The reintroduction of wolves created the chance to observe how a top predator influences its plant-eating prey and how changes in those prey influence plants.

By reducing the abundance of herbivores or changing their feeding behavior, predators free plants from being eaten. This series of effects is called a “trophic cascade.”

It is too soon to know for sure if this trophic cascade is actually happening, and how extensive it might be—or if it is just one of many factors at work. Ongoing, long-term scientific research will continue to examine the complex fabric of the Greater Yellowstone Ecosystem.

Effects of Wolves

Accumulated studies show that the loss of wolves from the food web on the northern range in the 1930s led to a loss of willows and other woody plants by allowing excessive grazing by elk.

Most researchers agree that reintroduced wolves have caused elk to change their behavior and many studies have shown a correlation between the presence of wolves and increased growth in willows. However, not all scientists agree that this relationship is causal.

For example, some researchers say elk don’t linger in willow or aspen areas where visibility is poor and that this behavioral change prevents them from eating as much willow or aspen. Other scientists argue that fluctuations in the availability of groundwater explain the growth patterns in woody vegetation.

Ecologists have documented a substantial rise in temperature in the northern range: from 1995 to 2005, the number of days above freezing increased from 90 to 110. Changes in precipitation and effects of global climate change are also affecting vegetation growth.

Wildlife Watching

The northern range of Yellowstone is one of the best places in the world to watch wolves.

Animals are Dangerous

• Do not approach or feed any animal.
• Bison and elk have injured people.
• Stay 100 yards (91 m) from bears and wolves.
• Stay 25 yards (23 m) from all other animals.

You are responsible for your safety.

Think Safety, Act Safely. Yellowstone is a Dangerous Place.