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Study Shows Wolves Improve Yellowstone Ecosystem


Trees, animal species booming in Yellowstone due to wolves' presence.

A new study by researchers at Oregon State University shows that predation on elk by wolves has been one of the major drivers of recovery for tree species at Yellowstone following decades of decline due to over-browsing.

The study, completed by College of Forestry professors Robert Beschta and William Ripple for the Journal of Biological Conservation analyzed previous streamside vegetation studies going back to 2001 and compared them to temperature, precipitation and snowpack records. What they found was that even in the face of long term warming trends species like cottonwood, willow and aspen are showing accelerated growth in the park since the reintroduction of wolves in 1995.

Dan Hartman Via PLOS Biology
Dan Hartman Via PLOS Biology

"I was skeptical that elk, a native ungulate, could stop nearly all cottonwood recruitment," said Beschta in an interview. "But it was the elk that had damaged plant communities during the period when wolves were absent, and the reductions in elk browsing, since wolves have returned, are allowing them to begin recovering."

The recovery of the riparian and cool season trees has resulted in the reestablishment of animal species like beavers and songbirds, long absent from the area since the 1930s.

"Research results following wolf reintroduction are generally supportive of the concept that the contemporary carnivore guild has, via a trophic cascade, mediated the effects of elk herbivory on riparian plant communities," the authors wrote. "The ongoing reduction in elk herbivory has thus been helping to recover and sustain these plant communities in northern Yellowstone, thereby improving important food-web and habitat support for numerous terrestrial and aquatic organisms."

The term "trophic cascade" is one that you increasing hear concerning the impacts reintroduced wolves are having on environments following decades of extirpation. What "trophic cascade" means in terms of Yellowstone wolves is that much like a stream cascading down from one source and splitting into different branches, a top predator effects the ecosystem on all levels, from insects to elk, from grasses to trees. With proper management this would ideally result in a balanced ecosystem with no single species becoming so dominate as to alter the landscape, like over browsing by elk.

No doubt about it, the elk population has dropped quite drastically since the wolves were brought into the park. According to the National Park Service, the winter count of elk on the northern range at the time of reintroduction in 1995 was 17,000. By 2003, that number had dropped to 10,000. The count continued to drop and by 2013, about 3,900 elk were identified, the lowest number since the 1960s. The Park Service has attributed the dropping numbers not only to the reintroduced wolves, but a large and robust bear population, increased hunter harvest outside of the park and a drop in pregnancy and survival rates due to drought.

So does this evidence really suggest that wolves are helping the biotic community at Yellowstone, or is this a case of "correlation does not imply causation?" While some hunters and anti-wolf  groups might disagree, for the success of Yellowstone the data seems to speak for itself.


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Study Shows Wolves Improve Yellowstone Ecosystem