Before the 1900's, wolves could be found all throughout the United States.
But then in the late 1800's people had started over hunting wolves in a predator control effort to protect their livestock.
The overhunting of the wolves caused them to go extinct. A consequential increase in elk occured due to them no longer having their main predator.
The increase in elk caused an increase in the number of plants they ate. They ate all the plants, making certain species of trees go extinct from the park, such as aspen. It also caused unwanted erosion and change in the environment.
That's when people realized something had to change. Researchers traced the erosion and tree disappearance back to the extinction of wolves from the park. They decided to re-introduce wolves to the area. They got 8 wolves from Canada and put them in the park to start a pack.