March 1, 2024

Apex predators are not a quick fix for restoring ecosystems, 20-year study finds

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In 2001, CSU researchers established four intensive study areas in Yellowstone to test the effects of reducing navigation through fencing and increasing water availability through creating simulated beaver dams. In this 2016 photograph, willow trees in the fenced area, which also had greater water availability, grew three times taller and much more densely than the unfenced areas without access to groundwater, shown in the foreground. Credit: David J. Cooper

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In 2001, CSU researchers established four intensive study areas in Yellowstone to test the effects of reducing navigation through fencing and increasing water availability through creating simulated beaver dams. In this 2016 photograph, willow trees in the fenced area, which also had greater water availability, grew three times taller and much more densely than the unfenced areas without access to groundwater, shown in the foreground. Credit: David J. Cooper

A Colorado State University experiment spanning more than two decades found that removing top predators from an ecosystem can create lasting changes that are not reversed upon their return — at least, not for long.

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

Researchers from CSU’s Warner College of Natural Resources examined the effects of three apex predators – carnivores at the top of the food chain not preyed on by other animals – in Yellowstone. Depleted populations of cougars and grizzly bears recovered naturally around the same time wolves were reintroduced to the park in 1995. The absence of these predators for nearly a century transformed the food chain and landscape.

Yellowstone’s north range has changed from willows and aspens along small streams with beaver activity to grassland due to heavy elk grazing. The widespread changes stabilized into an alternative ecological state that resisted a return to previous conditions after 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 adds evidence supporting the theory that ecosystem degradation cannot be reversed when harmful stressors are mitigated.

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

“We cannot rule out the possibility that the ecosystem will be restored in the next 40 years as a result of the return of apex predators. All we can be sure of is what is observable now: the ecosystem has not responded dramatically to the restoration of the food web.”

While it’s not a quick and easy solution, Hobbs said, restoring apex predators produces healthier ecosystems in the long run.

“The conservation message is, first and foremost, don’t lose them,” Hobbs said. “Keep the food chain intact, because there is no quick fix to losing top predators in ecosystems.”

Can Colorado learn from Yellowstone?

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

This study may contain lessons about how restoring apex predators affects the ecosystem, but Hobbs said the environmental degradation resulting from Yellowstone’s no-kill elk policy has never been replicated in Colorado.

“Unlike Yellowstone, Colorado’s landscapes have not experienced widespread overgrazing or elk grazing,” 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; just don’t expect them to cause immediate improvements in the ecosystem.

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

What do willow trees have to do with wolves?

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

Historically, beavers and willows depended on each other to thrive. Flooding caused by beaver dams created favorable soil moisture conditions for willows, and willows provided food and dam building materials for beavers. Without beaver-engineered floods, small streams in the North Range cut 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 the cougar and grizzly bear populations recovered on their own, the elk population declined due to predation and hunting by humans along the park’s borders.

However, global demand for woody food sources has not declined proportionately. As elk numbers declined, bison herds increased. Yellowstone carnivores typically do not attack bison because their large size makes them dangerous.

Long-term experiment

In 2001, CSU ecologists began an experiment to assess whether the Yellowstone ecosystem would recover due to the restoration of apex predators. They established four study areas in the northern range of the park, fenced eight lots to prevent navigation, and built mock beaver dams on some fenced and unfenced lots to raise the water table.

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

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

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

By manipulating one factor at a time – navigation and hydrology – in many locations over a long period of time, 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. “Disruptive 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 Yellowstone National Park senior wildlife biologist Daniel Stahler, to answer questions relevant to the park’s needs and share results to help guide policy. of the park.

“This research greatly contributes to our understanding of Yellowstone by revealing the degree to which complex links in a food web affect recovering ecosystems of native species,” Stahler said.

“Importantly, it is among the few studies published to date on the Yellowstone ecosystem that highlight that not just wolves, but multiple predator species in concert have contributed to changes in elk abundance. This point has ramifications for how we evaluate how complex ecosystems respond to carnivore presence and absence.”

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

More information:
N. Thompson Hobbs et al, Does restoring apex predators to food webs restore ecosystems? Large carnivores in Yellowstone as a model system, Ecological Monographs (2024). DOI: 10.1002/ecm.1598

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