[MUSIC PLAYING]
NARRATOR: Spring crests with a wave of green
that rises up from the valley floor
and rushes across the plateau.
Yellowstone's great herds are returning
to one of the last open ranges in North America.
In spring's fervor, a resurgent force works
to keep the system in balance--
the wolf pack.
This is the Wapiti Pack, the largest in Yellowstone.
A family and its prime, 20 members strong, and still
growing.
[BARKING]
They roam the vast interior of Yellowstone.
A territory of 200 square miles--
perfect wolf habitat.
But until recently, gray wolves were gone from this landscape.
A century ago, predators were seen as the enemy of progress.
Eliminated from nearly all of their former range.
Scientists had not yet discovered
that an ecosystem cannot thrive without both prey and predator.
In 1926, park rangers shot Yellowstone's last wolf.
No one understood the consequences.
70 years later in the winter of 1995,
two horse-drawn sleighs carried 15 gray wolves
captured in Canada, and released them in the Yellowstone back
country.
It was the beginning of one of the most ambitious restoration
projects in history.
No one knew if the wolves would stay,
if they could thrive in an unfamiliar place.
Or what the long term effects of their presence
would bring to the Yellowstone ecosystem.
But it didn't take long for the Wolves
to make Yellowstone their home.
The revival of the hunt returned the most primary dynamic
to Yellowstone--
a predator pursuing its prey.
Thanks to the return of the wolf, the world's
first national park is now recognized as one of the only
intact ecosystems of its kind.
[HOWLING]
Today, more than 100 descendants of this endeavor
roam Yellowstone.
The thriving wolf packs keep the grazing herds in check.