- The ‘How Wolves Change Rivers’ video is a Facebook favourite
- Based on a 2011 study that showcased how “top-down trophic cascade” can reinstate natural order
- New study takes care to pull focus away from wolves
Maybe the wolves have saved Yellowstone National Park’s rivers after all.
Back in 2014, a video titled “How Wolves Change Rivers“, narrated by British writer George Monbiot, was posted on YouTube and racked up 25 million views over the next year or two.
Way back then, that kind of explosive growth was considered “viral”. There’s an excellent chance you’ve watched a shared version on Facebook, but here it is anyway:
It’s beautifully made, and crucially, based on science.
The short version went:
- Wolves were reintroduced into Yellowstone National Park in 1995.
- Wolves started hunting elk that had been destroying willows and aspens along the park’s waterways
- The trees started coming back, and with them, beavers to build dams
- The rivers started to hold their course and less erosion was evident
- The improved habitat became a home for more mice and rabbits. More mice and rabbits meant more animals that hunted mice and rabbits.
Removing wolves – an apex predator – had sparked what it is known as a “top-down trophic cascade”. Reintroducing them sparked a reversal of that, and also inserted a revered tale into US society about how ecosystems work.
And two sides of science went to war, mainly because nothing in nature is ever that simple.
“This story — that wolves fixed a broken Yellowstone by killing and frightening elk — is one of ecology’s most famous,” field biologist Arthur Middleton wrote in the New York Times. “But there is a problem with the story: It’s not true.”
Not only that, the story was dangerous. It’s simple focus risked undermining the reputations of hard-working scientists and conservationists.
It was also a distraction from the many other challenges that natural environments faced, such as fungal infestations, gas drilling, climate change and much more devastating invasive species such as lake trout, that cannot be controlled so easily.
Wolves are back in Yellowstone. Nature fixed it. Let’s get back to our desks.
There were other factors, noted by other ecologists, as well.
The elk populations were getting hit at the same time from other directions. Severe drought, human hunting, cougars and bears, hungry from a decline in the population of cutthroat trout, were all working at the same time to reduce elk numbers.
Mountain lions and cougars also hunted elk. And as for elk themselves, “90% of what they eat is grass,” not willow, according to Utah State University ecologist Dan McNulty.
Right up to April this year, the argument continued when Colorado State University’s Natural Resource Ecology Laboratory professor Tom Hobbs said “it was important to put the wolves back, but it didn’t change the willows much”.
“It hasn’t even come close to recovery,” he said. And where it has, the recovery is more strongly linked to water supply as opposed to less elk herds munching on trees.
Now the authors of the original report that sparked it all, “Trophic cascades in Yellowstone: The first 15 years after wolf reintroduction”, are ready for round two.
“Can large carnivores change streams via a trophic cascade?” is the title of Robert Beschta and William Ripple’s submission published in Ecohydrology today.
Note “large carnivores”. Wolves are divisive, loved and hated in equal measure, especially on social media, and that kind of distraction is unhelpful.
So, having studied stream-bank willows over a 13-year period along two forks of a creek in Yellowstone National Park, first in 2004 and again in 2017, the Oregon State University scientists say the “return of large terrestrial carnivores” can indeed lead to improved stream structure and function.
“In the 1990s, elk were still keeping the willows short, usually less than 2 feet tall, and that led to stream widening – oversized cross sections of channel and a drastically reduced frequency of overbank flows,” Beschta said.
“But by 2017, willow heights greater than 6 feet were prevalent and canopy cover over the stream, which had essentially been absent in 1995, had increased to 43% and 93% along the west fork and east fork, respectively.”
Beschta does note that the healing “is in its early stages”, and that cougar numbers had increased, and “the bears have always been there”.
“It wasn’t until wolves were returned that we got this reshuffle in what elk were doing and we began to see improvement in plant communities and streams.
“This is the first study showing improving stream morphology in Yellowstone’s northern elk range, or anywhere else in the U.S. as it relates to the return of a large predator.”
This time around, however, the point is made that it’s not just the wolves sparking the recovery.
“Overall, results were consistent with a landscape‐scale trophic cascade whereby reintroduced wolves, operating in concert with other large carnivores, appear to have sufficiently reduced elk herbivory in riparian areas to initiate the recovery of Blacktail Deer Creek’s riparian plant communities and stream channels,” the pair wrote.