Wolves' Leftovers Are Yellowstone's Gain, Study Says

John Pickrell
for National Geographic News
December 4, 2003
Amid controversy, gray wolves (Canis lupus) were reintroduced to
Yellowstone National Park in 1995. Now, new studies are hinting at the
added benefits that the once-spurned carnivore have rapidly brought to
the preserve.

Research published last month revealed that—unlike other top predators—Yellowstone's wolves routinely leave unfinished elk (Cervas elephas) and moose (Alces alces) carcasses. These provide essential scraps for scavenging coyotes (Canis latrans), eagles, and other animals. Related work suggests that these carcasses provide dinner more consistently, and for more species, than remains discarded by human hunters.

Other recently published findings show that wolves may also be rebalancing Yellowstone's ecosystems. Some streamside trees, such as species of cottonwood (Populus) and willow (Salix) are growing vigorously once more in areas overgrazed for much of the last century, researchers wrote.

"Part of the integrity of the ecosystem has been restored," said Christopher Wilmers, an ecologist of the University of California, Berkeley, and lead author of two of the studies. The findings vindicate the decision to bring wolves back, he said.

Wiped Out

Wolves were systematically hunted in Yellowstone and much of the Western United States from the 1800s onwards. Yellowstone's last pack was eliminated in 1926.

"In the early 1900s no one stopped to consider the ecological role of wolves," commented Robert Beschta, a forestry scientist at Oregon State University in Corvallis. "Wolves were considered a predator with no value and seen as a huge constraint on allowing a productive ecosystem to flourish," he said. Wolves, mountain lions (Puma concolor), and coyotes (Canis latrans) were all targeted as threats to livestock and game, he said.

When the National Park Service first considered reintroducing wolves in the late 1980s, critics warned that the predators might seriously impact elk, deer, and moose herds. Nevertheless, 31 wolves were re-introduced in 1995 and 1996 to Yellowstone, which spans portions of Idaho, Montana, and Wyoming. That population, now living in the vast park and its surroundings, had swelled to 220 animals in 21 packs by the end of 2001.

While elk numbers have been reduced by around 18 percent since the mid-1990s, few devastating impacts associated with the reintroduction program have yet been recorded, said Wilmers.

Feeding the Masses

Instead, the park is now enjoying some wolf-related benefits. According to Wilmers and colleagues' study, published in the November Journal of Animal Ecology, wolves may be providing other scavengers with more regular meals than they've had since the early 20th century.

Mountain lions and grizzly bears (Ursus arctos) have a habit of sticking close by their kills, even when full. Wolves, on the other hand, tend to move away to "sleep off their meal," said Wilmers, making meat available for scavengers.

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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