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m field botanist; he would be through with his work in one minute; he would quickly identify the plant as Spartina alterniflora, press it, and be gone. Even the number of species of insects seems to be small enough so that one has hopes of knowing them all, something very difficult to do in most vegetation. ... The strong tidal fluctuations and salinity variations cut down on the kinds of organisms which can tolerate the environment, yet the marshes are very rich. Lots of energy and nutrients are available and lots of photosynthesis is going on so that the few species able to occupy the habitat are very abundant. There are great masses of snails, fiddler crabs, mussels, grasshoppers and marsh wrens in this kind of marsh. One can include a large part of the ecosystem in the study of single populations. Consequently, fewer and more intensive sampling and other methods can be used. ... In other words the saltmarsh is potentially to the ecologist what the fruit fly, Drosophila, is to the geneticist, that is to say, a system lending itself to study and experimentation as a whole. The geneticist would not select elephants to study laws and principles, for obvious reasons; yet ecologists have often attempted to work out principles on natural systems whose size, taxonomic complexity, or ecological life span presents great handicaps.

The science of ecology covers several hierarchical levels: individual organisms, species populations, communities, ecosystems, landscapes, and even the global scale. To some extent the science is fragmented because of this wide spectrum of hierar chical levels (Hedgpeth, 1978; McIntosh, 1985), and antagonistic attitudes arise sometimes between ecologists who specialize on one level. This situation is often the case between those studying the population and ecosystem levels. For example, some population ecologists do not even believe ecosystems exist because of their narrow focus on the importance of species to the exclusion of higher levels of organization. These kinds of antagonistic attitudes are counterproductive, and conscious efforts are being made to unify the science (Jones and Lawton, 1995; Vitousek, 1990). Ulanowicz (1981) likens the need for unification in ecology to the search for a unified force theory in physics (for gravitational, electromagnetic, and intranuclear forces), and he suggests network flow analysis as a solution. However, as noted by O'Neill et al. (1986): "Ecology cannot set up a single spatiotemporal scale that will be adequate for all investigations." In this regard, scale and hierarchy theories have been suggested as the key to a unified ecology (Allen and Hoekstra, 1992), but even this approach does not fully cover the discipline. Clearly, ecological engineers need more than just information on energy flow and nutrient cycles. Knowledge from all hierarchical levels of nature is required, and a flexible concept of the ecosystem is advocated in this book (Levin, 1994; O'Neill et al., 1986; Patten and J0rgensen, 1995; Pace and Groffman, 1998). Ecosystem science has become highly quantitative with the development of generalized models and relationships (DeAngelis, 1992; Fitz et al., 1996). Although not completely field tested and verified, this body of knowledge provides a basis for rational design of new, constructed ecosystems. Using analogies from physics, perhaps these models will fill the role of the "ideal gases" (Mead, 1971) or the "perfect crystals" that May (1973, 1974a) indicated in the following quote: "... in the long run, once the 'perfect crystals' of ecology are established, it is likely that a future 'ecological engineering' will draw upon the entire spectrum of theoretical models, from the very abstract to the very particular, just as the more conventional branches of science and engineering do today." In this text several well-known ecological models (such as the logistic population growth equation and the species equilibrium from island biogeography) are used throughout to provide a quantitative framework for ecological engineering design.

As a final aside to the discussion of the relationship of ecology to ecological engineering, an interesting situation has arisen with terminology. Lawton and others have begun referring to some organisms such as earthworms and beavers (Gurney and Lawton, 1996; Jones et al., 1994; Lawton, 1994; Lawton and Jones, 1995) as being "ecosystem engineers" because they have significant roles in structuring their ecosystems. While this is an evocative and perhaps even appropriate description, confusion should be avoided between the human ecological engineers and the organisms ascribed to similar function. In fact, this is an example of the fragmentation of ecology since none of the authors who discuss animals as ecosystem engineers seem to be aware of the field of human ecological engineering.

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