According to rewilding supporters, science clearly shows that reinstating missing species is key to a healthy ecosystem.

They weren't always welcome. In the 1920s gray wolves were basically eliminated in the Yellowstone Park area.

It is defined as "a conservation effort focused on restoring sustainable biodiversity and ecosystem health by protecting core wilderness areas."

In 1995, after more than a 70-year absence, gray wolves started roaming Yellowstone National Park once again.

At the heart of what some consider a fundamental idea is the belief that "nature knows best."

Currently these "ecosystem engineers" are being returned to forests and are transforming the land for the better.

In various places around the world, rewilding can be seen with favorable results.

This, in turn, stabilized the riverbanks and cleaned the water.

Thankfully, Europe's largest land mammal survived because of a captive breeding program.

In regions of southern Africa, cheetah and lion restoration programs have proven very successful.

In the 1990s conservationist Dave Foreman, co-founder of Wild Earth magazine, invented the term “rewilding.”

When gray wolves were reintroduced into Yellowstone Park, a shift in the landscape took place.