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, . ENDANGERED SPECIES 357 sity-stability concept as the most vexing and embarrassing example of conserva- tionists being provoked into exaggerating and distorting alleged values of "non- resources" (e.g., species, biotic communities, and ecosystems). He explained the issue as a case of inverted cause and effect; the most diverse communities simply have occupied the most stable environments for the longest periods of time. Stability within a community depends on species diversity (richness and compo- sition) and trophic complexity (number and strength of species interactions) of the community, the type of perturbation, and the species being perturbed (Kikkawa 1986). Goodman (1975) concluded that, even if the diversity-stability hypothesis were completely false, its basis of natural community preservation is correct, and the conservation principles it promotes are sound. Ehrlich and Ehrlich (1981) summarized ecosystem services, or ways in which ecosystems support human life. They stressed that maintaining genetic diversity is important to the continued functioning of ecosystems; only genetic variability allows evolution in response to environmental change. The world's gene pool (i.e., its species diversity) should be considered one of earth's most important and irreplaceable resources. Continuation of ecosystem services depends on the capacity of ecosystems to evolve, their resilience, and human willingness to lessen the assault on them. If the assault continues unabated, accelerated extinction may be counterbalanced by increased speciation, and a disproportion- ate number of opportunistic, r-selected species (which we often consider to be pests) may Occur in the biota our descendants inherit from us (Myers 1979a). Others have argued for preservation of biotic diversity on moral grounds. Ehrenfeld (1976) counseled against assigning economic values to nonresources. He coined the term "Noah principle" for the notion that communities and species should be conserved simply because they exist and have existed for a long time. Callicott (1986) reviewed arguments that humans have a moral obligation not to extirpate species, i.e., species have intrinsic value and therefore, a right to exist. He stated that the ethical argument for preservation of species may be most compeUing but has not been well articulated. 15.1.3 Slowing the Extinction Rate As the major causal agents of the current extinction spasm, we humans have a responsibility to reduce the rate of extinction. Fostering and applying a conser- vation or evolutionary ethic may be a good starting point, but practical means should also be used. Before they can be, the fundamental political, economic, and psychological resources of humankind must be mobilized (Ehrlich and Ehrlich 1981). The best way to safeguard living diversity-through reserves and parks- has been hampered by lack of knOWledge on which to base decisions about location and size of reserves. However, the extinction rate may be slowed by such recent developments as work of the Nature Conservancy, which tries to preserve diversity by preserving selected land areas; UNESCO's system of biosphere reserves; and emergence of a Society for Conservation Biology (Ehrlich 1987). Habitat in damaged ecosystems can sometimes be rehabilitated to enhance chances for survival of endangered or threatened species. However, restoration should not be considered a substitute for efforts to preserve natural ecosystems. Zoos, aquariums, and botanical gardens are useful for preserving organisms that cannot survive in the wild. The major disadvantages of depending on zoos as ~