in Science In addressing the question of what constitutes ecosystem health, it is tempting to make use of metaphors drawn from human medicine. The use of metaphor, it has been argued, has not only a role in poetry, but also a legitimate place in science [I]. Its function in both areas is to stimulate associations, bringing into juxtaposition phenomena that might at first appear to have little connection but can be seen to be in some way related. In poetry the association might be quite fanciful, while in science the value of metaphor lies in pointing to phenomena in apparently different spheres that bear some structural identity. What constitutes the health of Nature—that is, what are the suitable concepts and conventions to assess the condition of environment—is a question now being raised for the earth's major ecosystems (forests, lakes, seas, etc.) and indeed the entire biosphere. Ultimately, a healthy environment is essential for a healthy human population. It does not follow, however, that the appropriate standards for the health of nature need be based solely on criteria for human health. Ecosystems have a life of their own with or without human components, and it is this life that is receiving ever more attention as situations come to light in which ecosystems have become severely damaged [2]. What Are Ecosystems? Grappling with the notion of the ecosystem as an entity—something more than a community of species but less than the biosphere—is a difficult task. Ecologists differ among themselves as to the legitimization of the "ecosystem" concept. While few might hold the view that ecosysThe author thanks M. Hilden, S. Kerr, H. Regier, and R. Ryder for helpful comments on earlier drafts, and Annabella Elliot for help in preparation of the manuscript. ?Address: 10 Harvard Avenue, Ottawa, Ontario KlS 4Zl, Canada.© 1989 by The University of Chicago. AU rights reserved. 003 1-5982/90/330 1 -0665$0 1 .00 120 I DavidJ. Rapport ¦ Ecosystem Health terns are little more than somewhat fortuitous associations of species— forming, in association with the physical and chemical environment, biotic communities that lack overall integration—not all agree that nature is appropriately described at a supraorganismic level, exhibiting the kinds offeedbacks that are the hallmark ofself-regulating systems [3-6]. Spatially, the ecosystem proves difficult to "pin down." Boundaries of course can and are being drawn, but they are always, to a degree, arbitrary . In reality, ecosystems are open systems with important linkages to neighboring systems (via energy transfers and nutrient flows mediated by physical, chemical, and biological processes). Thus, in a way, ecosystems form a continuum that extends to encompass all of the biosphere. If ecosystems lack boundaries, and if a multitude of physical, chemical , and biological processes are making a mockery of arbitrarily drawn borders, such must be a fundamental consideration in reaching ajudgment on the health of some designated region. Can a part of nature be healthy yet contain unhealthy regions? Can a healthy ecosystem continue to remain so if it is surrounded by degraded systems? Questions of this sort are perplexing because all too little is known about the role of the dependencies between parts and wholes. Furthermore , the answers depend often on the detailed circumstances. For example , wetlands play a critical role in sustaining aquatic ecosystems by providing both feeding and breeding refuges for biota and carrying out other critical functions. Where wetlands are abundant, the loss of some small area may be inconsequential to the health of the aquatic system, while, in a region where wetlands are scarce, further reductions might cripple the system. Measures ofEcosystem Health There are three approaches commonly used to assess ecosystem health, all of which have parallels in the practice of human medicine. The first, and the one which is by far the most common, rests on the identification of critical characteristics that differentiate healthy from sick ecosystems. These we might refer to as the vital signs of ecosystems [7-8]. The second rests on measures of the counteractive capacity to handle stress loadings [9]. This refers to the ability of ecosystems to bounce back, or recover their equilibrium, after perturbations...

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