It is interesting to contemplate a tangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner, have all been produced by laws acting around us.
—Charles Darwin, 1876
Charles Darwin illustrated the action of natural selection with wolves and deer. If, he reasoned, the fleetest deer increased in numbers during the season when wolves are hardest pressed for food, then the swiftest wolves would have the best chance of surviving, and thus be favored by natural selection. But examples like that of wolves preying on deer is exactly why natural selection came to be seen as Lord Tennyson’s “Nature, red in tooth and claw.” How could grisly evolution by natural selection possibly provide a basis for ethics?
The answer to this question is usually found not in Darwin’s On the Origin of Species, from where the wolf and deer example is taken, but in Darwin’s The Descent of Man, where Darwin discusses (among other things) the evolution of a “moral sense” in humans and other species. However, in the mid-twentieth century, an influential scientist named Aldo Leopold found in Darwin a different answer to this question of an ethical foundation. Leopold—who at various points in his life was a forester, a wildlife manager, a conservationist, and a professor—developed what he called a land ethic, whereby we would extend our ethical respect, and community of moral responsibility, not just to other humans, but to other species and to the land community as a whole. Leopold thought that our ethics, dealing only with the relations between humans or between humans and society, were incomplete, and so he developed a land ethic to supplement them. And, although this is not well recognized, Leopold’s land ethic was influenced by Darwin’s idea of a struggle for existence, discussed in On the Origin of Species.
Darwin indicated that the struggle for existence could take many forms: two canines fighting, a plant on the edge of a desert struggling against the drought, or plants struggling against other plants for space. But within this struggle, he clarified, there was also dependence, usually between distantly related species. For example, mistletoe is dependent on the various trees on which it lives as well as on birds that disseminate mistletoe seeds. He similarly suggested that the numbers of prey, such as partridges, grouse, and hare, depend on the numbers of “vermin” (i.e., predators) that are in the area. For Darwin, “plants and animals, remote in the scale of nature, are bound together by a web of complex relations,” where even something as simple as the presence or absence of cattle could have a huge effect on the number of other species present in an area.
Darwin’s idea of dependence formed the backbone for Leopold’s land ethic. Leopold saw an ethic, in ecological terms, as “a limitation on freedom of action in the struggle for existence.” He stated:
All ethics so far evolved rest upon a single premise: that the individual is a member of a community of interdependent parts... The land ethic simply enlarges the boundaries of the community to include soils, waters, plants, and animals, or collectively: the land.
Leopold described a “land pyramid” where the bottom layer is soil, on which rests a layer of plants, on which rests a layer of insects, on which rests a bird and rodent layer, and through various animal groups to the apex where the largest carnivores are. According to Leopold, “each successive layer depends on those below it for food and often for other services, and each in turn furnishes food and services to those above.” The land pyramid consists of a “tangle” of food chains, which are just “lines of dependency” for food and other services. A functioning land pyramid, Leopold tells us, depends on the cooperation and competition of its diverse parts, parts which, it is critical to note, include abiotic components like soil and water as well as non-human organisms.
Importantly, humans are also very much a part of these food chains and the land community more generally. In a lecture in 1941, Leopold tells the story of a Wisconsin farmer who wants more cows and so clears a slope in order to have more pasture space. But the clearing of the slope causes gullies to be formed, leading to soil erosion and flooding, and eventually the trout in nearby streams become suffocated with mud. As a result, everyone suffers: the farmer who has lost soil fertility, taxpayers and neighbors who have to pay the cost of flooding, fishers who have no more trout to fish, and wildflowers, partridge, and woodcock who disappear along with their lost habitat. Each of these members of the land community—human, non-human, and abiotic—is interdependent; effects on one have downstream effects on others.
We are used to developing systems of rules to guide ethical relations between interdependent individuals in a human community and we are used to valuing our human communities for their own sake. What Leopold is telling us is that we need to recognize that our communities are larger than we often recognize: They extend beyond the human members to include all interdependent beings. As undeniable moral agents and members of this land community, we have ethical obligations toward other species and toward the land community as a whole. Therefore, we need to extend our ethics, the scope of our morality, to all the interdependent members of our land community and to the land community as a whole–for their sake, for the community’s sake, and for our own sake. We need to learn to “think like a mountain,” recognizing that just as a deer herd lives in mortal fear of its wolves, so does a mountain live in mortal fear of its deer. The fear of the mountain stems from the damage that an overpopulated deer herd can cause in the absence of the wolf predators that previously kept its population size in check but whom we humans extinguished.
If we are to take the lessons of the land ethic seriously, we need to act so as to preserve interdependencies, to maintain the food chains that allow our land communities to continue to function, cycling energy and nutrients through the land pyramid and retaining the capacity of land to self-renew. In other words, we have a moral obligation to preserve land health and should act accordingly.
Yes, nature can be red in tooth and claw. But those very fights to the death, at the core of the struggle for existence and natural selection, are among the interdependencies that sustain us and the whole community of life. This is a lesson that we ignore at our own peril. Humans are “only fellow-voyagers with other creatures in the odyssey of evolution.” As we contemplate ever more drastic modifications to the environment that will degrade land health, whether through climate change, habitat destruction, or other means, we had best take Leopold’s land ethic, a necessary expansion of our moral thinking, to heart.
 Darwin, C.R. (1874). The Descent of Man, and Selection in Relation to Sex. (2nd ed.). London: John Murray.
 Here and throughout this essay the term “ethic” or “ethics” should be seen as synonymous with the term “morality.”
 For a more extensive discussion of this claim see, Millstein, R.L. (2015). Re-examining the Darwinian Basis for Aldo Leopold’s Land Ethic. Ethics, Policy & Environment, 18, 301–317.
 Darwin, C. (1876). The origin of species by means of natural selection, or the preservation of favoured races in the struggle for life. (6th ed.). London: John Murray.
 Leopold, A. (1949). A Sand County Almanac and Sketches Here and There. New York: Oxford University Press.
 Leopold, A. (1941). Conservation of natural resources. The Aldo Leopold Archives, University of Wisconsin, Madison, WI.http://digicoll.library.wisc.edu/cgi-bin/AldoLeopold/AldoLeopold-idx?type=turn&entity=AldoLeopold.ALYale.p0289&id=AldoLeopold.ALYale.
Thanks to Ted Geier, Curt Meine, and Bob Sandmeyer for helpful comments.
Image credit: Carpenter Bee with Pollen courtesy of Wikimedia Commons.