How 14 Wolves Moved Rivers In Yellowstone

by Eric Daniels

The term trophic cascade refers to an ecological phenomenon. This effect occurs when a predator leaves or enters the food chain. Because of the predator’s presence, other plants and animals in the ecosystem naturally respond by moving, eating, and breeding differently.

Trophic cascades can have fascinating and drastic effects on an environment. A negative trophic cascade nearly destroyed Yellowstone National Park, and a positive trophic cascade brought it back.

Yellowstone Before 1995

Yellowstone National Park in 1995 was a very different place than the one you can visit today. The hills were dry and desolate, the trees were sparse, and the rivers were flat and shallow.

This version of Yellowstone was caused by a natural imbalance. For over 70 years, there had been no wolves in Yellowstone. They had all been hunted away, either killed for their pelts or to keep them away from local farms.

Because there were no wolves, the elk had been allowed to run rampant. Elk are beautiful creatures, but when left unchecked, they consume every green thing in sight. The plants and trees couldn’t keep up, and Yellowstone was drying out.

The Reintroduction of Wolves

In January of 1955, 14 gray wolves were captured from across the continent and reintroduced to Yellowstone National Park. Biologists held their breath as the wolves began to do their work.

Wolves hunt. Specifically, wolves hunt elk and deer. Luckily for these wolves, the elk at Yellowstone hadn’t seen a natural predator in generations. They were easy to kill, and the brand-new wolf pack flourished.

So the wolves had cubs. And they ate more elk. And things started to change more quickly than anyone expected.

The Behavior of Smart Elk

It didn’t take long for the elk to figure out that a new predator was in town. The local herds suddenly decreased in population, and the remaining elk needed to find a new way to survive.

The elk’s first line of defense was to stop spending time in open areas where they could be easily hunted. They began to act like their ancestors did, hiding in the forest and coming out to graze when the coast was clear.

The Land Begins to Regenerate

Without deer eating all of the vegetation, the open valleys and gorges of Yellowstone started to regenerate. Grass grew back first, followed by trees, bushes, and flowers.

The elk didn’t dare eat these new plants; if they came out in the open, they might get caught. The trees grew taller, put down strong roots, and made Yellowstone green again.

Songbirds and Beavers Return

Green land meant that new animals can survive. First, the birds came back, flying in huge flocks to build nests on the newly-grown trees. Then the beavers returned, and they used the trees to build river dams.

These dams created deep pools in the river where otters and fish could thrive. Just a few years after the wolves returned, Yellowstone’s entire ecosystem was back in action.

Killing Off the Predators

The wolves also did something else that was amazing. Wolves and coyotes don’t get along, so the wolves killed off most of the coyote packs that inhabited Yellowstone.

Coyotes can’t hunt elk, but they can hunt off other small creatures. Without coyotes, Yellowstone once again became home to squirrels, field mice, and countless other tiny creatures that love to call the park home.

The Rivers Start to Reshape

All of these changes had an effect that the biologists weren’t planning to see. Before 1995, Yellowstone’s rivers were in a terrible state. The soil was slowly eroding, allowing the rivers to get wider and more shallow. Shallow water doesn’t travel as far, and the land downstream was starting to dry up.

But the elk stopped eating the trees, and the trees put down new roots. These roots held the topsoil firmly in place and prevented the gradual erosion. Over a few years, the rivers filled back up, and the ground around them became covered in healthy grass and vegetation.

A Balanced Natural Environment

All of these changes were caused by a single pack of 14 wolves. Thank to their efforts, thousands of other plant and animal species were allowed to thrive in Yellowstone National Park.

Every ecosystem requires balance. If you remove one element, the rest might not be able to survive. Even though wolves are a dangerous predator, they’re also extremely important to the balance of the natural world.

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