The goal of restoration ecology is the restoration of a degraded ecosystem or the creation of a new ecosystem to replace one that was lost. The primary purpose of these actions is to add ecological value for its own sake, rather that to provide some useful function for society. In this sense, restoration ecology differs in emphasis from other subdisciplines of ecological engineering such as treatment wetlands or soil bioengineering where ecosystems are constructed to provide a useful function first (i.e., wastewater treatment or erosion control) and to add ecological value as a secondary objective. In fact, restoration ecology sometimes attempts to restore or replace ecosystems to a natural state that existed before human presence was dominant (except for aboriginal peoples). Thus, there is a direct and logical connection between restoration ecology and environmentalism because of the primary focus of restoring systems to their natural condition.
In general terms, environmentalism is a popular movement that arises from the social desire to maintain natural ecosystems within landscapes that are dominated by humans. In the past, when human population densities were relatively low, this movement was motivated by idealism. However, as population densities have increased, there is now a growing awareness that natural ecosystems provide real life support functions for humanity as a by-product of their natural existence, which makes the past idealism become pragmatic and adaptive. Environmentalism takes many forms, ranging from the establishment of parkland in urban environments through the protection of wilderness and endangered species, to the rise of political parties based on this theme. Here, two dimensions are explored that can be tied to engineering.
At one end of the environmentalism continuum is the application of scientific approaches to conserving biodiversity, which is the concern of the field of conservation biology. This important field combines elements of ecology and genetics along with public policy analysis for maintaining as much of the Earth's biodiversity as possible. Restoration ecology and conservation biology are related because the restored ecosystems provide habitat for species threatened by human impacts (Dob-son et al., 1997; Jackson, 1992; Jordan et al., 1988). One activity in conservation biology that has some similarities with engineering practice is the design of preserves based on island biogeography (see Chapter 4). The similarities involve the use of theoretical equations for design, which justifies reviewing the topic here.
The theory of island biogeography was outlined in the 1960s by Robert MacArthur and E. O. Wilson (MacArthur and Wilson, 1963, 1967). It basically described the origin and maintenance of ecological species diversity on oceanic islands with extrapolations to habitat islands, such as patches of forest in an agricultural landscape. The theory was explosively popular among academic scientists who applied it to a tremendous number of situations in the 1970s and 1980s, such as caves (Culver, 1970), mountaintops (Brown, 1971), reefs (Molles, 1978; Smith, 1979), lakes (Keddy, 1976; Lassen, 1975), rivers and streams (Minshall et al., 1983; Sep-koski and Rex, 1974), plant leaves (Kinkel et al., 1987), host-parasite systems (Tallamy, 1983), and artificially constructed habitat islands (Cairns and Ruthven, 1970; Dickerson and Robinson, 1985; Schoener, 1974; Wallace, 1974). The simplest expression of the theory explained the number of species that could be supported on an island as a function of the area of the island and its proximity to other islands which act as sources of species that might immigrate. The equilibrium number of species that could be supported is a function of the number of species available to immigrate and the balance between immigration and extinction rates, as given by the following equation dS/dt = k(ST - S) -k2 S (5.1)
where kj(ST - S) = immigration rate k2S = extinction rate S = the number of species on the island ST = the total number of species on nearby islands that can immigrate to the island k1 and k2 = proportionality constants t = time
Thus, when an island is first exposed to colonization, as might occur after a hurricane removes the biota, the number of species increases due to an excess of immigration over extinction until a dynamic equilibrium between the two processes is reached. The number of species could decrease (or "relax") if the area of the island declines, as occurs when sea level rises forming land bridge islands. In this case extinction exceeds immigration until a new equilibrium is established. The theory also drew on the species-area curve. Area figures into the equation indirectly with the values of the proportionality constants. In general, the extinction rate decreases as island area increases, while immigration rate increases as island area increases. The proximity to source islands also leads to increased immigration rate.
Together, these expressions formed the quantitative foundation for the island biogeographic theory of MacArthur and Wilson. They were tested in many settings, and generally they were found to provide explanations for species diversity patterns. Not unexpectedly, the theory was also quickly applied to the problem of reserve design in conservation biology, which was just emerging in the 1970s. This was an obvious application because a reserve is like an island of natural species within a surrounding landscape of agricultural, urban, or other human-dominated land use. In the mid-1970s a number of papers were presented that applied island biogeogra-phy theory to reserve design (Diamond, 1975; Diamond and May, 1976; May, 1975; Sullivan and Shaffer, 1975; Terborgh, 1975; Wilson and Willis, 1975). Rules of reserve design evolved from the theory of island biogeography in a systematic fashion. Of course, the species-area equation indicated that reserves with larger areas would support greater numbers of species, which was a desirable objective. The species equilibrium equation also indicated that the number of species supported in a reserve could be increased by increasing the immigration rate. This could be achieved by placing the reserve near other reserves that provide a source of species
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