Wolves' Leftovers Are Yellowstone's Gain, Study Says

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With funding from the Park Service, Wilmers and colleagues based at Berkeley, Yellowstone, and elsewhere, set out to test the effect of reintroduction on the park's carrion-eating species. The team radio-tagged and observed wolves at kills over winter and spring between 1998 and 2001.

They found that a pack of wolves, each eating only 20 pounds (9 kilograms) or so of an adult elk weighing up to 700 pounds (320 kilograms), leave rich pickings for coyotes, magpies (Pica pica), golden eagles (Aquila chrysaetos), and other animal scavengers.

What's more, this bounty is distributed more evenly across the year: In the absence of wolves, herbivores weakened by winter hunger tend to die in a "boom" of carrion near the end of the colder months, leaving little for smaller meat eaters during the rest of the year. Prior to wolves' reintroduction, "It was the feast followed by the famine," said study co-author Wayne Getz, a researcher also at the University of California, Berkeley. "Having a large pulse of food accumulate at the end of winter is not conducive to maintaining a strong scavenger population."

Human hunters have not been effective surrogates for wolves since the 1920s either. In a second study published by Wilmers and colleagues in the November edition of the science journal Ecology Letters, the team showed that a similar problem was presented by human hunters during the January to February annual elk shooting season. Human hunters leave up to 73,000 pounds (33,000 kilograms) of entrails from dressed carcasses in the backcountry over just a six week period—far more than can be consumed by scavengers during the time span. (The seasonal hunt accounts for 80 percent of annual hunter kills in all of Yellowstone's northern range.)

In the area studied, reintroduced wolves now leave an additional 29,000 pounds (13,000 kilograms) or so of carrion, which is distributed throughout the year. Wolf kills also provide for a more diverse range of animals, said Wilmers. Since so much meat—typically aggregated in space as well as time—is left from human-hunter kills, it's not local scavengers that benefit most, but rather highly-mobile and fast-moving species such as bald eagles (Haliaeetus leucocephalus) and ravens (Corvus corax), that travel from far away.

"Ecological Chain Reaction"

This research on scavenging "illustrates important ecological benefits from the reintroduction of wolves," commented William Ripple, a forestry biologist at Oregon State University in Corvallis. It also "documents an important ecological chain reaction which is taking place because of wolf restoration," he said.

Wolves may also be vital to the maintenance of some riverside tree species. These have been slowly declining in Yellowstone, according to Ripple and Beschta's own reports, published in the October editions of science journals Forest Ecology and Management and Ecological Applications.

Beschta has documented a decline in abundance of mature aspen, willow, and cottonwood trees during the 20th century. That decline started soon after the extirpation of wolves in the 1920s, and is thought likely to be due to elk overgrazing. Others species have suffered, according to the recent studies: The decline has also been linked to falling beaver (Castor canadensis) numbers.

Research shows that browsing has suppressed the development of adult cottonwoods in particular. In some sites, scientists found thousands of small seedlings and mature trees older than 70 years but very little of intermediate age. With no fear of wolves "[elk] browsing had been preventing any seedlings from getting taller," said Beschta.

However, according to Ripple and Beschta's data, the situation has changed since 1997, in so-called "high risk" riverside sites. These are sites where foliage limits a herbivore's view, or escape is hindered by riverbanks, gullies, dead trees, and similar features. Here, cottonwood and willow have grown to 6.5 to 13 feet (2 to 4 meters) in height. In contrast "low risk" sites with few obstructions, have changed little over the same period.

All of these studies document "complimentary examples of wolves indirect effects on the ecosystem," said Ripple. "There are undoubtedly many more ecological effects of wolves that we still need to learn about from this grand [reintroduction] experiment."

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