Polar bears act as crucial providers for Arctic species

New study shows polar bears annually provide millions of kilograms of food, supporting a vast Arctic scavenger network

Peer-Reviewed Publication

San Diego Zoo Wildlife Alliance

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Two-year-old polar bears with bearded seal carcass and ivory gulls

Image Credit: Wayne Lynch

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Credit: Photo Credit: Wayne Lynch

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SAN DIEGO (Oct. 28, 2025) – A new study published in the scientific journal Oikos reveals for the first time the critical role polar bears play as carrion providers for Arctic species. Researchers from University of Manitoba and San Diego Zoo Wildlife Alliance, alongside researchers from Environment and Climate Change Canada, and the University of Alberta, have estimated that polar bears leave behind approximately 7.6 million kilograms of their prey annually, creating a massive and vital food source for a wide network of arctic scavenger species. 

This research demonstrates that these apex predators are a crucial link between the marine and terrestrial ecosystems. By hunting seals on the sea ice and abandoning the remains, polar bears transfer a substantial amount of energy from the ocean to the ice surface, making it accessible to other animals. The study identifies at least 11 vertebrate species known to benefit from this carrion, including Arctic foxes and ravens, with an additional eight potential scavenger species. 

“Our findings quantify for the first time, the sheer scale of polar bears as a food provider to other species and the interconnectedness of their ecosystem,” says Holly Gamblin, lead author of the study and PhD Candidate in the Department of Biological Sciences at the University of Manitoba. “What is apparent from this review is that there is no other species that adequately replaces how a polar bear hunts, in which they drag their prey from the water to the sea ice and leave substantial remains for other species to access.” 

Past research has emphasized that continued warming in the Arctic and the resulting loss of sea ice directly endanger polar bear populations. However, this new research highlights that a decline in polar bears would not only impact the species itself but the loss of the carrion they provide could have significant consequences for the entire Arctic ecosystem. 

“Our research highlights the important role of polar bears as carrion providers,” says Dr. Nicholas Pilfold, Scientist in Population Sustainability at San Diego Zoo Wildlife Alliance. “The sea ice acts as a platform for many species to access scavenging resources provided by polar bears, and ultimately, declines in sea ice will reduce access to this energy source. Our findings indicate that documented declines in polar bear abundance in two subpopulations have already resulted in the loss of more than 300 tonnes of food resources for scavengers annually.” 

These findings highlight the interdependence of arctic wildlife species and their shared vulnerabilities in the face of rapid environmental change. With polar bear populations continuing to decline, this research underscores the urgency of conservation efforts to protect them, not only for their own sake but for the species that rely on them. 

Report overstated the ecological effects of wolf recovery in Yellowstone National Park.

 A new peer-reviewed analysis challenges one of the most publicized claims about Yellowstone’s wolves.

In a detailed comment published in Global Ecology and Conservation, researchers from Utah State University and Colorado State University demonstrate that the 2025 study by Ripple et al. overstated the ecological effects of wolf recovery in Yellowstone National Park.

“Ripple et al. argued that carnivore recovery produced one of the world’s strongest trophic cascades,” said Dr. Daniel MacNulty, lead author and wildlife ecologist at Utah State University. “But our re-analysis shows their conclusion is invalid because it relies on circular reasoning and violations of basic modeling assumptions.”

Ripple et al. based their conclusion on a 1,500 percent increase in willow crown volume, calculated from plant height data using a regression model that defines and predicts volume from the same variable. “Because height was used both to compute and to predict volume,” MacNulty explained, “the relationship is circular—mathematically guaranteed to look strong even if no biological change occurred.”

The authors identified several additional issues:

• The height-to-volume model was applied to heavily browsed willows with distorted shapes, violating model assumptions and exaggerating apparent growth.
• Willow plots compared between 2001 and 2020 were largely unmatched, conflating ecological change with sampling bias.
• Global comparisons of trophic cascade strength ignored equilibrium assumptions that do not apply to Yellowstone’s still-recovering, non-equilibrium system.
• Selective photographic evidence and omission of key factors such as human hunting further distorted causal interpretation.

“Once these problems are accounted for, there is no evidence that predator recovery caused a large or system-wide increase in willow growth,” said Dr. David Cooper, co-author and emeritus senior research scientist at Colorado State University. “The data instead support a more modest and spatially variable response influenced by hydrology, browsing, and local site conditions.”

The authors emphasize that their critique does not diminish the ecological significance of large carnivores but underscores the need for rigorous methods when evaluating complex food-web interactions.

“Our goal is to clarify the evidence, not downplay the role of predators,” MacNulty said. “Predator effects in Yellowstone are real but context-dependent—and strong claims require strong evidence.”

The study reconciles conflicting interpretations of the same dataset. Ripple et al. (2025) concluded that carnivore recovery produced a strong trophic cascade, whereas Hobbs et al. (2024), who collected the data through two decades of field experimentation, found only weak cascade effects.

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Journal

Global Ecology and Conservation

DOI

10.1016/j.gecco.2025.e03899 

The ‘big bad wolf’ fears the human ‘super predator’ – for good reason

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Western University professor Liana Zanette sets up an automated camera-speaker system.   

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Fear of the fabled ‘big bad wolf’ has dominated the public perception of wolves for millennia and strongly influences current debates concerning human-wildlife conflict. Humans both fear wolves and, perhaps more importantly, are concerned about wolves losing their fear of humans – because if they fear us, they avoid us and that offers protection.

A new Western University study shows that even where laws are in place to protect them, wolves fully fear the human ‘super predator.’

These findings by Western biology professor Liana Zanette – in collaboration with one of Europe’s leading wolf experts, Dries Kuijper from the Polish Academy of Sciences, and others – were published today in Current Biology.

Zanette and her colleagues conducted an unprecedented experiment across a vast 1,100 sq. km area in north-central Poland, demonstrating that wolves fully retain their fear of humans, even where laws exist to protect them. To conduct their experiment, the team deployed hidden, automated camera-speaker systems at the intersection of paths in the Tuchola Forest that, when triggered by an animal passing within a short distance (10 metres), filmed the response of the animal to hearing either humans speaking calmly in Polish, dogs barking or non-threatening controls (bird calls).

Wolves were more than twice as likely to run, and twice as fast to abandon the site, after hearing humans compared to control sounds (birds). The same was true of wolves’ prey (deer and wild boar).

By demonstrating experimentally that wolves fear humans, the study verifies that fear of humans, who are predominantly active in the daytime, forces wolves to restrict their activities to the night. Wolves were 4.9 times more nocturnal (active at night) than humans. In fact, wolves are not just nocturnal where Zanette and her team did their study, but everywhere humans are present, as shown in a recent continent-wide survey. This new experiment establishes that the reason is because wolves everywhere are fearful of humans.  

“Wolves are not exceptional in fearing humans – and they have good reason to fear us,” said Zanette, a renowned wildlife ecologist. “Global surveys show humans kill prey at much higher rates than other predators and kill large carnivores like wolves at on average nine times the rate they die naturally, making humans a ‘super predator.’”

Consistent with humanity’s unique lethality, growing experimental evidence from every inhabited continent demonstrates that wildlife worldwide, including other large carnivores like leopards, hyenas and cougars, fear the human ‘super predator’ above all else.

Legally protected but still fearful 

“Legal protection does not change wolves’ fear of humans because legal protection does not mean not killing wolves, it means not exterminating them. This is an important distinction,” said Zanette.

Humans remain very much a ‘super predator’ of wolves even where wolves are strictly protected, such as in the European Union, where humans legally and illegally kill wolves at seven times the rate they die naturally. France, for example, allows up to 20 per cent of the wolf population to be legally killed every year. Human killing of wolves in North America is comparable.

“At these rates, any truly fearless wolf that did not avoid humans would very soon be a dead wolf,” said Zanette.

Legal protection leading to fearless wolves – not scientifically supported

Wolves are now reoccupying areas in Europe and North America where they had been exterminated, leading to increased human-wolf encounters. This increase in encounters has been attributed to legal protection allowing the emergence of fearless wolves, but these new experimental results demonstrate this assumption is not scientifically supported.

“For wolves – like all creatures great and small – fear is primarily about food, specifically, how to avoid becoming food while trying to find food. Focusing on this fundamental risk-reward trade-off is critical,” said Zanette. “The certainty that wolves fear humans means we need to re-focus attention on what counterbalances this fear, rather than whether wolves are fearless.”

Humans are both uniquely lethal and unique in being normally surrounded by super-abundant, super high-quality food. Results of the study strongly indicate any apparently fearless wolf is actually a fearful wolf risking proximity to humans to get a bite of our ‘superfoods.’

The real problem, said Zanette, is how to keep the wolf from our human food.  

“The critical significance of our study lies in re-focusing the discourse on human-wolf conflict toward public education on food storage, garbage removal and livestock protection – reducing wolf access to human foodstuffs,” said Zanette. “What our study establishes is that there is no alternate problem to contend with. There is no ‘big bad wolf’ unafraid of the human ‘super predator.’”

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Journal

Current Biology

DOI

10.1016/j.cub.2025.09.018 

Coyote populations surge, rebound quickly

New study reveals challenges associated with management of the predator

Peer-Reviewed Publication

University of Georgia

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Trying to curb coyote populations may be a lost cause, according to a new University of Georgia study.

After careful counting of the animal across the Southeast, researchers found that Eastern coyote populations stabilize faster than they can be reduced.

“In general, predator populations are contentious to manage, but coyotes are a lot harder to manage than a lot of other predators due to their really unique, amazing ability to reproduce. They can bounce back very rapidly,” said Heather Gaya, corresponding author of the study and a postdoctoral research associate in the Warnell School of Forestry.

The analysis suggests a need for alternatives when it comes to habitat management and biodiversity.

Coyotes may be more prevalent in wooded areas than previously thought

Using cameras, categorizing different howls and other biological elements, researchers quantified coyotes per square mile in the Savannah River Site and beyond in South Carolina.

They found between 45 and 50 coyotes every 38 square miles. That’s more than one coyote per every square mile.

This finding was particularly surprising, the researchers said, because of where SRS is located. Coyotes typically favor open habitats, not forested areas.

“Coyotes have the ability to occupy and adapt to many different habitats, and SRS is apparently one that can sustain a lot of coyotes with enough prey and resources for a long time,” said Gino D’Angelo, co-author of the study and an associate professor in the Warnell School.

Part of what drives the coyotes’ success is low competition from other species and lots of available prey.

“For over 75 years, we didn’t have a lot of apex predators, so coyotes started to fill that void,” D’Angelo said. “We had naive prey populations not ready for a predator at such a high abundance. That can have real dire effects on populations that aren’t used to predatory pressure.”

Population control is costly, unsustainable

The study also included an 18-year analysis of coyote populations and how they changed over time.

Researchers found that despite repeated removal efforts over the years, coyote totals rebounded — and sometimes even spiked — shortly after.

Coyotes from neighboring states also made their way into areas with reduced coyote numbers.

These control methods cost $30,000 to $50,000. So the researchers recommended investing in other solutions. That could look like adjusting hunting regulations or enhancing habitats to support other species under continued coyote presence.

“The cost and man-hours that it takes to actively remove those coyotes is something that’s just not sustainable or not practical on a large scale,” Gaya said. “I think that when we’re managing coyotes, we have to consider if it’s worth it to put in all of that time and money for what seems to be short-term gain. And if we’re not able to sustain that in the long term, maybe we should be thinking about other options.”

This research was also co-authored by UGA alumnus Jordan Youngmann, an associate research scientist for the Odum School of Ecology, and Stacey Lance and John Kilgo.

 

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Journal

Ecosphere

DOI

10.1002/ecs2.70339 

 

When bison are room to roam, they reawaken the Yellowstone ecosystem

 On Aug. 28, scientists from Washington and Lee University, the National Park Service and the University of Wyoming published research in Science magazine shedding new light on the value of bison recovery efforts in Yellowstone National Park.

Bill Hamilton, John T. Perry Jr. Professor in Research Science at Washington and Lee University, and Chris Geremia, a researcher with the National Park Service at Yellowstone, served as co-first authors, with co-author Jerod Merkle, associate professor and Knobloch Professor in Migration Ecology and Conservation at the University of Wyoming.

While momentum is building to restore bison across North America, most efforts focus on small, managed herds, leaving it unclear how large, migrating herds shape landscapes and whether their effects enhance or degrade ecosystems. The study by Geremia, Hamilton and Merkle considered large migrating herds of bison at Yellowstone National Park, observing their grazing habits as they concentrate in the park’s river valleys during spring and summer.

With a population of around 5,000 animals (stabilized since the mid-2010s, after recovering from a low of 23 animals in 1902), bison today travel about 1,000 miles each year, making back-and-forth movements along a 50-mile migration route. Along their route, the bison graze intensely, consuming young growing plants emerging after snow melts; to many, this might look overgrazed, but they found that it is far from the whole story.

The research team’s study suggests that bison speed up the nitrogen cycle as they graze; the plants grow as much as they would if they weren’t grazed but, strikingly, are 150% more nutritious.

The nitrogen cycle is how nitrogen moves between the plants and animals of the ecosystem and the air and soil. Microbes in the soil recycle nitrogen from decaying plants and animals into forms typically seen in common fertilizers (ammonium and nitrate). Those forms are favored by plants to reuse. The authors found that as bison graze they increase the amount of microbes, resulting in more nitrogen available for plants to use and, ultimately, more nutritious plants for plant-eating animals.

As the bison’s grazing continues throughout the summer, this cycle is reinforced, keeping plants short, dense and nitrogen rich.

“What we’re witnessing is that as bison move across the landscape, they amplify the nutritional quality and capacity of Yellowstone,” Hamilton said. “Their grazing likely has important consequences for other herbivores and for the food web as a whole, similar to the changes that occurred in the Serengeti when the wildebeest population recovered.”

To understand how bison create these changes in the nitrogen cycle, the researchers conducted field experiments from 2015 to 2021, monitoring plant growth, nutrient cycling, plant and soil chemistry, herbivory, plant community composition and soil microbial populations. They used movable exclosures to compare grazed and ungrazed conditions, and they combined these experiments with landscape-scale satellite imagery and GPS collar data. This allowed them to map bison impacts across the entire migratory landscape.

“With the current large herds of bison, Yellowstone grasslands are functioning better than in their absence,” Hamilton said. “And this version is a glimpse of what was lost when bison were nearly wiped out across North America in the late 1800s.”

Over the course of the study, the researchers found that the soils sustained their nutrient storage. Plant communities changed in some areas, but productivity was maintained, and the biodiversity of plants increased across the migration corridor. Overgrazing is often characterized as a landscape where plant productivity and diversity go down and soils become compacted with resulting reductions in nutrient storage and cycling.  “Yet, we found pretty much the opposite of that” said co-author Merkle. “The return of a large-scale bison migration provides clear benefits to the ecosystem services that underlie Yellowstone. Heterogeneity is what bison seem to provide. When I look out across the bison migration, there is strong variation in the amount of grazing — some places appear to be very short lawns while others remain untouched.”

Yellowstone has long served as a model of ecological restoration, and this new research highlights the overlooked power of restoring large herbivores in large, free-moving numbers. Unlike traditional bison conservation efforts focused on small, fenced areas and controlled numbers based on grazing management principles, Yellowstone shows the value of restoring movement and scale. Migrating bison reshape the land not by being managed — but by moving.

Hunting wolves reduces livestock deaths measurably, but minimally

 Wolf hunting has prevented livestock loss in a measurable way, but it is by no means a silver bullet, according to an international research team led by the University of Michigan.

"Hunting, on the whole, is not removing negative impacts associated with wolves. It does have some effect on rates of livestock loss, but the effect is not particularly consistent, widespread or strong," said Neil Carter, associate professor at the U-M School for Environment and Sustainability and senior author of the new study published in the journal Science Advances. 

As governmental protections have helped rebuild once-dwindling wolf populations, it's become increasingly likely that the predators encounter domesticated animals to prey on. Hunting is often presented as a remedy, but it's a contentious and polarizing proposal. Advocates on both sides of the issue have active lawsuits across the U.S. aiming to relax or redouble regulations.

But several northwestern states already allow hunting in some capacity, which has given researchers an opportunity to bring new information to the issue. The new study, supported by the U.S. National Science Foundation, showed that, on average, each wolf that was killed by a hunter was associated with a 2% reduction in predation.

The team also analyzed how hunting affected "lethal removals." These are expensive operations led by government agencies, targeting specific wolves, typically after multiple or severe predation events, said lead author Leandra Merz, assistant professor at San Diego State University.

Based on the team's study, hunting led to no reduction in lethal removals.

"We're not necessarily saying that we shouldn't be hunting and I want to be clear about that, because there are other motivations for hunting," said Merz, who worked on the project as postdoctoral scholar at U-M's School for Environment and Sustainability. "But if the goal is to reduce livestock predation and we're using hunting for that, it's not as effective as we would like."

The research team also included collaborators from the University of Idaho, Washington State University, Ohio State University and the GESIS Leibniz Institute for the Social Sciences in Germany. The researchers said this is an important part of a small but growing body of research that brings relevant information to a charged debate about wolf management strategies.

"A lot of uncertainty exists about the utility of public wolf hunting in reducing negative impacts from these animals as their populations recover," Carter said. "In an issue that's divisive and contentious, that uncertainty is something that we should try to minimize, because we could be making decisions that are just not as efficacious as they should be nor in the public’s best interest."

And out come the wolves

The Endangered Species Act, or ESA, has protected gray wolves across the contiguous U.S. for decades and, in the 1990s, the U.S. launched a successful wolf reintroduction campaign in the Northern Rocky Mountains. Within about 20 years, the Northern Rocky wolf population grew to where some states rolled out legalized hunting programs.

Since then, wolf hunting has been characterized by starts and stops driven by litigation brought by parties on both sides. In fact, in 2020, the U.S. Fish and Wildlife Service determined that gray wolf populations had recovered significantly enough to remove federal protections under the ESA before a court order reversed that decision in 2022. And the debate is far from over.

"There are timely things happening related to wolf management, both domestically and internationally," Carter said, pointing to Michigan and Europe as regions where the issue is especially being pressed. "Stakeholders involved in wolf management are bringing up the topic of hunting wolves and it's an imminent conversation we'll be having."

Yet the actual impact of wolf hunting on protecting livestock was unknown, meaning there have been a lot of assumptions involved in the discourse, Carter said. So he, Merz and their colleagues saw an opportunity to shed light on that using information from areas where wolf hunting was legal—Idaho and Montana—and from areas where wolf hunting was not legal, specifically Oregon and Washington.

The research analyzed data available for those areas from 2005 and 2021 using an array of mathematical models to connect hunting to its influence on depredation and lethal removal events. No hunting was permitted prior to 2009, helping researchers establish a control scenario.

Again, hunting showed no impact on the number of lethal removal operations, but predation was more nuanced. The average county in the analysis lost roughly 3 to 4 livestock animals per year to wolves. With the 2% reduction mentioned earlier, that translates to about 0.07 animals protected per wolf hunted, Carter said.

But the team also emphasized there can be huge variations from that average. For instance, Merz knows of an Idaho rancher that lost 65 sheep in one night to wolves. And a loss need not be that severe to have devastating economic consequences and steep psychological tolls.

"The cost can be really high to an individual rancher, even over very short time periods," Merz said. "We don't want to minimize that."

From a policy perspective, though, it's helpful to ask what are the best management methods to spread that cost effectively and equitably, Merz said. Hunting, as the study shows, relieves a minimal burden from ranchers, but at no direct cost to them.

Then there are nonlethal methods that are proving to be effective, Merz said, including fladry, a combination of flags and barriers that can be electrified. Even increasing the presence of humans or human activities helps (drones blasting AC/DC and arguments from the movie "Marriage Story" recently made news). But the cost of implementing and maintaining these measures almost exclusively falls on ranchers.

"There's not going to be an easy solution either way. If there were, we would have figured it out by now and we'd be using it," Merz said. "But the upside is that people are really creative. We just need to be a little bit more creative in how we redistribute some of the costs and benefits. I think outside of managing wildlife, we do that a lot in society."

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Journal

Science Advances

DOI

10.1126/sciadv.adu8945