Oregon State University researchers have documented the first new generation of overstory aspen trees in Yellowstone’s northern range in 80 years, three decades after wolves were reintroduced to the nation’s oldest national park. Photo provided by Luke Painter, OSU College of Agricultural Sciences.
Credit: Photo provided by Luke Painter, OSU College of Agricultural Sciences.
Yellowstone National Park is celebrating an ecological milestone along with a key anniversary this summer, Oregon State University researchers report.
A paper published today in Forest Ecology and Management documents the first new generation of overstory aspen trees in Yellowstone’s northern range in 80 years, three decades after wolves were reintroduced to the nation’s oldest national park.
Without predation pressure from wolves, which had been extirpated from the park by 1930, elk populations grew to the point that their browsing was thwarting the growth of young aspen. The ecosystem effects were widespread as aspen stands support a range of species including beavers and cavity-nesting birds.
With wolves back in the mix along with bears and cougars, a nearly extirpated predator whose numbers increased along with wolf reintroduction, elk numbers have been reduced and aspen are once again working toward becoming full-grown trees.
“The reintroduction of large carnivores has initiated a recovery process that had been shut down for decades,” said the study’s lead author, Luke Painter, who teaches ecology and conservation in the OSU College of Agricultural Sciences. “About a third of the 87 aspen stands we examined had large numbers of tall saplings throughout, a remarkable change from the 1990s when surveys found none at all.”
Another third of the surveyed stands had patches of tall saplings growing into new overstory trees, he added, and the rest remained suppressed by herbivory.
“Increasing numbers of bison may be emerging as a new constraint to aspen in some areas,” Painter said.
The fact that stands with many tall saplings have low rates of browsing, whereas other stands continue to be suppressed, indicates aspen recovery is happening because of a trophic cascade and not other factors such as climate or site productivity, he said.
In a trophic cascade, a change at the top of a food web causes ripple effects throughout an ecosystem, altering its structure and balance. In Yellowstone, top predators have reduced herbivory by elk, allowing aspen to begin to recover.
“This is a remarkable case of ecological restoration,” Painter said. “Wolf reintroduction is yielding long-term ecological changes contributing to increased biodiversity and habitat diversity.”
Collaborating with Painter were Robert Beschta and William Ripple of the OSU College of Forestry. The Ecosystem Restoration Research Fund of the Oregon State University Foundation supported the research.
In late 2020, a female coastal wolf collared for a study on predation patterns unexpectedly died in southeastern Alaska.
The wolf, No. 202006, was only four years old.
“We spent quite a bit of time trying to figure out the cause of her death by doing a necropsy and different analyses of tissues,” says Gretchen Roffler, a wildlife research biologist with the Alaska Department of Fish and Game.
“What finally came up was really unprecedented concentrations of mercury in this wolf’s liver and kidneys and other tissues.”
Roffler was put in touch with Dr. Ben Barst, PhD, an assistant professor in the Faculty of Science at the University of Calgary who was working at the University of Alaska Fairbanks at the time.
Barst, an expert in ecotoxicology, says mercury is a naturally occurring element humans release from the Earth’s crust through coal combustion and small-scale gold mining.
“It’s a really weird metal in that it’s liquid at room temperature or it can be a vapour,” he says. “When it gets into the atmosphere in its elemental form, it can travel for really long distances.”
Barst says it also gets converted into methyl mercury when it gets into aquatic environments.
“It’s an organic form of mercury that really moves quite efficiently through the food web, and so it can reach high concentrations in predators that are tapped into aquatic food webs," he says. "So, we see higher concentrations in wolves that are tapped into a marine system.”
The latest research compares wolves from Pleasant Island — located in the Alaska Panhandle region, west of Juneau — with the population on the mainland adjacent to the island, as well as wolves from interior Alaska.
“The highest concentrations are the wolves from Pleasant Island,” says Barst, noting that the mainland population mostly feeds on moose and the odd sea otter.
He says there could be a number of factors driving the higher concentrations of mercury, but they are still researching several possibilities.
Mercury-wolf health impact examined
Researchers are also doing more work to determine mercury’s role in impacting wolf health, as it remains unclear exactly what caused the death of Wolf No. 202006.
Barst notes, however, that years of data collected by Roffler show that 70 per cent of the island wolves’ diet is sea otters.
“They're eating so many sea otters that they're just getting this higher dose of mercury and it accumulates over time,” he says.
Roffler says there are other populations of wolves in Alaska as well as in B.C. that appear to be eating sea otters.
“It turns out that this might be a more widespread phenomenon than we thought originally,” she says. “At first I was surprised it was happening at all.”
It’s not yet known whether the sea otters off the B.C. coast also contain high levels of mercury.
Potential link to climate change
Back in Alaska, Barst says there’s a potential link to climate change due to the state's shrinking glaciers.
“We know that glaciers can release a tremendous amount of mercury,” he says. “In coastal Alaska, glaciers are retreating at some of the most rapid rates in the world.
“With that melting of glaciers, you get release of the particulate bedrock and some of that bedrock contains mercury – and so we don’t really know the fate of that mercury. It may just get buried in sediments or it may actually be available for conversion to methyl mercury and get into the food web.
The Endangered Species Act (ESA), now 50 years old, was once a rare beacon of bipartisan unity, signed into law by President Richard Nixon with near-unanimous political support. Its purpose was clear: protect imperiled species and enable their recovery using the best available science to do so. Yet, as our case study on the grizzly bear in the Greater Yellowstone Ecosystem reveals, wildlife management under the ESA has changed, becoming a political battleground where science is increasingly drowned out by partisan ideology, bureaucratic delays, power struggles, and competing political interests. The survival of the ESA, a wildlife policy mimicked all over the world, may depend on our ability to navigate these waters.
The grizzly bear, a cultural symbol of the American West, embodies this shift. Listed as threatened in 1975 when its numbers dwindled to fewer than 1,000 and its range contracted by 98%, the species has managed to come back from the brink. In the Greater Yellowstone Ecosystem, the population now exceeds 700, a number that surpassed recovery goals set by the federal wildlife management agency tasked with recovery, the US Fish & Wildlife Service. By the ESA’s own metrics, this is a success story, which now means the grizzly bear is eligible for ‘delisting’. Yet, attempts to remove federal protections in 2007 and 2017 were overturned by courts, not because the science was lacking, but because the process has become a lightning rod for political interests.
Our study looks at 750 documents and 2,832 stakeholder quotes to track this politicization. Historically, wildlife management is the strict domain of agency scientists in the executive branch. These scientists are experts trained to interpret interdisciplinary scientific data and balance both human and ecological needs.
Our work shows that today, the most dominant voices belong to legislators, legal advocates, and non-governmental organizations (NGOs) who are increasingly crowding out the agency scientists. Senators, elected politicians, like Wyoming’s John Barrasso proclaim, “The grizzly is fully recovered in Wyoming. End of story,” pushing for state control and criticizing the ESA as sluggish and outdated. Can you blame him though? Senator Barraso advocates for his Wyoming constituents who have collaborated in grizzly recovery and are now on the frontlines of human-wildlife conflict issues where grizzlies might harm livestock or tourists. All the while, population targets set by the ESA have been met, and the species remains listed.
Meanwhile, NGOs and their attorneys, such as the well-known environmental advocacy group Earthjustice, argue that delisting is premature, citing ‘political pressure’ overriding ‘biological evidence.’ The courts, too, have flexed their muscle, with rulings hinging on genetic connectivity’s role in population recovery. Ranchers with increasing grizzly conflict see these scientific developments as intentional delays to delisting rather than advancements in the field of conservation. There are no easy answers.
Wildlife management turned politics
This conflict reveals a stark reality: wildlife management is no longer just about science, it's about who dominates the political discourse, and the power that accompanies it. Legislators see delisting as a way to reclaim state authority from what they consider federal overreach. Their rhetoric, steeped in populist appeals to the Western ranching community, frames grizzlies as a recovered species with bureaucrats in Washington stalling the process of handing management back over to the states.
Montana Senator Steve Daines, for instance, highlights ‘skyrocketing’ livestock losses and bears roaming beyond their historic range. These issues resonate with rural constituents tired of federal wildlife law superseding local management by trusted state agencies. On the other hand, NGOs and legal advocates rely on the courts to maintain federal oversight, warning that state management could unleash ‘trigger-happy’ hunting seasons and jeopardize long-term survival. These advocates argue that we are facing a generational extinction crisis, where every decision we make about imperiled species could approach extinction, a route that we cannot come back from. The public, caught in the middle, may be unaware that conversations over wildlife protection have shifted from credentialed agency biologists and scientists over to politicians.
Our data underscore this shift in power. While executive branch officials, with whom scientific expertise resides, once dominated the discourse (eg fish and wildlife agency personnel at the federal and state level), they are no longer the leading voices in ESA recovery conversations. Elected politicians now lead the charge. Their influence is growing threefold over time compared to scientific agency voices. Legal advocates and NGOs, meanwhile, are shaping the debate over wildlife science with their roles amplified by lawsuits that keep grizzlies listed. Even tribes, historically sidelined, find their strongest platform in court, a sign that political systems still fail to integrate Indigenous perspectives outside litigation.
What’s lost in this debate is the ESA’s original intent: a science-driven process to recover species and then allow federal agency experts to step back so that states, who may better represent local interests, can manage species.
The path forward
This politicization threatens the ESA’s future. When politicians outshout scientists, when courts dictate biology or delay timely management responses, and when recovery becomes a bargaining chip, the law risks losing its credibility with the public. The grizzly saga suggests a path forward: agencies must adapt to this political reality, not retreat from it. Scientists can’t afford to ‘stay out of politics’ when protected species like grizzlies are lightning rods for political debate. Multi-stakeholder groups, like the Interagency Grizzly Bear Committee, offer a model bridging agencies, states, tribes, and NGOs to tackle thorny issues like genetics collaboratively rather than through unending lawsuits in the courts.
The grizzly bear’s fate isn’t just about one species: this pattern is playing out across a range of species in the West and beyond. It will prove itself as the greatest challenge for wildlife managers in an era of increased polarization. If the ESA is to endure another 50 years, it must evolve beyond a scientific ideal into a framework that navigates the messy, human politics of conservation. Otherwise, the grizzly’s roar will be drowned out by an even greater sound: the chaos of our own imperfect politics.
Wolf populations in Europe increased by nearly 60% in a decade, according to a study led by Cecilia Di Bernardi and Guillaume Chapron at the Swedish University of Agricultural Sciences, published in the open-access journal PLOS Sustainability and Transformation.
Large carnivore populations are declining worldwide. However, in Europe, conservation policies have supported the recovery of wolves (Canis lupus) in recent decades. To understand current trends in their populations, researchers collated data on wolf numbers in 34 countries across Europe. They found that by 2022, at least 21,500 wolves lived in Europe — an increase of 58% compared to the estimated population of 12,000 a decade earlier. In most countries analyzed, wolf populations were increasing, with only three countries reporting declines over the previous decade. The researchers also investigated sources of conflict between humans and wolves, such as livestock deaths. They estimated that in the European Union, wolves killed 56,000 domestic animals per year, out of a total population of 279 million livestock. Although the risk varied between countries, on average, livestock faced a 0.02% chance of being killed by wolves each year. Compensating farmers for these losses cost European countries 17 million euros annually. Still, wolves can also have positive economic impacts, such as reducing traffic accidents and damage to forestry plantations by controlling wild deer populations. However, there wasn’t enough data available to quantify these benefits.
Considering Europe’s large human population and the widespread alteration of landscapes for agriculture, industry and urbanization, the rapid recovery of wolves over the last decade highlights their extraordinary adaptability. However, as conservationists transition from saving endangered populations to sustaining a successful recovery, the challenge will be to adapt national and international policies to ensure that humans and wolves can coexist sustainably in the long term, the authors say.
The authors add: “The recovery of wolves across human-dominated landscapes of Europe has been continuing during the past decade, with their population growing to over 21,500 individuals by 2022 – a 58% increase in a decade. Ongoing and future challenges include damages directly caused by wolves and broader socio-political issues.”
The freely available article in PLOS Sustainability and Transformation:https://plos.io/41wNLjq
Corvallis, OR — February 6, 2025 — A new study reveals the profound ecological effects of wolves and other large carnivores in Yellowstone National Park, showcasing the cascading effects predators can have on ecosystems. In Yellowstone, this involves wolves and other large carnivores, elk, and willows. The research, which utilized previously published data from 25 riparian (streamside) sites and collected over a 20 year period, from 2001 to 2020, revealed a remarkable 1,500% increase in willow crown volume along riparian zones in northern Yellowstone National Park, driven by the effects on elk due to a restored large carnivore guild following the reintroduction of wolves in 1995–96, and other factors. The study was led by Dr. William J. Ripple of Oregon State University and the Conservation Biology Institute in Corvallis, OR, and published today in Global Ecology and Conservation.
Trophic cascades, the effects of predators on herbivores and plants, have long been a topic of ecological interest. The study quantifies the strength of this phenomenon for the first time using willow crown volume as a proxy for aboveground biomass, demonstrating a significant three-dimensional recovery of riparian vegetation represented by the growth in both crown area and height of established willows. The strength of the Yellowstone trophic cascade observed in this study surpasses 82% of strengths presented in a synthesis of global trophic cascade studies, underscoring the strength of Yellowstone’s willow recovery process. The authors note that there is considerable variability in the degree of recovery and not all sites are recovering.
Even though riparian areas in the western United States comprise a small portion of the landscape, the study has particular relevance since these areas provide important food resources and habitat for more wildlife species than any other habitat type. These areas also connect upland and aquatic ecosystems and are widely known for their high diversity in species composition, structure, and productivity.
“Our findings emphasize the power of predators as ecosystem architects,” said William Ripple. “The restoration of wolves and other large predators has transformed parts of Yellowstone, benefiting not only willows but other woody species such as aspen, alder, and berry-producing shrubs. It’s a compelling reminder of how predators, prey, and plants are interconnected in nature.”
Wolves were eradicated and cougars driven to low numbers from Yellowstone National Park by the 1920s. Browsing by elk soon increased, severely damaging the park’s woody vegetation, especially in riparian areas. Similar effects were seen in places like Olympic National Park in Washington, and Banff and Jasper National Parks in Canada after wolves were lost. While it’s well understood that removing predators can harm ecosystems, less is known about how strongly woody plants and ecosystems recover when predators are restored. Yellowstone offers a rare opportunity to study this effect since few studies worldwide have quantified how much plant life rebounds after large carnivores are restored.
“Our analysis of a long-term data set simply confirmed that ecosystem recovery takes time. In the early years of this trophic cascade, plants were only beginning to grow taller after decades of suppression by elk. But the strength of this recovery, as shown by the dramatic increases in willow crown volume, became increasingly apparent in subsequent years,” said Dr. Robert Beschta, an emeritus professor at Oregon State University. “These improving conditions have created vital habitats for birds and other species, while also enhancing other stream-side conditions.”
The research points to the utility of using crown volume of stream-side shrubs as a key metric for evaluating trophic cascade strength, potentially advancing methods for riparian studies in other locations. It also contextualizes the value of predator restoration in fostering biodiversity and ecosystem resilience.
