Two new papers out of Colorado State University cast more doubt on long-held beliefs that the reintroduction of wolves to Yellowstone in the mid-1990s has caused a widespread “trophic cascade,” particularly within riparian areas.
The wolf-driven trophic cascade is one of the most-told ecological success stories in the Greater Yellowstone Ecosystem: A long-absent carnivore returned to the landscape and changed the behaviors and whereabouts of elk and other ungulates that had been eating willows and other lowland plants to death. Absent the overabundance of carefree browsers, river bottom plant communities bounced back and an array of wildlife returned, ultimately bringing the ecosystem into balance.
It’s not nearly that simple, says Tom Hobbs, a CSU ecology professor whose laboratory has studied willow growth in Yellowstone National Park since well before the reintroduction of wolves.
“There’s is a phenomenon in science where if you tell a story — particularly if you tell it in the popular press — and if you tell it frequently enough it becomes true,” Hobbs said in an April interview. “I think that’s what happened here.
“The story is really attractive,” Hobbs said, “but the science just doesn’t support it.”
A September 2013 paper Hobbs co-authored that ran in the Proceedings of the Royal Society rounded up the results of 30 years of willow-monitoring data in Yellowstone’s Northern Range. The study found that willows grew back significantly only when they were protected from heavy browsing and also had adequate water.
“Our landscape-level observations suggest that recovery of tall willows following reintroduction of wolves is the exception rather than the norm,” Hobbs and co-authors Kristin Marshall and David Cooper wrote. “The frequency distribution of willow heights closely resembles the distribution observed before wolves were reintroduced.”
The missing ingredient for the recovery of willows is beavers, which have been extirpated from parts of Yellowstone, Hobbs said. The large aquatic rodents’ dams stimulate willow growth.
“The reason is that willows like to have their toes wet,” Hobbs said. “If they don’t have enough water, it doesn’t really matter if they’re being browsed or not.”
Beavers have not successfully recolonized any of the areas they inhabited in the 1920s that Hobbs and colleagues studied, the Royal Society paper said.
“Beavers are there [in Yellowstone], but they’re not building dams,” Hobbs said. “There’s just not enough lumber along those small streams to build dams.”
“We went up Slough Creek, where there has been some beaver activity in building small dams,” he said. “I looked at a couple of those and they can only been described as heroic. There are beaver there, they’re making a valiant effort, but there is not the habitat that beavers need.”
A second CSU paper, also written jointly by Hobbs, Cooper and Marshall, looked at willow growth in Yellowstone over the last century.
The paper, “Interactions among herbivory, climate, topography and plant age shape riparian willow dynamics in northern Yellowstone National Park,” was published in the Journal of Ecology in February.
“We do show some evidence that there’s a correlation between elk numbers and the establishment of willows,” Hobbs said. “Establishment has gone up as elk numbers have gone down.”
The youngest age class of willows responded most to declining elk numbers, Hobbs said.
But in all other willow age classes, he said, “there is no difference” since the 1995-96 reintroduction of grey wolves.
“Our results contribute to a growing body of evidence showing that changes in growth of woody, deciduous plants following the reintroduction of wolves cannot be explained by the trophic cascade model alone,” Hobbs, Cooper and Marshall wrote in closing.
Wolves were absent from the Greater Yellowstone Ecosystem for 70 years, Hobbs said, and it took just as long for riparian areas and willow stands to degrade to the state they were in by the mid-1990s.
“It took almost a century for the effect of [not having] wolves to be expressed,” he said. “It took decades for those to develop.
“Why would we expect that they would be quickly reversed?” Hobbs asked. “Why would we expect to see immediate changes?”
Hobbs’ CSU lab, the Natural Resources Ecology Labratory, will continue monitoring willows in Yellowstone’s Northern Range for another decade, he said.
Meanwhile some mainstream media are continuing to propagate the popular theory of a wolf-driven trophic cascade in Yellowstone wetlands. One recent example is a four-minute YouTube video, “How Wolves Change Rivers,” that went viral and generated more than 4 million views.
Its British narrator and creator, George Monbiot, is essentially telling lies, Hobbs said.
“It is true that wolves eat coyotes,” he said, “and just about every other statement in that video is false.
“All of the claims about the explosion of the riparian communities ... there’s not a shred of scientific evidence that supports them,” Hobbs said. “This kind of story is a distraction from the real problems that Yellowstone faces.”