Desuetude"is a state of being unused. Leopold's description reveals what happens when predators are removed from an ecosystem. Herbivores overpopulate and eventually overex-ploit their food resources. Left with no food, they die. since Leopold wrote that essay in the 1940s, biologists have learned that wolves typically kill sick, elderly, or very young prey. Rather than harming herds of deer, elk, and caribou, this strengthens the population. In general, predators-sharks, hawks, snakes, and others-take only what they need to survive. That's a good lesson for humans.