A Sand County almanac, and sketches here and there.

This combination of natural history, philosophy, and poetic writing informed the environmental movement. It is perhaps best known for the following quote, which defines Leopold's land ethic: "A thing is right when it tends to preserve the integrity, stability, and beauty of the biotic community. It is wrong when it tends otherwise." In his chapter entitled "Thinking Like a Mountain" Leopold set out the concept of a trophic cascade, pointing out that killing a predator wolf carries serious implications for the rest of the ecosystem.