Did Wolves Really Start a Trophic Cascade in Yellowstone?

Did Wolves Really Start a Trophic Cascade in Yellowstone?

You’ve seen the video. Narrated by British environmental activist George Monbiot, “How Wolves Change Rivers” tells the incredible story of how gray wolves sparked a cascading series of ecological benefits for the Greater Yellowstone Ecosystem.

“We all know that wolves kill various species of animals,” Monbiot begins. “But perhaps we’re slightly less aware that they give life to many others.”

Prior to wolf reintroduction in 1995, Monbiot explains, increasing elk herds overgrazed the riparian areas around Yellowstone’s rivers and streams, which led to erosion, habitat loss, and difficulty for the area’s animals and plants. Then, when wolves entered the landscape, they pushed the herds away from those rivers and streams, allowing the plants and trees to recover and restoring the area’s natural biodiversity.

If this sounds compelling, you’re not alone. The video has racked up over 43 million views as of this writing, and nearly as many (seemingly) articles and papers have been published highlighting Yellowstone’s “trophic cascade” catalyzed by increasing wolf packs.

There’s just one problem: It’s not entirely true. Elk herds did decline following wolf reintroduction, and those declines have spurred ecological benefits for the region (though even this is up for debate). But many biologists hold that wolves can’t be credited with Yellowstone’s recovery. A host of complex and interlocking factors have led to a rebound for some animal and plant species in the GYE, and wolves are only one small part of that story.

What is a Trophic Cascade? The term “trophic cascade” refers to how animals at the top of a food chain affect plants and animals farther down the line. Trophic cascades have been well-documented among food chains involving spiders, fish, insects, and snails that exist in relatively small spaces such as lakes.

In the early 2000s, just a few years after wolves were reintroduced in Yellowstone, scientists began asking the question: Could wolves catalyze a trophic cascade and bring ecological benefits to the region?

The idea isn’t entirely unfounded. Researcher Mark Hebblewhite documented such a cascade in Canada’s Banff National Park in a paper published in 2005. Another scientist, David Mech, who counts himself among the skeptics of a trophic cascade in Yellowstone, called Hebblewhite's study “the only one that has provided seemingly irrefutable evidence of a true trophic cascade from wolves through prey, vegetation and song birds.”

Since then, scientists have credited gray wolves with bringing back everything from willow trees to songbirds to insects. As in the video above, wolves have also been credited with halting erosion around rivers by encouraging beaver habitat and keeping ungulates off the riverbanks.

Wolves accomplish this miraculous feat, according to a 2004 study by William Ripple and Robert Beschta, by preying on elk herds and creating a “landscape of fear” that discourages these cervids from over-browsing plants and trees around Yellowstone’s rivers.

“Can predation risk structure ecosystems? Our answer—based on theory involving trophic cascades, predation risk, and optimal foraging, in addition to a developing body of empirical research—is yes,” the authors conclude.

Further studies were published since the early 2000s, and media outlets and pro-wolf activists gravitated to the narrative. If wolves are the primary drivers of the resurgence of biodiversity in Yellowstone, the trophic cascade theory provided a simple, accessible scientific framework within which to argue for the increased wolf presence on the landscape.

Correlation vs. Causation There’s no question that cervid herds have declined in Yellowstone since wolves were reintroduced in 1995. But to blame (or credit, depending on your perspective) wolves with that decline is more difficult than it sounds. It isn’t enough to point out that wolf packs increased at the same time herd sizes decreased; scientists must prove that wolves caused the herd decreases.

Researcher Benjamin Allen and his colleagues argue in a 2017 paper that many trophic cascade studies fail because they cannot prove this causal connection.

“The fallacies lie in coming to a conclusion based on the order or pattern of events, rather than accounting for other factors that might rule out a proposed connection,” they say. “Unfortunately, but perhaps motivated by the dire status of many carnivore populations, a growing number of studies rely on weak inference when valuing the roles of large carnivores in ecosystems.”

As for those first trophic cascade papers in the early 2000s, Jim Heffelfinger, the wildlife science coordinator for the Arizona Game and Fish Department, says researchers jumped to conclusions based on correlation rather than causation. “They saw some things, and they correlated it with other things without necessarily having the scientific connection between the two,” he said in an interview with MeatEater.

Furthermore, if scientists want to justify the pro-wolf hype, they must also prove that other factors played little or no role in the trophic cascade process. Researchers have pointed out, however, that elk herds are affected by a wide range of factors, including drought, winter severity, human hunting, cougars, coyotes, black bears, and grizzly bears—all of which were factors in the decades after wolf reintroduction.

“In many areas, bears are important contributors to limiting ungulate numbers,” Mech said. “Ferreting out the role of each of these factors in the [Yellowstone National Park] elk decline is a complex task that has yet to be accomplished.”

By limiting elk herds, wolves were also credited with restoring beavers by giving them larger, healthier trees to use in their dams. But this phenomenon may have a much simpler explanation: between 1986 and 1999, Mech reports, biologists released 129 beavers on the Gallatin National Forest in seven drainages just north of the Yellowstone boundary. (Ironically, wolves have more recently been accused of killing too many beavers in another national park in Minnesota, which has “altered the very landscape” of the park.)

While it’s no doubt true that wolves played a role in lowering elk numbers, no one has been able to quantify that role.

“Wolves contributed to that decline in elk, but wolves didn’t cause that decline in elk,” Heffelfinger said. “They were just part of it, and nobody knows what percent. It’s almost impossible to sort that out. People who are lifelong Yellowstone wolf researchers haven’t been able to figure it out.”

Landscape of Fear? Even if wolves haven’t singlehandedly decimated elk herds, trophic cascade proponents argue that they have contributed to a “landscape of fear” that keeps cervids away from Yellowstone’s rivers.

There may be some evidence for this in certain circumstances. According to a 2010 paper by John Laundre and his colleagues, two separate studies in the early 2000s documented elk shifting towards forest edges in response to predation by wolves. We also covered a recent study that found mountain lion numbers are dwindling in part because wolves are pushing elk into areas that are more difficult for lions to hunt.

The question, of course, is whether a landscape of fear (such as it exists) has allowed the aspen, willow, and cottonwood trees to recover in the park. “How Wolves Change Rivers” depicts this recovery as incontrovertible, but most recent studies have cast doubt on even this basic fact.

Mech points out that after Ripple and Beschta published photographs in 2004 documenting willow increase, another set of researchers led by Danielle Bilyeu came along in 2008 and published photos purporting to refute the increase.

Another 2010 study by Matthew Kauffman found that aspen trees hadn’t regrown in Yellowstone even as the elk population declined by 60%.

“Even in areas where wolves killed the most elk, the elk weren’t scared enough to stop eating aspens,” biologist Arthur Middleton wrote in the New York Times in 2014. “Other studies have agreed. In my own research at the University of Wyoming, my colleagues and I closely tracked wolves and elk east of Yellowstone from 2007 to 2010 and found that elk rarely changed their feeding behavior in response to wolves.”

Some researchers have found that the preferred forage of deer is more abundant in areas with longer-term wolf occupancy than in areas with shorter-term occupancy. This may suggest a certain amount of wolf-induced trophic cascade, but it’s no guarantee.

In fact, as biologist Adam Ford noted in a 2015 article, that phenomenon could also be explained in the reverse: “The prediction that the preferred forage of deer is more abundant in areas with longer-term occupancy by wolves is equally valid for a bottom-up driven food web: abundant forage could increase the deer population, thereby stabilizing wolf occupancy.”

Even if scientists acknowledge that certain tree species have made a recovery in Yellowstone, as with beavers, there is another, simpler explanation than a the “landscape of fear” hypothesis. As the work of Thomas Hobbs has shown, water table levels are far more important to willow recovery than the presence or absence of wolves.

Why Hunters Should Care None of this is to say that wolves have had zero effect on the Greater Yellowstone Ecosystem. Heffelfinger acknowledged that some vegetation has recovered in Yellowstone due to declining elk numbers, and wolves have definitely played a role in that phenomenon.

But to overstate the benefits of wolves may well backfire on those who wish to preserve the species.

“Perhaps the greatest risk of this story is a loss of credibility for the scientists and environmental groups who tell it,” Middleton writes in the New York Times. “We need the confidence of the public if we are to provide trusted advice on policy issues. This is especially true in the rural West, where we have altered landscapes in ways we cannot expect large carnivores to fix, and where many people still resent the reintroduction of wolves near their ranchlands and communities.”

Hunters, Heffelfinger argues, also have a vested interest in telling the true story of the Yellowstone wolf. In order to secure the continued support of the non-hunting public, hunters should strive to be seen as responsible, truthful conservationists.

“Especially as hunters who talk about how we’ve recovered elk and pronghorn and geese and mule deer, we should also be bragging about how we’ve brought the native predators back,” he said. “It may cost us some elk. But we can’t expect the 95% of the public that doesn’t hunt to continue to support us if we say we don’t want wolves on the landscape. That’s not going to generate support from non-hunters.”

“The future of hunting is not in the hands of anti-hunters and their criticisms,” Heffelfinger concluded. “The future of hunting is in the hands of hunters who must continue to be seen as a positive force for overall conservation by the general public.”

To do that, Heffelfinger says, we shouldn’t oversimplify the role of wolves in Yellowstone even if that story might convince some people to support reintroducing the native species.

After all, as Middleton put it in a 2016 piece for National Geographic, “It’s an unproven theory that gets undue attention in the quest to have wolves shine rainbows out of their asses.”

Feature image via Yellowstone National Park.

You’ve seen the video. Narrated by British environmental activist George Monbiot, “How Wolves Change Rivers” tells the incredible story of how gray wolves sparked a cascading series of ecological benefits for the Greater Yellowstone Ecosystem.

“We all know that wolves kill various species of animals,” Monbiot begins. “But perhaps we’re slightly less aware that they give life to many others.”

Prior to wolf reintroduction in 1995, Monbiot explains, increasing elk herds overgrazed the riparian areas around Yellowstone’s rivers and streams, which led to erosion, habitat loss, and difficulty for the area’s animals and plants. Then, when wolves entered the landscape, they pushed the herds away from those rivers and streams, allowing the plants and trees to recover and restoring the area’s natural biodiversity.

If this sounds compelling, you’re not alone. The video has racked up over 43 million views as of this writing, and nearly as many (seemingly) articles and papers have been published highlighting Yellowstone’s “trophic cascade” catalyzed by increasing wolf packs.

There’s just one problem: It’s not entirely true. Elk herds did decline following wolf reintroduction, and those declines have spurred ecological benefits for the region (though even this is up for debate). But many biologists hold that wolves can’t be credited with Yellowstone’s recovery. A host of complex and interlocking factors have led to a rebound for some animal and plant species in the GYE, and wolves are only one small part of that story.

What is a Trophic Cascade? The term “trophic cascade” refers to how animals at the top of a food chain affect plants and animals farther down the line. Trophic cascades have been well-documented among food chains involving spiders, fish, insects, and snails that exist in relatively small spaces such as lakes.

In the early 2000s, just a few years after wolves were reintroduced in Yellowstone, scientists began asking the question: Could wolves catalyze a trophic cascade and bring ecological benefits to the region?

The idea isn’t entirely unfounded. Researcher Mark Hebblewhite documented such a cascade in Canada’s Banff National Park in a paper published in 2005. Another scientist, David Mech, who counts himself among the skeptics of a trophic cascade in Yellowstone, called Hebblewhite's study “the only one that has provided seemingly irrefutable evidence of a true trophic cascade from wolves through prey, vegetation and song birds.”

Since then, scientists have credited gray wolves with bringing back everything from willow trees to songbirds to insects. As in the video above, wolves have also been credited with halting erosion around rivers by encouraging beaver habitat and keeping ungulates off the riverbanks.

Wolves accomplish this miraculous feat, according to a 2004 study by William Ripple and Robert Beschta, by preying on elk herds and creating a “landscape of fear” that discourages these cervids from over-browsing plants and trees around Yellowstone’s rivers.

“Can predation risk structure ecosystems? Our answer—based on theory involving trophic cascades, predation risk, and optimal foraging, in addition to a developing body of empirical research—is yes,” the authors conclude.

Further studies were published since the early 2000s, and media outlets and pro-wolf activists gravitated to the narrative. If wolves are the primary drivers of the resurgence of biodiversity in Yellowstone, the trophic cascade theory provided a simple, accessible scientific framework within which to argue for the increased wolf presence on the landscape.

Correlation vs. Causation There’s no question that cervid herds have declined in Yellowstone since wolves were reintroduced in 1995. But to blame (or credit, depending on your perspective) wolves with that decline is more difficult than it sounds. It isn’t enough to point out that wolf packs increased at the same time herd sizes decreased; scientists must prove that wolves caused the herd decreases.

Researcher Benjamin Allen and his colleagues argue in a 2017 paper that many trophic cascade studies fail because they cannot prove this causal connection.

“The fallacies lie in coming to a conclusion based on the order or pattern of events, rather than accounting for other factors that might rule out a proposed connection,” they say. “Unfortunately, but perhaps motivated by the dire status of many carnivore populations, a growing number of studies rely on weak inference when valuing the roles of large carnivores in ecosystems.”

As for those first trophic cascade papers in the early 2000s, Jim Heffelfinger, the wildlife science coordinator for the Arizona Game and Fish Department, says researchers jumped to conclusions based on correlation rather than causation. “They saw some things, and they correlated it with other things without necessarily having the scientific connection between the two,” he said in an interview with MeatEater.

Furthermore, if scientists want to justify the pro-wolf hype, they must also prove that other factors played little or no role in the trophic cascade process. Researchers have pointed out, however, that elk herds are affected by a wide range of factors, including drought, winter severity, human hunting, cougars, coyotes, black bears, and grizzly bears—all of which were factors in the decades after wolf reintroduction.

“In many areas, bears are important contributors to limiting ungulate numbers,” Mech said. “Ferreting out the role of each of these factors in the [Yellowstone National Park] elk decline is a complex task that has yet to be accomplished.”

By limiting elk herds, wolves were also credited with restoring beavers by giving them larger, healthier trees to use in their dams. But this phenomenon may have a much simpler explanation: between 1986 and 1999, Mech reports, biologists released 129 beavers on the Gallatin National Forest in seven drainages just north of the Yellowstone boundary. (Ironically, wolves have more recently been accused of killing too many beavers in another national park in Minnesota, which has “altered the very landscape” of the park.)

While it’s no doubt true that wolves played a role in lowering elk numbers, no one has been able to quantify that role.

“Wolves contributed to that decline in elk, but wolves didn’t cause that decline in elk,” Heffelfinger said. “They were just part of it, and nobody knows what percent. It’s almost impossible to sort that out. People who are lifelong Yellowstone wolf researchers haven’t been able to figure it out.”

Landscape of Fear? Even if wolves haven’t singlehandedly decimated elk herds, trophic cascade proponents argue that they have contributed to a “landscape of fear” that keeps cervids away from Yellowstone’s rivers.

There may be some evidence for this in certain circumstances. According to a 2010 paper by John Laundre and his colleagues, two separate studies in the early 2000s documented elk shifting towards forest edges in response to predation by wolves. We also covered a recent study that found mountain lion numbers are dwindling in part because wolves are pushing elk into areas that are more difficult for lions to hunt.

The question, of course, is whether a landscape of fear (such as it exists) has allowed the aspen, willow, and cottonwood trees to recover in the park. “How Wolves Change Rivers” depicts this recovery as incontrovertible, but most recent studies have cast doubt on even this basic fact.

Mech points out that after Ripple and Beschta published photographs in 2004 documenting willow increase, another set of researchers led by Danielle Bilyeu came along in 2008 and published photos purporting to refute the increase.

Another 2010 study by Matthew Kauffman found that aspen trees hadn’t regrown in Yellowstone even as the elk population declined by 60%.

“Even in areas where wolves killed the most elk, the elk weren’t scared enough to stop eating aspens,” biologist Arthur Middleton wrote in the New York Times in 2014. “Other studies have agreed. In my own research at the University of Wyoming, my colleagues and I closely tracked wolves and elk east of Yellowstone from 2007 to 2010 and found that elk rarely changed their feeding behavior in response to wolves.”

Some researchers have found that the preferred forage of deer is more abundant in areas with longer-term wolf occupancy than in areas with shorter-term occupancy. This may suggest a certain amount of wolf-induced trophic cascade, but it’s no guarantee.

In fact, as biologist Adam Ford noted in a 2015 article, that phenomenon could also be explained in the reverse: “The prediction that the preferred forage of deer is more abundant in areas with longer-term occupancy by wolves is equally valid for a bottom-up driven food web: abundant forage could increase the deer population, thereby stabilizing wolf occupancy.”

Even if scientists acknowledge that certain tree species have made a recovery in Yellowstone, as with beavers, there is another, simpler explanation than a the “landscape of fear” hypothesis. As the work of Thomas Hobbs has shown, water table levels are far more important to willow recovery than the presence or absence of wolves.

Why Hunters Should Care None of this is to say that wolves have had zero effect on the Greater Yellowstone Ecosystem. Heffelfinger acknowledged that some vegetation has recovered in Yellowstone due to declining elk numbers, and wolves have definitely played a role in that phenomenon.

But to overstate the benefits of wolves may well backfire on those who wish to preserve the species.

“Perhaps the greatest risk of this story is a loss of credibility for the scientists and environmental groups who tell it,” Middleton writes in the New York Times. “We need the confidence of the public if we are to provide trusted advice on policy issues. This is especially true in the rural West, where we have altered landscapes in ways we cannot expect large carnivores to fix, and where many people still resent the reintroduction of wolves near their ranchlands and communities.”

Hunters, Heffelfinger argues, also have a vested interest in telling the true story of the Yellowstone wolf. In order to secure the continued support of the non-hunting public, hunters should strive to be seen as responsible, truthful conservationists.

“Especially as hunters who talk about how we’ve recovered elk and pronghorn and geese and mule deer, we should also be bragging about how we’ve brought the native predators back,” he said. “It may cost us some elk. But we can’t expect the 95% of the public that doesn’t hunt to continue to support us if we say we don’t want wolves on the landscape. That’s not going to generate support from non-hunters.”

“The future of hunting is not in the hands of anti-hunters and their criticisms,” Heffelfinger concluded. “The future of hunting is in the hands of hunters who must continue to be seen as a positive force for overall conservation by the general public.”

To do that, Heffelfinger says, we shouldn’t oversimplify the role of wolves in Yellowstone even if that story might convince some people to support reintroducing the native species.

After all, as Middleton put it in a 2016 piece for National Geographic, “It’s an unproven theory that gets undue attention in the quest to have wolves shine rainbows out of their asses.”

Feature image via Yellowstone National Park.