And the result of that was fairly dramatic.
The researchers found that areas abandoned by elk were filled with new growth, not just of willows and aspen that elk normally feed on, but bushes and grasses and smaller plants that protect stream banks.
"Once browsing decreases, the plants are free to grow, and a number of different ecosystem functions can be restored," Ripple says.
Beavers, which had become rare in some areas, moved back en mass. Their dams created new ponds and flooded wetlands that had been dry for decades, paving the way for the return of many other species, even insects and migratory birds.
It was, Ripple says, an amazing thing to see. And totally unanticipated.
The return of a feared predator had lead to the restoration of various habitats, thus becoming a critical ally in the effort to maintain bio-diversity.
It's an interesting footnote to a tragic era. Wolves weren't the only targets in the early 1900s.
In the Yellowstone area alone, 121 mountain lions were known to have been killed from 1904 through 1925, and 4,350 coyotes were exterminated from 1907 through 1935. And 136 wolves were killed from 1914 through 1926, by which time they were completely wiped out.
But Canis lupus is back.
The U.S. Fish and Wildlife Service and the National Park Service began restoring gray wolves to Yellowstone and other areas in 1995. Fourteen wolves were brought in from Canada, held in temporary pens for 10 weeks as they acclimated, and then released into Yellowstone.
Others were brought in later, and they rapidly reproduced. By the end of last year, there were at least 174 wolves in the region, a total of 13 or 14 packs.
Of course, a lot of elk died along the way, and nearby ranchers still blame stray wolves for the loss of some livestock, so the program is still hotly debated.
But what is emerging quite clearly is we really don't know what kind of impact we're likely to get when we just get out of nature's way. One thing seems clear. The natural balance in Yellowstone is returning.
Just ask the beavers. And the birds. And a bunch of other critters that have returned to their ancestral homes.
Lee Dye's column appears weekly on ABCNEWS.com. A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.