In 1995, grey wolves were reintroduced to Yellowstone National Park after having been absent from the area since the early 1930s. Their 65-year absence was the result of a series of predator control programs led by the US Government. The Animal Damage Control Act of 1931, for example, granted the Secretary of Agriculture the power to:

"promulgate the best methods of eradication, suppression, or bringing under control ... of mountain lions, wolves, coyotes, bobcats, prairie dogs, gophers, ground squirrels, jack rabbits, brown tree snakes, and other animals ... for the protection of stock and other domestic animals." (Animal Damage Control Act 1931) .

Thus, by the early 1930s, grey wolves had been eliminated from Yellowstone and most of the United States in order to keep the region amenable to the widespread raising of livestock. The only area in the country where wolves remained was in Minnesota.

But attitudes towards wildlife management changed gradually over the years. Management approaches became more hands-off and shifted to a reliance on natural processes (Smith 2003). Burgeoning populations of ungulates (such as bison and elk) were seen not as a problem solved by culling, but as an imbalance that could be swayed by the reintroduction of once native predators such as wolves.

Between 1995 and 1996, 31 wolves were reintroduced back into Yellowstone National Park in Wyoming with the hope of bringing the region's ecosystem back into balance. In the past decade, the Yellowstone wolf population has grown to 130 wolves that have formed 13 packs. And those 130 wolves have directly and indirectly altered the face of Yellowstone. Wildlife biologists have noted changes in elk numbers (the population dropped from 19,000 to 11,000), vegetation structure (willow thickets, cottonwoods, and aspen have reclaimed some areas where elk grazing previously excluded them from the landscape), and changes in coyote numbers (which have fallen by half). These changes may all be related in some way to the renewed presence of wolves in the park.

The reintroduction of wolves has initiated a ripple of effects felt through the Yellowstone ecosystem. These effects have impacted predators-prey relationships, grazing behavior, and vegetation structure within the park and reveal the complex role wolves play in their ecosystem.

Find out more:

• Animal Damage Control Act. 1931. Section 426. Predatory and Other Wild Animals 7 USC 426-426c.
• Hunting Habits of Wolves Change Ecological Balance in Yellowstone (New York Times)
• Smith DW, Peterson RO, Houston, DB. 2003. Yellowstone After Wolves. Bioscience 53(4):330-340.

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