The National Park Service has been able to increase the biodiversity in Yellowstone Park by introducing wolves.
Wolves have allowed many river areas that were almost completely barren to grow back with diverse plant life by decreasing elk populations.
These elk once congregated around river areas, leaving them constantly grazed. By reintroducing their natural predators, the ecosystem has been improved.
Wolf population was originally reduced in the early agriculture days of the Western United States to protect livestock. In 1978, in all of the lower 48 states (except Minnesota) EVERY wolf subspecies was endangered.
In 1995, scientists released wolves into Yellowstone that had been caught in Canada.
Not only have they helped restore river plant species, they help feed scavenging species.