In 1926, there were no longer wolves in Yellowstone, once the natural habitat of this species. Between 1977 and the re-introduction in 1995, we have reliable reports of wolves being seen throughout the park.Most of them were either lone wolves or pairs, probably only transiting.
Finally, in 1995, grey wolf packs were reintroduced in the Lamar Valley of Yellowstone National Park and Idaho. The effects were more striking than anyone could have expected. The entire ecosystem of the national park transformed and it went so far that even the rivers changed.