Apex predators not a silver bullet for restoring ecosystems, 20-year study finds

An experiment conducted by Colorado State University, spanning more than two decades, found that removing top predators from an ecosystem can create lasting changes that are not reversed after their return, from at least not for a very long time.

The study, published in Ecological monographschallenges the widely held belief that the reintroduction of wolves to Yellowstone National Park restored an ecosystem degraded by their absence.

Researchers at CSU’s Warner College of Natural Resources examined the effects of three large predators – carnivores at the top of the food chain that are not preyed upon by other animals – in Yellowstone. Decimated cougar and grizzly bear populations naturally recovered around the same time wolves were reintroduced to the park in 1995. The absence of these predators for nearly a century transformed the food web and landscape .

Yellowstone’s northern range has shifted from willow and aspen stands along small streams with beaver activity to grasslands due to heavy elk grazing. The widespread changes stabilized in an alternative ecological state that resisted a return to previous conditions once carnivores were restored, according to study authors Tom Hobbs and David Cooper.

This designed experiment, conducted in Yellowstone, is the longest of its kind and supports the theory that ecosystem degradation cannot be reversed when harmful stressors are mitigated.

“When you disrupt ecosystems by changing the composition of a food web, it can cause lasting changes that are not quickly corrected,” said Hobbs, lead author and professor emeritus in the Department of Ecosystem Science and Sustainability and the Natural Resource Ecology Laboratory. .

“We cannot rule out the possibility that the ecosystem will be restored over the next 40 years as a result of the return of top predators. All we can be sure of is what is observable now: the ecosystem n ‘has not responded dramatically to the recovery. Internet food.’

Although it’s not a quick and easy solution, Hobbs said, restoring top predators produces healthier ecosystems in the long term.

“The conservation message is don’t lose them in the first place,” Hobbs said. “Keep the food web intact, because there is no silver bullet for losing ecosystems’ top predators.”

Can Colorado learn from Yellowstone?

Colorado Parks and Wildlife introduced five wolves to the state on Dec. 18 and plans to attract more in coming years. Wolves were eradicated in the state in the mid-1940s, but Colorado voters approved their restoration by a narrow margin in 2020.

This study could hold lessons about how restoring top predators affects the ecosystem, but Hobbs said the environmental degradation resulting from Yellowstone’s policy of not culling elk has never been replicated in Colorado.

“Unlike Yellowstone, Colorado’s landscapes have not experienced excessive grazing or browsing by elk,” Hobbs said. “The state has done a good job managing elk populations through hunting.”

Hobbs and Cooper said there are many good reasons to restore wolves; don’t expect them to result in immediate ecosystem improvements.

“Our work supports the fact that wolves are important components of ecosystems,” said Cooper, a distinguished research scientist in the Department of Forest and Range Management. “They will have some benefits to the ecosystem by reducing some large populations of herbivores. Over the next hundred years, they will play a larger role in regulating some of the ecological processes we study.”

What do willows have to do with wolves?

Wolves and cougars were exterminated in Yellowstone in the early 1920s. Without large predators or human hunters to control their population, elk fed on the willows along the small streams of Yellowstone’s northern range, depleting food supplies. and construction materials from beavers and forcing them to abandon streams in favor of more suitable areas.

Historically, beavers and willows depended on each other to thrive. Flooding from beaver dams created soil moisture conditions favorable for willows, and willows provided food and dam building materials for beavers. Without beaver-driven flooding, small streams in the North Range move deeper into the landscape, disconnecting willow roots from groundwater. The willows never regained their former height and density.

Following the reintroduction of wolves to the park in 1995, as cougar and grizzly bear populations recovered on their own, the elk population declined both due to predation and hunting by humans along the park boundaries.

However, overall exploitation of woody food sources has not declined proportionately. As elk numbers declined, bison herds increased. Yellowstone carnivores generally do not prey on bison because their large size makes them dangerous.

Long term experience

In 2001, CSU ecologists began an experiment to assess whether the Yellowstone ecosystem would recover through the restoration of top predators. They established four study areas in the northern part of the park, fenced eight plots to prevent grazing, and constructed simulated beaver dams in some fenced and unfenced plots to raise the water table.

They also left the control zones unchanged. In 2009, they added 21 additional control plots to ensure their experiment results were representative of the landscape.

If predators regulated the elk population, preventing them from cutting down willows, the landscape would hypothetically return to its previous state. Instead, willows remained deficient in control plots, while fenced sites with simulated dams showed dramatic recovery.

Willows grew more than three times taller in fenced and diked areas than in control plots, indicating the importance of access to groundwater in addition to grazing mitigation.

By manipulating one factor at a time – grazing and hydrology – at many sites over an extended period of time, the researchers were able to show that carnivores were not causing landscape restoration.

“We learned from science that it was much more complicated,” Cooper said.

“Our result is well supported by ecological theory and empirical results from around the world,” Hobbs added. “Disruption of food webs can cause persistent changes in ecosystems.”

Research in Yellowstone is common, but this study was rare in its manipulation of the landscape and its duration. Hobbs and Cooper worked closely with park management and biologists, including Daniel Stahler, Yellowstone National Park’s senior wildlife biologist, to answer questions relevant to the park’s needs and share results to help guide the park policy.

“This research contributes greatly to our understanding of Yellowstone by determining how the complex connections of a food web affect ecosystems as part of the recovery of native species,” Stahler said.

“Importantly, this is one of the few studies published to date on the Yellowstone ecosystem that highlights that not only wolves, but also several predator species contributed to changes in elk abundance “This point has implications for how we assess how complex ecosystems respond to the presence and absence of carnivores.”

He continued: “This long-term research by the CSU team also highlights the value of national parks in helping us understand ecological processes, to better protect ecosystems. We should not only cherish our national parks because they protect, preserve, and allow people to enjoy nature, but because they provide a place where well-designed science can elevate our understanding of its complexity.