Final answer:
The reintroduction of wolves to Yellowstone National Park had a positive impact on the ecosystem by altering the foraging behavior of elk and promoting the regrowth of important vegetation.
Step-by-step explanation:
One positive impact of human alteration on ecosystems is the reintroduction of wolves into Yellowstone National Park. Biologists found that by preying on elk, wolves changed the foraging behavior of the elk and allowed the regrowth of important vegetation. This had a large positive impact on the ecosystem as a whole. When a relatively small number of individuals, like wolves, have disproportionate impacts on the ecosystem, they are referred to as keystone species.