There are sounds that wake you up out of a deep sleep, only to be dismissed as you fall back to sleep. And then there are sounds that rivet you, make you sit bolt upright.

That was the type of sound that woke us while we were deep in the backcountry of Yellowstone National Park. Sunrise hadn't yet come, yet we were wide awake, listening to one of the most mesmerizing sounds you can encounter in the wilds: The melodious rising and falling howl of a wolf.

It was late summer in 2008 when two friends and I were lucky enough to catch that howling. Had it been 20 years earlier, there would have been an audible hole in the park sky because there were no wolves in Yellowstone in 1988.

It was an effort launched early in the 1990s that returned the predators to the park in January 12, 1995 – 30 years ago – when 14 wolves trapped in Canada were brought into Yellowstone to kick off an audacious effort to see healthy wolf packs loping through the park.

How have they done? To find out, our guest today is Eric Clewis, the Northern Rockies senior representative for Defenders of Wildlife.