Ecological sustainability as the fourth landmark in the development of conservation ethics.

Aldo Leopold, in "The Land Ethic," made 2 important contributions to conservation ethics: he emphasized the community and ecosystem levels of organization and he explicitly included people as members of the biotic community. Leopold's writings remain eloquent, inspirational, and influential, but the ideas he describes are inherently complex, and ecological science has continued to evolve since "The Land Ethic" was published in 1949. We used 4 sets of quotations from Leopold's essays to develop our commentary on the meaning of and challenges in interpreting his work and to explore the ongoing development of conservation ethics: the "A-B cleavage" (Leopold's description of the contrast between utilitarian value versus a broader definition of value in nature), "land health" and the rightness of human action, the right of all species to continued existence in natural populations "at least in spots," and humans as "plain member[s] and citizen[s]" of the "land-community." We define the broader function of land and land health in "The Land Ethic" as including completeness, dynamic stability, and self-renewal in a way that incorporates the needs of humans and all other species. We argue that the consequences of implementing Leopold's land ethic include multiple conservation goals nested within an overall systems approach and that conservation science must clarify the implications of Leopold's ethic by quantitatively investigating and defining large-scale, system-level ecological sustainability. At this scale, land use will encompass areas ranging from large expanses of wilderness to areas dominated by humans.