What the Deer Are Telling Us
In 1909, a United States Forest Service officer named Aldo Leopold shot a mother wolf from a perch of rimrock in the Apache National Forest in Arizona. It was a revelatory moment in the life of the young naturalist. “In those days we never heard of passing up a chance to kill a wolf,” Leopold wrote in an essay called “Thinking Like a Mountain,” later included in his Sand County Almanac, published posthumously after his death in 1948 and which went on to sell several million copies. “We reached the old wolf in time to watch a fierce green fire dying in her eyes. I realized then, and have known ever since, that there was something new to me in those eyes—something known only to her and to the mountain.”
Leopold, who today is revered among ecologists, was among the earliest observers of the impact of wolves on deer abundance, and of the impact of too many deer on plant life. In “Thinking Like a Mountain,” he outlined for the first time the basic theory of trophic cascades, which states that top-down predators determine the health of an ecosystem. The theory as presented by Leopold held that the extirpation of wolves and cougars in Arizona, and elsewhere in the West, would result in a booming deer population that would browse unsustainably in the forests of the high country. “I now suspect that just as a deer
