More diversity, less disease in wild animal populations? | Outside
With its diversity of predators, could Yellowstone National Park be a disease-free island in a surrounding sea of chronic wasting disease?
That’s speculation based on a recent study by Ellen Brandell who, along with other scientists, built a model to analyze the role predators play in removing diseased animals from the environment.
“The Yellowstone ecosystem is an exciting area to study this because there is a rich community of predators and CWD has just started infecting elk and deer,” Blandell wrote in a blog post on the Animal Ecology in Focus.
Dan MacNulty of the Department of Wildland Resources and Ecology Center at Utah State University has studied wolves in Yellowstone for years. He collaborated with Blandell on his study and said, “There is a growing body of scientific research, including our recent study, that predicts that Yellowstone National Park will become a mostly CWD-free island in a sea of CWD infection. thanks to its diversity and abundant community of large carnivores.
Doug Smith, biologist responsible for wolves in Yellowstone National Park, also collaborated on the study. He said the modeling was missing key information: how effectively wolves and cougars prey on CWD-infected elk and deer.
If predators identify a sick animal early in its infection, the effect will be much greater, he said.
Another question is how many infected animals will the predators kill? And will the mortality rate of sick animals be higher than that of healthy animals?
Despite shortcomings in some knowledge, Smith said the model shows that predators could help slow the spread of chronic wasting disease by clearing diseased animals from the landscape.
“I would say that’s one of the benefits of having an intact predator community,” he said.
By collecting brainstem material from elk and deer that predators have killed and analyzing it for CWD, Yellowstone researchers are trying to uncover infection and mortality rates in the park. Smith and his colleagues also capture elk every winter and test them for disease.
Smith also hopes that over time evidence can be gathered to support the CWD theory.
“The utility of models like this is a deeper understanding of complex ecological relationships, but the models also require simplifying certain processes and should be interpreted with some caution,” Brandell wrote.
Paul C. Cross, Will Rogers, Nathan L. Galloway, Daniel R. Stahler, John Treanor, and Peter J. Hudson also collaborated on Brandell’s modeling.
Scientists already know that predators often kill sick, old, young, and weak prey that are easier to take down. So maybe mountain lions and wolves could play a role in eliminating elk, deer, or moose that have chronic wasting disease, according to Brandell’s study hypothesis.
A 2010 study in Colorado’s Front Range “showed that mule deer killed by cougars were more likely to be infected with CWD than mule deer killed by hunters,” Colorado State University reported. A 2008 study, however, found that “such selective predation by mountain lions…did not limit CWD transmission in deer populations with high infection rates.”
The theory was that because lions ambush their prey, instead of chasing like wolves, cougars might be “less likely to detect sick animals than wolves”.
Reducing the number of animals, with or without chronic wasting disease, is one of the tools national wildlife agencies use to reduce the spread of the disease. This is based on the fact that CWD disperses more easily if animals are in close contact. He also acknowledges that hunters can remove sick animals from the population, sometimes before they show symptoms of infection.
Viewing predators as another tool to keep disease prevalence low is an argument wolf advocates have long made to the Montana Fish and Wildlife Commission. With that in mind, groups like Wolves of the Rockies have sought to reduce wolf hunting and trapping quotas in the state, so far to no avail.
In Yellowstone, wolves primarily eat elk, making up about 96% of their diet in winter and 85% in summer, according to the National Park Service. Cougars rely on elk for about 55% of their diet, with about 45% coming from mule deer, the Yellowstone Cougar Project found.
Chronic wasting disease tends to infect adult male deer more often than females. Elk appear somewhat resistant to the disease, although Montana detected its first infected elk in 2019 and a Wyoming elk shot in Grand Teton National Park tested positive for CWD in 2020.
The first detected case of chronic wasting disease in Montana in wild deer occurred in 2017 in Carbon County in south-central Montana. In Wyoming, the disease has been making its way north and west since it was first identified in 1985.
The disease, which damages the brain of the infected animal, is always fatal. The abnormal proteins that cause CWD, called prions, spread from the bodily fluids or feces of an infected animal. Once in the environment, prions can survive for years, making their eradication difficult.
So far, 27 states and two Canadian provinces have detected animals infected with CWD. The disease has also been discovered in South Korea and Norway.
Brandell’s study comes following the October release of a US Geological Survey study documenting the benefits of scavengers — from ravens and crows to coyotes and golden eagles — as landscape sanitizers. The study placed healthy bovine fetuses in different types of habitats. Cameras were installed to film which animals arrived to feed and how long it took before the fetuses were eaten.
The goal was to mimic when an elk, which may be a carrier of brucellosis, aborts its fetus. Birthing equipment is thought to be one of the main ways brucellosis spreads.