Wolves were reintroduced to Yellowstone National Park in the (Rocky Mountains USA) in 1995 amidst much controversy. The last of the naturally occurring wolves in Yellowstone had been exterminated in the 1930s. While 7 years is not really a large enough span of time for anyone to honestly assess the full impact this has had on the ecology of the park but a New Scientist report in November 2002 indicates that so far the experiment has been a great success. The wolves have thrived and there are now more than 200 wolves in 19 packs living in the park. Surprisingly to some people, the Elk are still thriving as well. Depending on the weather wolves succeed in killing between 15 and 26 percent of the wolves they attack, and they don't attack adults that aren't already sick or injured. Several wolves have died from making just this mistake. It took about a year for the two species to adapt to each other.
Other side benefits include a 50% reduction in the Coyote population, which had risen far higher than its natural level in the wolves absence. Wolves not only out compete coyote for various resources, they actively kill coyotes that get to close. This has had a beneficial knock-on effect on other smaller predators such as red foxes and possibly wolverines and lynx which were suffering under the Coyote dominance. Biologists who have been monitoring the wildlife of the park state that returning the wolves has helped increase the biodiversity balance in the park, a good thing in anybody's point of view.
The only negative consideration is that the wolves do not always stay in the park and the surrounding buffer zones, occasionally bringing them into conflict with the livestock farmers of Idaho. However so far the cost has been small and the benefits large, wolves are a functional part of the world we live in.