Trophic Cascade

A young forest service employee named Aldo Leopold, charged with killing wolves in New Mexico in the early 1900s, started to notice that as the wolves died off, the deer population boomed and ate all the plants to nothing. In his groundbreaking piece “Thinking Like a Mountain,” from his book Sand County Almanac, Leopold put forth an idea 50 years ahead of his time: predators regulate ecosystems.

Since the reintroduction of wolves to Yellowstone National Park in 1995, we have learned much about the effect large carnivores have on an ecosystem. In the past, it was largely thought that an ecosystem was built from the bottom up, with plant life as the foundation from which everything else grew. Once healthy plants were established, insects, small rodents, birds, larger herbivores, and finally the top predators, all fell into balance with each other. Almost all conservation and species reintroduction efforts were based on this theory. In a damaged area, biologists would first try to rebuild the plant life before doing anything else. However, some ecosystems could not be fixed before reintroducing an endangered top-level animal. In Yellowstone National Park, the U.S. Fish and Wildlife Service was required by the Endangered Species Act to reintroduce wolves before balancing the plant base and herbivore populations.

In the years since the wolf reintroduction, Yellowstone has become a prime scientific laboratory for wilderness observation and ecosystem recovery. Scientists have come from around the world to watch the effect wild wolves have on the park. We have discovered that an ecological effect called the “trophic cascade” has taken over Yellowstone, with the wolves initiating a more natural ecosystem balance than has been seen in over 65 years.

The idea of a “trophic cascade” is relatively basic. The term “trophic” refers to the different levels of a food chain (with plants being one trophic level, insects the next, all the way up the ladder to mid-level and top predators). However, the “cascade” forces us to look at the traditional food chain from a different perspective. Picture a small stream flowing through the woods — then the stream comes to a waterfall, or cascade. As the stream falls over the edge of the cliff, it hits a rock and splits, then each of those waterfalls hit another rock and splinter again. You end up with a single stream at the top scattering out into many different and unique tributaries, inlets, and cascades. Now, put the two terms together: trophic cascade. We are learning that a large carnivore at the top of the food chain functions as the origin of this little stream — its effects on the rest of the ecosystem splinter out over all of the trophic levels. In other words, when wild wolves return to an ecosystem, by chasing and hunting their prey and competing with other species, they help restore a natural balance.

Since wild wolves have returned to Yellowstone, the elk and deer are stronger, the aspens and willows are healthier, and the grasses taller. When wolves chase elk during a hunt, the elk are forced to run faster and farther. As the elk run, their hooves aerate the soil, making it prime for water retention and allowing more grasses to grow. Since the elk cannot remain stationary for too long, aspens and willows in one area are not heavily grazed, and can therefore fully recover between migrations. As with the rest of the country, coyote populations were nearly out of control in Yellowstone before the wolves returned. Now, the coyotes have been out-competed and essentially reduced by nearly 80% in areas occupied by wolves. The coyotes that do remain are more skittish and wary. With fewer coyotes hunting small rodents, raptors like the eagle, hawk, and osprey have more prey and are making a comeback. The endangered grizzly bears successfully steal wolf kills more often than not, and thus have more food to feed their cubs. The grizzly bears also benefit from the vegetation regrowth, and in turn, as top predators, help reinforce the effect of the wolves on prey species. In essence, we have learned that by starting recovery at the top of the food chain, with predators like wolves, the whole system benefits. A wild wolf population actually makes for a stronger, healthier, and more balanced ecosystem. From plant, to insect, to people — we all stand to benefit from wolves.

With only 5% of our nation’s wilderness left, people are recognizing the importance of complete ecosystems in keeping all of us healthy. With new knowledge of trophic cascades, we can now begin to focus wilderness recovery efforts on a wider variety of ecosystems. Using Yellowstone as an example, we can teach the world about the wolf’s positive and vital role in nature.