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 

River otters unfazed by feces and parasites while eating… and that’s good for ecosystems

 North American river otters have terrible hygiene when it comes to their food. They eat, play and defecate in the same place. But their unhealthy habits make them ideal for detecting future health threats in the environment, according to scientists. In a new study published Aug. 14, Smithsonian scientists analyzed the otters’ diets and “latrine” habitats in the Chesapeake Bay for the first time. They discovered river otters often eat food riddled with parasites—and that may not be a bad thing for the larger ecosystem.

“River otters are impressive apex predators that play a vital role in ecosystems,” said Calli Wise, lead author of the study and a research technician at the Smithsonian Environmental Research Center (SERC). “The parasites consumed by river otters may also teach us about the health of the environment.”

River otters are among the most elusive animals in the Chesapeake. They’re nocturnal, semi-aquatic and generally shy around people, so live sightings are rare. Once abundant across North America, their numbers dwindled due to the fur trade and habitat degradation. A Maryland reintroduction program in the mid-1990s helped their populations rebound across the state. But even as they bounce back, scientists still don’t have precise estimates as to their population numbers in the Bay region. And many other aspects of their behavior and diets remain obscure.

“It is shocking how little information there is about their biology and ecology,” said Katrina Lohan, co-author and head of SERC’s Coastal Disease Ecology Lab.

Since live otters are difficult to observe, biologists rely on what they leave behind. Specifically, their feces. Otters leave the water periodically to congregate at latrines—sites on land where they eat, socialize and leave fresh droppings as scent marks for other otters. By studying the feces (or “scat”) from otter latrines, scientists can get a sense of what the otters are eating.

The latest study, published in the journal Frontiers in Mammal Science, looked at scat from 18 active latrines on the SERC campus in Edgewater, Maryland. Most were natural sites, such as beaches or riverbanks, but a few latrines appeared on manmade structures like docks or boardwalks. The biologists took the scat samples back to the lab, where they surveyed the samples under the microscope and ran DNA analyses using a technique called metabarcoding.

Finfish and crabs formed the staples of otter diets—accounting for 93% of all prey items in the DNA analysis. The otters also ate amphibians, worms and the occasional bird. The researchers even found evidence that otters ate two invasive fish: the common carp and the southern white river crayfish.

But the DNA analyses also uncovered a host of parasites from six different taxonomic classes teeming in the otter scat. The vast majority were trematodes—parasitic flatworms also known as “flukes.” Other parasites included microscopic dinoflagellates and other flatworms known to infect the gills, skin or fins of fishes. Most of the parasites likely infected the otters’ prey, not the otters themselves—and the otters probably weren’t any worse off for eating them. In fact, Lohan suggested, otters may be helping the overall prey populations by eating parasite-infected animals, since this weeds out sicker fish and crabs. Meanwhile, parasites may be helping the otters catch prey that would otherwise elude them.

“While parasites have negative impacts on individuals, they are extremely important in food webs,” Lohan said. “It is possible that river otters, like other top predators, wouldn’t be able to find enough food to eat without parasites.”

However, a few parasites in the study, such as roundworms and single-celled apicomplexans, are known to infect mammals. The scientists suspect these parasites directly infected the otters themselves, rather than their prey. This study did not detect any parasites in river otters than can infect humans. But some of the parasites were closely related to ones that can cause human disease, including the gastrointestinal disease cystoisosporiasis. As river otters are appearing more often in urban and suburban areas, the likelihood of them encountering something that could affect human health is also rising.

“As river otters move into more urban waterways, they will be increasingly exposed to pollutants and parasites of concern to humans,” Wise said. “As mammals, river otters may be disease sentinels that we can study to learn more about environmental risks to humans.”

Researchers from Frostburg State University, Johns Hopkins University and the University of the Pacific also contributed to this study. A copy of the study will be available on the journal’s website after publication. For photos, an advance copy of the study or to speak with one of the authors, contact Kristen Goodhue at GoodhueK@si.edu.   

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Journal

Frontiers

DOI

10.3389/fmamm.2025.1620318 

Strategically bringing back beavers could support healthy and climate-resilient watersheds

• Ponds created by beaver dams can help increase freshwater storage, boost biodiversity, contain wildfires, and improve water quality.

• Beaver populations in North America have fallen from an estimated 60-400 million before European colonization to roughly 10-15 million today because of extensive hunting, habitat degradation, and trapping.

• Better maps could help watershed managers prioritize areas for beaver reintroduction that would maximize benefits while highlighting trade-offs for water users.

After enduring centuries of hunting, habitat loss, and disease, North American beavers (Castor canadensis) are making a comeback – and bringing benefits for both humans and nature with them.

Equipped with findings from a new study published Aug. 11 in Communications Earth & Environment, a team of researchers from Stanford and the University of Minnesota aims to ensure that beavers return to or establish new homes in areas with the biggest bang for their buck (teeth).

Supported in part by a grant from the Stanford Woods Institute for the Environment’s Environmental Venture Projects program, the research reveals some of the factors that determine how well beavers can function within a given watershed. The findings could inform decisions about how to manage habitats, wildlife, and waterways.

“Our findings can help land managers figure out where beaver activity will have the biggest impact,” said lead study author Luwen Wan, a postdoctoral fellow in Earth system science at the Stanford Doerr School of Sustainability and the Institute for Human-Centered Artificial Intelligence. “It gives them a practical tool for using nature to solve water and climate problems.”

Although beavers often receive a bad reputation when their dams flood a farmer’s field or block drainage from a busy highway, their dynamic and rapid dam construction makes them superheroes in natural watershed management. Beaver dams create cool ponds that foster biodiversity, improve water quality, and even limit the spread of wildfires. They frequently construct multiple dams within an area, creating a wetland network of surface water and vegetation known as “beaver wetland complexes.”

These complexes provide long-term freshwater storage and recharge groundwater – a crucial benefit, especially in the American West, where dwindling surface water supplies are the result of years of sustained climate change-driven drought and over-allocation of surface water supplies, as seen in the Upper Colorado River Basin.

“Beavers are naturally doing a lot of the things that we try to do as humans to manage river corridors,” said study senior author Kate Maher, a professor of Earth system science at the Stanford Doerr School of Sustainability and a senior fellow at the Woods Institute for the Environment. “Humans will build one structure, leave it there, and hope it lasts for many decades. Beavers on the other hand, build little, tiny dams where they're needed and flexibly manage what's going on with the water in their environment.”

Maher and Wan collaborated with Emily Fairfax, a beaver expert at the University of Minnesota who has mapped beaver dams through topographic surveys and remote sensing imagery for years. However, traditional surveys in remote areas limit the scale and detail needed to holistically map beaver ponds and their impact on hydrology and ecology. Additionally, dams and ponds are often too small for satellite imagery to capture.

The new study details how the team mapped more than 80 beaver pond complexes across diverse regions in Colorado, Wyoming, Montana, and Oregon using high-resolution aerial imagery from the USDA National Agricultural Imagery Program. They then identified key factors influencing variations in  beaver dam length and pond area. 

Their approach allowed the researchers to link pond size to unique landscape features like topography, vegetation, climate, soil characteristics, and stream hydrology. For instance, they found that longer dams were correlated with larger ponds, which in turn could increase ecosystem benefits like cooler local air temperatures and more fish habitat.

Despite the potential for wetland resilience and restoration, beaver activity can create problems for nearby communities. New dams can temporarily reduce water flows, putting stress on downstream water users already struggling to find sufficient surface water supplies during drought conditions. Unmanaged beaver populations can pose a flooding threat to homes, crops, and infrastructure.

“There's definitely a lot of exuberance around reintroducing beavers, and it may not be that every beaver reintroduction project is the right one to pursue,” said Maher. “It’s important to understand those trade-offs and the risks and rewards from either intentionally reintroducing beavers, or just their natural return to watersheds.”

The team's research highlights the possibility of achieving dual benefits by relocating so-called “nuisance beavers” to watersheds with the capacity to support a beaver population and maximize the natural benefits beavers create. Wan also notes that the approach could help decision-makers understand the impact of beaver-inspired human structures like beaver dam analogues (BDAs) and other nature-based water management structures.

Moving forward, Wan and Maher are eager to collaborate with Jeannette Bohg, an assistant professor of computer science in the Stanford School of Engineering and co-investigator on the project, to apply machine learning methods to their mapping. Ultimately, the researchers envision dynamic risk maps that policymakers, watershed managers, and ecologists can use to quantitatively evaluate where, when, and how to bring back beavers.

Pups in tow, Yellowstone-area wolves trek long distances to stay near prey

 

For the first time, a UC Berkeley-led research team has observed gray wolves outside of the Arctic migrating during pup-rearing season

Peer-Reviewed Publication

University of California - Berkeley

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In a new study, a UC Berkeley-led team of biologists observed gray wolves near Yellowstone National Park traveling 20 kilometers or more over rugged, mountainous terrain, with very young pups in tow. It is the first time gray wolves outside of the Arctic have been observed migrating, or shifting their territorial range, to be closer to prey during pup-rearing season. This photo from the Thorofare Wilderness on the southeast side of Yellowstone National Park shows the summer range of one of Yellowstone’s migratory elk herds.

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Credit: Avery Shawler

Berkeley — Gray wolf pups are born nearly helpless: blind, deaf and lacking the acute sense of smell of their elders. They usually remain in the safe confines of their den until they are at least three weeks old.

That is why UC Berkeley biologists were surprised to observe gray wolves near Yellowstone National Park traveling 20 kilometers or more over rugged, mountainous terrain, with very young pups in tow. 

“The first time I saw a camera trap photo of a wolf carrying its pup, I just cracked up because the pup is being carried by its butt,” said Avery Shawler, first author of a new study presenting the findings, which appeared online today (Aug. 1) in the journal Current Biology. “You can picture a squirming child and the mom just being like, ‘All right, we're doing this.’”

Shawler and the other researchers believe wolves undertook these risky journeys to move their packs closer to elk, their preferred prey, during the elk spring migration to higher altitudes. The study is the first time gray wolves outside of the Arctic have been observed migrating, or shifting their territorial range, to be closer to prey during pup-rearing season. 

“Our findings counter years of assumptions by researchers that migratory hoofed mammals can escape predation in spring because [their predators] are tied to dens and immobile offspring,” said study senior author Arthur Middleton, a Berkeley professor of environmental science, policy and management.

Understanding how wolves are adapting to the movements of their prey is key to the conservation of both species, Shawler said. It can help land managers understand seasonal patterns of human-wildlife conflict in an ecosystem that includes both ranches and wilderness, where wolves may view livestock as a tasty alternative to elk. 

“In the U.S., more wolves live outside of protected areas than within protected areas, and these wolves are going to overlap with humans and livestock,” said Shawler, who completed a Ph.D. at UC Berkeley in 2024. “Our research provides some insight into the behavior of wolves living on working landscapes and how they've had to adapt to an environment that is different from what wolves were dealing with 100 years ago.”

Gray wolves were reintroduced to Yellowstone National Park in 1995, and researchers estimate that their population in the Greater Yellowstone Ecosystem has since grown to around 500. Tens of thousands of partially migratory elk also inhabit the region. 

Climate change and shifts in land use are putting pressure on both species and leading them to adapt accordingly. Earlier research led by Middleton showed that the timing of annual elk migrations is currently in flux, with elk arriving at their winter ranges up to 50 days later in 2015 compared to 2001.

To explore how elk migration patterns impact wolf behavior, the researchers used GPS collars to track the movements of 19 gray wolves and 99 elk in the eastern Greater Yellowstone Ecosystem between 2019 and 2021. 

They found that wolves are surprisingly adaptable to the movements of their prey. Some elk herds in Yellowstone only migrate short distances in the spring, and the wolf packs that tracked them generally stayed in the same territory where they first established their dens. Other elk herds travel much longer distances in the spring, and wolf packs that tracked them had to get more creative, engaging in behaviors the researchers called “commuting” and “migrating.”

The researchers used the term “commuting” to describe temporary forays taken outside of the wolves’ home territories, usually to track migrating elk herds. 

Wolves “migrated” when they moved to an entirely new seasonal range, following migrating elk up to 50 km. Sometimes they carried small pups as far as 20 km from their original dens to new pack “rendezvous” sites. 

“In Yellowstone, research has shown how a lot of wolf mortality can come from other packs coming in and killing pups, because there's a lot of packs competing for space and food,” Shawler said. “It's pretty wild that this risky behavior of moving young pups is even occurring when that's happening next door.”

The findings can inform conservation efforts and land management in any region that has gray wolves — including California, which is home to approximately 10 packs after wolves began recolonizing the state in 2011. Middleton is co-leading the new California Wolf Project, which aims to understand the social and ecological factors that are shaping these wolf populations.

“While it’s still early days, our partners in California have a strong hunch that the numbers and movements of deer and elk are playing into wolf behavior, including livestock predation,” Middleton said. “The work around Yellowstone sharpens our ideas and approaches as we grow the project in California.”

Additional study co-authors include Kristin J. Barker of UC Berkeley and Beyond Yellowstone Living Lab; Wenjing Xu of the Senckenberg Biodiversity and Climate Research Centre in Germany; and Kenneth J. Mills and Tony W. Mong of the Wyoming Game and Fish Department.

The study was supported by the National Geographic Society, Knobloch Family Foundation, George B. Storer Foundation, the Wyoming Game and Fish Department, UC Berkeley, and the USDA National Institute of Food and Agriculture, and conducted from a base at the Buffalo Bill Center of the West in Cody, Wyoming.

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Journal

Current Biology

DOI

10.1016/j.cub.2025.07.015