These diseases struck the wolves much faster than scientists expected, said Penn State ecologist Peter Hudson, who is a coauthor on the study, published in the September issue of the Philosophical Transactions of the Royal Society B. Hudson still expects the wolves to pull through in the long run.
"Healthy ecosystems are full of parasites," he said. The reintroduced wolves had no natural immunity to these diseases, but eventually he thinks the disease organisms and the wolves will strike a balance.
By tracking the way outbreaks are spreading among the reintroduced wolves, he and his colleagues hope to learn lessons they can apply to understanding the dynamics of other emerging diseases.
"People used to think parasites and pathogens were of no importance to ecology," he said. Now, scientists are realizing the pathogens are part of the environment and can cause animal populations to go through big oscillations.
One thriving wolf pack, called the Druids, was hit with mange in 2009 and died out the next year, said Penn State ecologist Emily Almberg, who was lead author on the paper.
The impact of the wolves continues to be dramatic. Since their reintroduction, she said, exploding populations of elk have declined, allowing some plants to rebound, along with animal species that depended on them.
No one knows exactly how many wolves lived in the region that became Yellowstone National Park before the late 1800s, when humans wiped them out. The wolves were exterminated in a massive, government-led campaign that employed a combination of shooting, germ warfare, and poison.
By the 1940s, wildlife conservation pioneer Aldo Leopold first suggested that wolves played a role in the ecology and should be reintroduced, said Daniel Stahler, a biologist with the Yellowstone Wolf Project and the Yellowstone Center for Resources.
There was tremendous resistance from ranchers, who feared wolves would leave the park and kill their cattle. Meanwhile, elk populations were exploding.
Finally, in 1995 and 1996, 41 wolves were brought from Canada and released into the park, said Stahler. By 2003, they reached a peak of 170. Now, they are down to 100, and Stahler agrees with the Penn State researchers that the numbers are likely to fluctuate but not collapse.
The elk population has dropped from 20,000 in the mid-1990s to 8,000 today, he said. Scientists don't know for sure whether the drop is entirely caused by wolf predation or also by other factors such as drought, which affects the plants the elk depend upon for food.
The decline in wolves may also reflect a rebalancing of the ecosystem. Wolves are territorial; their leading cause of death is being killed by another wolf in territorial disputes.
While the wolves have eaten enough elk to reduce their numbers, they have also changed the elks' behavior, said Penn State's Almberg. The wolves keep the elk on the move, "so they can't keep hammering at one willow or aspen stand" and destroying it.
Meanwhile, ecologists have seen a cascade effect on the landscape, with more willow and aspen surviving, which in turn benefits warblers, beavers, and other animals that depend on those trees.
Scavengers such as ravens and grizzly bears also benefit from the carcasses left by wolf kills.
The researchers saw the first mange cases in 2002, said Almberg, and the first outbreaks of canine distemper in 2005.
Distemper, she said, is a highly contagious viral disease that affects the nervous system and kills some adult wolves and most wolf pups.
Mange is a skin disease caused by mites, and while it's not fatal by itself, infected wolves can bite and scratch themselves so much that they die from secondary infections, or lose so much fur that they freeze to death. The outbreak, she said, went through most of the park by 2007.
In the future, Almberg and Hudson are calling on locals and tourists to become citizen scientists and e-mail pictures they take of the wolves to email@example.com. "Visitor photographs are really helpful. . . . So many people are out there every day taking photos," Almberg said. "There's no way our staff could re-create that field effort."
Contact Faye Flam at 215-854-4977 or firstname.lastname@example.org, or follow on Twitter @fayeflam. Read her blog at philly.com/evolution.