Broken by bison, aspen saplings having a tough time in northern Yellowstone

 

 

IMAGE: A BISON BULL BREAKING ASPEN SAPLINGS AND EATING ASPEN IN THE LAMAR VALLEY IN NORTHERN YELLOWSTONE NATIONAL PARK. OVERSTORY ASPEN TREES HAVE DIED AND FALLEN TO THE GROUND AS SEEN IN THE PHOTO, AND TALL SAPLINGS HAVE GROWN SINCE THE EARLY 2000S. BROKEN STEMS OF ASPEN SAPLINGS ARE VISIBLE IN THE FOREGROUND, AND SILHOUETTED AGAINST THE BODY OF THE BISON (PHOTO BY LUKE PAINTER, 2020). THESE BROKEN STUMPS SHOW THAT SAPLINGS RECENTLY COVERED MUCH MORE OF THE STAND AREA, BUT THEY HAVE BEEN BROKEN AND KILLED BY BISON, AND BISON CONTINUE TO BREAK THE REMAINING SAPLINGS. THESE SAPLINGS WERE TALLER THAN 4 METERS, NO LONGER VULNERABLE TO SUPPRESSION BY ELK OR BISON EATING THEIR TOP BRANCHES, BUT BISON BROKE THEM OFF AT A LOW HEIGHT. view more 

CREDIT: A BISON BULL BREAKING ASPEN SAPLINGS AND EATING ASPEN IN THE LAMAR VALLEY IN NORTHERN YELLOWSTONE NATIONAL PARK. OVERSTORY ASPEN TREES HAVE DIED AND FALLEN TO THE GROUND AS SEEN IN THE PHOTO, AND TALL SAPLINGS HAVE GROWN SINCE THE EARLY 2000S. BROKEN STEMS OF ASPEN SAPLINGS ARE VISIBLE IN THE FOREGROUND, AND SILHOUETTED AGAINST THE BODY OF THE BISON (PHOTO BY LUKE PAINTER, 2020). THESE BROKEN STUMPS SHOW THAT SAPLINGS RECENTLY COVERED MUCH MORE OF THE STAND AREA, BUT THEY HAVE BEEN BROKEN AND KILLED BY BISON, AND BISON CONTINUE TO BREAK THE REMAINING SAPLINGS. THESE SAPLINGS WERE TALLER THAN 4 METERS, NO LONGER VULNERABLE TO SUPPRESSION BY ELK OR BISON EATING THEIR TOP BRANCHES, BUT BISON BROKE THEM OFF AT A LOW HEIGHT.

CORVALLIS, Ore. – In northern Yellowstone National Park, saplings of quaking aspen, an ecologically important tree in the American West, are being broken by a historically large bison herd, affecting the comeback of aspen from decades of over-browsing by elk.

Findings of the research led by Luke Painter of Oregon State University were published today in Ecology and Evolution.

The study comes five years after Painter, who teaches ecology and conservation in the OSU College of Agricultural Sciences, published a paper in Ecosphere showing that wolf reintroduction in Yellowstone had been a catalyst for aspen recovery both outside and inside park boundaries.

“I’ve studied the response of aspen in northern Yellowstone to the reduction in elk after the wolves were brought back and found that during this time, bison increased and have begun to affect aspen,” Painter said. “Now we’re showing strong evidence of a previously unreported behavior of bison bulls breaking aspen saplings.”

The saplings were tall enough to escape most browsing by elk and thus likely to grow into trees, but bison broke them off at a low height, he said. Other saplings were killed when bison scraped off the bark with their horns.

Quaking aspen largely reproduces by root sprouts, a process known as suckering, and stands of aspen are often a single organism connected by the trees’ common root system. Fire stimulates aspen reproduction from both roots and seeds.

For much of the 20th century, Painter said, aspen sprouts were unable to grow into trees because they were eaten by elk during winter. But at the end of the century, when wolves were reintroduced and the numbers of other large predators such as grizzly bears and cougars increased, elk numbers in the northern part of Yellowstone went down, bringing relief to the aspen.

“Some young aspen began growing into saplings – young trees taller than 2 meters – which was an indication they were no longer being consumed by elk and were likely to grow into mature trees,” Painter said. “It was a trophic cascade that changed the Yellowstone ecosystem, creating conditions that could bring it closer to what it was historically, with more aspen, willow and beaver, which depend on these plants. But the tremendous increase in bison over the last two decades has added a new turn to the story.”

Bison have long been known to have strong effects on their environment, Painter said. Among those is removing and suppressing shrubs and trees by eating, trampling and breaking them – and as bison numbers have greatly risen in northern Yellowstone in the last two decades, their effects on plants have also increased.

In places where bison are present in large numbers, like Yellowstone’s iconic Lamar Valley, they are hindering some aspen stands from replacing their dying trees, he said.

The Yellowstone bison herd is divided into central and northern herds, and this study was in the range of the northern herd. The northern herd’s numbers were generally less than 1,000 until 2005 and then increased, for reasons that aren’t fully understood, to about 4,000 during the last decade, Painter said.

Painter and OSU College of Forestry collaborators Robert Beschta and William Ripple examined a random sampling of plots in 87 randomly selected aspen stands, and 18% of saplings had been broken. They may resprout from their base, but the sapling height has been lost and new sprouts are vulnerable to being eaten by bison or other herbivores, the researchers note.

Multiple lines of evidence support attributing the breakage to bison, Painter said.

“Most broken saplings were in areas of high bison density and low elk density, and they were broken in summer when elk wouldn’t have been foraging on them,” he said. “Plus we directly observed bison breaking aspen saplings. The purpose of the behavior doesn’t seem to be about accessing food, and we observed only bulls engaging in this behavior, so it may be related to displays of aggression.”

Painter noted that Yellowstone bison are managed under an agreement with the state of Montana that requires them to remain in or very near the park – those that stray are killed, captured or hazed back into the park, in large part because they can carry bacteria that cause brucellosis, a threat to Montana’s cattle industry.

Elk also carry brucellosis and have passed it to cattle, but the same restrictions are not applied to them. Thus, unlike other wildlife bison are not allowed to disperse to other areas as their numbers and density increase.

The conservation of bison in Yellowstone, whose numbers plummeted nationally from over-hunting in the 1800s, is a big success story, Painter noted – and so is the recovery of aspen and other deciduous woody plants that began when the park’s large predators made their comeback.

“Thus, one important conservation goal is affecting another important conservation goal,” he said. “Researchers are only beginning to understand how these conservation goals have overlapped and affected each other. We reported a piece of this complex puzzle, describing and quantifying one way that bison shape their habitat by suppressing trees.”

The study was supported in part by the National Science Foundation and the Ecosystem Restoration Research Fund of the Oregon State University Foundation.

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JOURNAL

Ecology and Evolution

DOI

10.1002/ece3.10369 

Gray wolf recovery is a success—is that a problem?

 

Peer-Reviewed Publication

AMERICAN INSTITUTE OF BIOLOGICAL SCIENCES

Over the past 30 years, efforts to recover gray wolf populations in the United States have been broadly successful, with many regions now sporting robust populations of the charismatic carnivore. Writing in BioScience, wolf experts David E. Ausband and L. David Mech describe the conservation landscape and also the obstacles that wolves face as their populations expand into their historical ranges.

                "Remarkable wolf conservation success yields remarkable challenges," say the authors, as 6000 wolves now occupy habitat across 11 states. These growing populations now face significant threats as they attempt to colonize human-dominated areas, among them "fragmented habitats and barriers to dispersal, as well as increased encounters with humans, pets, and livestock."

                In response to those concerned about wolves’ potential impacts to prey populations and domestic livestock production, many jurisdictions have ramped up wolf efforts. For instance, in Wisconsin, "the legislature requires a public hunting or trapping season whenever wolves are delisted from the US Fish and Wildlife Service’s (USFWS) list of Endangered species." In contrast, wolves are seen as desirable in other areas, such as Colorado, where voters recently passed a ballot initiative to reintroduce them in the state. The authors caution that such pro-reintroduction initiatives, which may seem initially promising for wolves, could have the unintended consequence of setting precedent for laws barring reintroduction and thus complicate management. An uncertain regulatory regime, say Ausband and Mech, could cause major fluctuations in wolf populations, with dire consequences for conservation efforts.

                The answer to this quandary, the authors suggest, is thoughtful management that carefully considers the needs of diverse stakeholders: "Future wolf conservation in the United States will be affected by the ability of managers to predict colonization and dispersal dynamics, to reduce hybridization and disease transmission, to mitigate and deter wolf–livestock conflicts, to harvest wolves sustainably while satisfying diverse stakeholders, to avert a reduction in tolerance for wolves due to a disinterest in nature, and to engage diverse stakeholders in wolf conservation to avoid management by ballot initiative or legislative and judicial decrees." Only through such science-informed management, argue Ausband and Mech, can the present success of wolf conservation be built on in the future.

***

David E. Ausband is affiliated with the US Geological Survey’s Idaho Cooperative Fish and Wildlife Research Unit, at the University of Idaho, in Moscow, Idaho, and L. David Mech is affiliated with the US Geological Survey’s Northern Prairie Wildlife Research Center, in Jamestown, North Dakota, and with the University of Minnesota in St. Paul.

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JOURNAL

BioScience

DOI

10.1093/biosci/biad053 

Moose can play a big role in global warming

Peer-Reviewed Publication

NORWEGIAN UNIVERSITY OF SCIENCE AND TECHNOLOGY

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IMAGE: THIS BIG BOY CAN BE RESPONSIBLE FOR HUGE AMOUNTS OF CARBON EMISSIONS, SIMPLY BY EATING YOUNG VEGETATION THAT SPROUTS AFTER A CLEARCUT AND THAT IF LEFT ALONE, WOULD GROW UP AND STORE CARBON. view more 

CREDIT: PHOTO: ENDRE GRÜNER OFSTAD

One of the biggest potential single sources of carbon emissions from wooded parts of Norway has four legs, weighs as much as 400-550 kg and has antlers.

That’s right — moose can reduce carbon storage in clearcut sites equivalent to as much as 60 per cent of the annual fossil fuel carbon emissions from a region, a new study shows.

“Moose are an ecosystem engineer in the forest ecosystem, and strongly impact everything from the species composition and nutrient availability in the forest,” said Gunnar Austrheim, an ecologist at the NTNU University Museum who was one of the study’s co-authors. “A grown animal can eat 50 kilograms of biomass each day during summer.”

That consumption represents roughly 10 per cent of what the Norwegian forest industry itself harvests, he said.

And therein lies the reason why moose can be responsible for such a large additional amount of carbon emissions, said Francesco Cherubini, director of NTNU’s Industrial Ecology (IndEcol) Programme, and co-author of the paper.

Moose influence vegetation growth and more

Moose like to eat young deciduous trees, like birch, rowan and willow. So the young saplings that would normally sprout in the forest after a timber company clearcuts an area never get the chance to grow.

When saplings grow into mature trees, they bind up CO2 in their trunks, leaves and roots. Moose essentially gobble up that possible source of carbon storage.

“It was really a surprise to see how much moose can influence vegetation growth, the carbon cycle and the climate system,” said Xiangping Hu, a researcher at IndEcol and co-author of the study.

Scientists have known that browsing by large animals like moose could be an unaccounted-for source of additional carbon emissions, but there are very few studies with actual numbers to say precisely how much, Hu said.

Filling in the unknowns in climate modelling

Researchers use computer models to try to predict future climate scenarios, based on current and expected emissions of carbon dioxide and other greenhouse gasses.

That’s basically the information we get from the Intergovernmental Panel on Climate Change, or IPCC.

Most recently, the IPCC said humanity is on track to raise the Earth’s average temperate by 2.4 C, which is quite a bit higher than the 1.5 C goal that scientists agree we should aim for.

The challenge is that climate modelling is imperfect. It’s getting better, but there are areas where researchers simply don’t have enough information yet.

They know there are factors that should be in their climate models, but they simply don’t have enough data to include those factors in a realistic way.

The effects of large animals are one of those factors, Cherubini said.

“One of the biggest unknowns that we have in our understanding of the climate system and the carbon cycle is potentially the effect of larger animals, and how they interact with carbon storage in vegetation,” he said.

“This study gave us a great opportunity to quantify this effect,” he said. “We have some numbers that we can relate to the regional carbon budget, and which actually show the importance of large animals like the moose.”

Good for the forest industry, maybe not so good for the climate

The researchers were able to discover the importance of moose on climate as a result of a different, but related study that began in 2008.

At that time, researchers at NTNU and NINA ( the Norwegian Institute for Nature Research) wanted to know what effect moose had on the forest ecosystem after an area had been clearcut. They looked at vegetation regrowth, species diversity and soil nutrient dynamics. Clearcutting is a process where essentially all trees are harvested from an area.

So the researchers set up 47 paired plots in areas that had been clearcut in the previous three years. One of the plot pairs was fenced off so that moose couldn’t browse on the tasty new saplings that naturally spring up after an area has been cut.

The second, nearby plot pair, was open to moose but marked so that researchers could go back year after year to see what happened to tree regrowth and other ecosystem metrics as moose feasted on the vegetation.

What they found was that moose were doing Norway’s forest industry a huge favour, Cherubini said.

“So the forest industry prefers coniferous species, they prefer pine, they prefer spruce. So moose to some extent, are helping them because they’re reducing competition,” by thinning out the deciduous trees and partly pine, leaving the spruce, he said.

Win for the climate, biodiversity and forest management?

The researchers realized they could revisit the plots to study effects on carbon emissions by calculating the differences in aboveground carbon content between browsed and unbrowsed plots.

That enabled them to see potential additional carbon emissions that moose caused by eating deciduous saplings.

Birch and other highly selected deciduous species such as rowan, willow and aspen may also help contribute to the biodiversity of an area, Austrheim said, which moose also affect by removing those species.

So while moose were relatively good for the forest industry, they aren’t necessarily that good for the climate or biodiversity.

But there’s good news.

Finding a balance

Almost all of Norway’s productive forests are harvested using clearcuts.

“These clearcuts provide a lot of good food for moose,” Austrheim said.  Moose are also very heavily managed in Norway, he said.

“We don’t only regulate the amount of animals, we very carefully regulate the proportion of females, males and calves. So there’s a  stronger management for moose than for most livestock in Norway,” he said.

That means it should be possible to find the right balance between moose numbers and how forested lands are managed. That, in turn, could make it possible to limit excess carbon emissions, boost biodiversity and increase forest productivity, the researchers said.

“I think as we get more of an understanding of how all these different things are interrelated, land managers could come up with an optimal plan,” Cherubini said. “That could be a much needed win-win solution for climate, for biodiversity and for timber value.”

Reference: Salisbury, J., Hu, X., Speed, J. D. M., Iordan, C. M., Austrheim, G., & Cherubini, F. (2023). Net climate effects of moose browsing in early successional boreal forests by integrating carbon and albedo dynamics. Journal of Geophysical Research: Biogeosciences, 128, e2022JG007279. https://doi.org/10.1029/2022JG007279

 

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JOURNAL

Journal of Geophysical Research Biogeosciences

DOI

10.1029/2022JG007279 

New study identifies key success factors for large carnivore rewilding efforts

 

• The findings are based on data from almost 300 relocations of large carnivores, from wolves to bears;
• Relocations showed a high overall success rate, and a significant increase over recent years;
• Key factors that boosted survival of relocated animals included using younger animals, using wild-born animals, and including an acclimatisation period;
• Low mating success of relocated animals indicates ongoing challenges for rewilding programmes.

New research published today has identified the top factors that determine whether efforts to relocate large carnivores to different areas are successful or not. The findings could support global rewilding efforts, from lynx reintroductions in the UK to efforts to restore logged tropical forests.

 As apex predators, large carnivores play crucial roles in ecosystems, however their numbers have plummeted over recent decades. Relocating large carnivores can support their conservation, for instance to reintroduce a species to an area where it has been exterminated, or to reinforce an existing population to increase its viability. But to date, there has been little information about what factors determine whether these (often costly) efforts are successful or not.

The study was carried out by an international team led by researchers at the University of Oxford’s Department of Biology, Wildlife Conservation Research Unit (WildCRU), and School of Geography and the Environment. The group analysed data from almost 300 animal relocations which took place between 2007 and 2021. These spanned 22 countries in five continents, and involved 18 different carnivore species, including bears, hyaenas, big cats, and wild dogs.

Key findings:

• Overall, two thirds (66%) of the relocations were successful (where the animal survived in the wild for over 6 months).
• Success rates for large carnivore relocations have increased significantly since before 2007. For wild-born carnivores, success rates increased from 53% pre-2007 to 70%; and for captive-born animals, success rates doubled from 32% in pre-2007 to 64%.
• The species with the highest success rates included maned wolves, pumas, and ocelots which had a 100% success rate. The species with the lowest success rates (around 50%) were African lions, brown hyenas, cheetahs, Iberian lynx, and wolves.
• Overall, using a ‘soft release’ increased the odds of success by 2.5-fold. This involves acclimatising the animal to the new environment before it is fully released.
• Releasing younger animals (particularly 1 -2 year olds), also increased success rates. This may be because younger animals have greater behavioural plasticity to adapt to new environments, and they are less likely to have developed homing behaviours.
• For animals born in captivity, the success rate decreased by 1.5-fold, compared with animals born in the wild.
• However, just over a third (37%) of the relocated animals were observed to find a mate and/or raise a cub in their new habitat.

Although the fact that most relocated animals survived is encouraging, the authors say that the low mating success shows the ongoing challenges facing rewilding efforts and, crucially, the importance of protecting habitats that already exist.

Lead author Seth Thomas (Department of Biology, University of Oxford) remarked: ‘In the last 15 years we have become more successful at translocating and reintroducing large carnivores. This allows us to be optimistic for the future of rebuilding damaged ecosystems around the globe, but we must remember that it is always more important to protect large carnivore populations where they are now before we lose them. Even as we have grown to be more successful, 34% of individual translocations fail and they cannot be seen as a replacement for immediate conservation action to save these populations.’

In the near future, relocating large carnivores may become increasingly necessary as habitats become altered due to climate change, and if land use changes increase conflict between humans and animals.

In the UK, one of the most nature-deprived countries in the world, there have been calls to reintroduce formerly native apex predators, such as wolves and the Eurasian lynx.

Professor David Macdonald (WildCRU, Department of Biology, University of Oxford), a co-author for the study, said: ‘As the UN decade of ecosystem restoration gets underway, the ecological need and political appetite for relocations of large carnivores has never been greater, and they have the potential to contribute more now than ever before to biodiversity conservation. By scrutinising the most geographically comprehensive sample of relocated large carnivores to date, our study makes plain to conservationists and policy makers the urgency of improving rewilding efforts.’

Professor Alastair Driver, the Director of the charity Rewilding Britain (who were not directly involved in the study) said: ‘This study could not come at a better time here in the UK, with the devolved governments at last consulting positively on the merits of species reintroductions and various groups working hard on the feasibility of reintroducing species such as the European Wildcat and Eurasian Lynx. We still have a long way to go to overcome the misconceptions which dominate societal concerns around sharing our human-dominated landscape with other apex predators, but this report and the successes which it documents, will be hugely valuable in securing a more "grown-up" discussion on the subject. I have no doubt that this will, in turn, lead to well-planned and implemented carnivore reintroductions which only 10 years ago, I would have thought inconceivable in my lifetime.’

Dr. Miha Krofel (University of Ljubljana), a co-author who worked on lynx reintroductions included in the study said: ‘The main reason that allowed us to quantify the higher rate of success is the wider applicability of tracking technology compared to 15 years ago. Nowadays, many practitioners and scientists fit animals with tracking tags for better post-release monitoring of the translocated individuals. This allows us to learn from past releases to improve our interventions in the future.’

Caribou have been using same Arctic calving grounds for 3,000 years

 

IMAGE: ALASKA'S BARREN-GROUND CARIBOU HAVE BEEN USING THE SAME PARTS OF THE ARCTIC NATIONAL WILDLIFE REFUGE TO GIVE BIRTH TO THEIR CALVES FOR AT LEAST 3,000 YEARS, ACCORDING TO RESEARCHERS. view more 

CREDIT: MICHAEL MILLER

Caribou have been using the same Arctic calving grounds for more than 3,000 years, according to a new study by the University of Cincinnati.

Female caribou shed their antlers within days of giving birth, leaving behind a record of their annual travels across Alaska and Canada’s Yukon that persists on the cold tundra for hundreds or even thousands of years. Researchers recovered antlers that have sat undisturbed on the arctic tundra since the Bronze Age.

“To walk around the landscape and pick up something that’s 3,000 years old is truly amazing,” said Joshua Miller, an assistant professor of geosciences at the University of Cincinnati.

He has been leading summer expeditions to the Arctic National Wildlife Refuge since 2010, using rafts to navigate remote rivers to search for caribou antlers exposed on the tundra.

“We think about having to dig down into the soil to find that kind of ecological history, but on the Coastal Plain, the vegetation grows extremely slowly,” Miller said. “Bones dropped by animals that lived dozens or even hundreds of generations in the past can provide really meaningful information.”

The study demonstrates how important the area is for an animal that native Alaskans and Candians still depend on for sustenance, even as energy companies seek to exploit oil and gas resources in this protected area.

The Biden Administration in 2021 suspended drilling leases in the Arctic National Wildlife Refuge, the largest tract of undeveloped wilderness in the United States. 

“We know this region of the Arctic National Wildlife Refuge has been an important area for caribou for millennia,” Miller said. “That should give us pause on how we think about those landscapes.”

The study was published in the journal Frontiers in Ecology and Evolution.

Barren ground caribou undertake nature’s longest overland migration, traveling as far as 800 miles each year to reach their spring calving grounds in the Arctic National Wildlife Refuge and Canada’s Ivvavik National Park. The largest herd in this area, named for the Porcupine River, numbers in the hundreds of thousands of animals.

Scientists think caribou use these areas because they have fewer predators and offer seasonal vegetation near places where they can avoid the worst of the mosquitoes.

“The mosquitoes are horrible,” Miller said. “You get swarmed — literally covered in them. They can do significant damage to a young calf.”

Whatever the reason, the antlers they leave behind provide a physical record of their epic yearly travels that researchers can unlock through isotopic analysis. 

Caribou antlers, like those of elk, deer and moose, are made of fast-growing bone that the animals shed each year and regrow the following year.

“It is amazing to think that the oldest of the antlers found in our study were growing approximately the same time Homer was penning ‘the Iliad’ and ‘the Odyssey,’” study co-author Patrick Druckenmiller said.

He is director of the University of Alaska Museum and professor of the Department of Geology and Geophysics at the University of Alaska Fairbanks. Eric Wald from the U.S. Fish and Wildlife Service also co-authored the study.

The antler surveys in the vast expanse of the Arctic refuge require meticulous logistical planning, Miller said. Small planes deposit researchers and their gear deep in the interior, where they have to be watchful for grizzly and polar bears. They pilot rafts to the Beaufort Sea, conducting a  grid search of suitable caribou habitat identified in advance using aerial photography.

“We search for antlers along old river terraces, walking back and forth, covering every inch of habitat to find those ancient treasures,” Miller said. 

While male caribou antlers span four feet and weigh more than 20 pounds, female caribou antlers are much smaller. The antlers contain nutrients such as phosphorus and calcium that are important to plants and animals.

The dropped antlers create “nutrient sinks,” which could have a profound effect on the area’s vegetation. Miller said the caribou’s migration serves as a nutrient “conveyor belt” that might even draw caribou back to reap the benefits of this fertilizer in a reinforcement loop.

Caribou and other mammals are known to chew on dropped antlers for their valuable minerals. This could be an important dietary supplement for new caribou moms.

“We’d like to know to what degree this conveyor belt influences why caribou are going there in the first place,” Miller said.
The study was supported by the U.S. Fish and Wildlife Service, the National Geographic Society, the National Science Foundation, the Wildlife Society and the UC Office of Research.

Miller said the Arctic is warming faster than other parts of the globe. Parts of the Arctic that were once barren tundra are sprouting new spruce forests.

“We were in Arctic Village this summer, just south of the calving grounds, talking to elders about the changes they have seen,” Miller said. “Where once it was open tundra, large stretches of this barren ground are now full of trees everywhere. What will happen to the barren ground caribou as this habitat gets converted into forest?”

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JOURNAL

Frontiers in Ecology and Evolution

DOI

10.3389/fevo.2022.1059456 

Wolves eliminate deer on Alaskan Island then quickly shift to eating sea otters

 

 Wolves on an Alaskan island caused a deer population to plumet and switched to primarily eating sea otters in just a few years, a finding scientists at Oregon State University and the Alaska Department of Fish and Game believe is the first case of sea otters becoming the primary food source for a land-based predator.

Using methods such as tracking the wolves with GPS collars and analyzing their scat, the researchers found that in 2015 deer were the primary food of the wolves, representing 75% of their diet, while sea otters comprised 25%. By 2017, wolves transitioned to primarily consuming sea otters (57% of their diet) while the frequency of deer declined to 7%. That pattern held through 2020, the end of the study period.

“Sea otters are this famous predator in the near-shore ecosystem and wolves are one of the most famous apex predators in terrestrial systems,” said Taal Levi, an associate professor at Oregon State. “So, it’s pretty surprising that sea otters have become the most important resource feeding wolves. You have top predators feeding on a top predator.”

The finding were published today in PNAS.

Historically, wolves and sea otters likely lived in the study area, Pleasant Island, which is located in an island landscape adjacent to Glacier Bay about 40 miles west of Juneau. The island is about 20 square miles, uninhabited and accessible only by boat or float plane.

During the 1800s and much of the 1900s, populations of sea otters in this region were wiped out from fur trade hunting. Unlike wolves in the continental USA, Southeast Alaskan wolves were not hunted to local extinction. Only in recent decades, particularly with the reintroduction and legal protection of sea otters, have the populations of both species recovered and once again overlapped, providing new opportunities for predator-prey interactions between the two species.

The researchers studied the wolf pack on Pleasant Island and the adjacent mainland from 2015 to 2021. Gretchen Roffler, a wildlife research biologist with the Alaska Department of Fish and Game, and others from the department collected 689 wolf scats, many along the island’s shoreline.

Once the scat is collected, members of Levi’s lab in Oregon used molecular tools, such as DNA metabarcoding and genotyping of the scat, to identify individual wolves and determine their diets.

Roffler also captured and placed GPS collars on four wolves on the island and nine on the mainland. The researchers were curious whether wolves were traveling between the mainland and island, considering other scientists have found they are capable of swimming up to eight miles between land masses. Both the GPS collar data and genotypes of the scats confirmed they were not, indicating that the island wolf pack is stable and that the island is not a hunting ground for mainland wolves.

Locations from the GPS-collared wolves also provide evidence that the wolves are killing sea otters when they are in shallow water or are resting on rocks near shore exposed at low tide. Roffler and her crew have investigated wolf GPS clusters on Pleasant Island for three, 30-day field seasons since 2021 and found evidence of 28 sea otters killed by wolves.

“The thing that really surprised me is that sea otters became the main prey of wolves on this island,” Roffler said. “Occasionally eating a sea otter that has washed up on the beach because it died, that is not unusual. But the fact that wolves are eating so many of them indicates it has become a widespread behavior pattern throughout this pack and something that they learned how to do very quickly.

“And from the work we are doing investigating kill sites, we are learning that wolves are actively killing the sea otters. So, they aren’t just scavenging sea otters that are dead or dying, they are stalking them and hunting them and killing them and dragging them up onto the land above the high tide line to consume them.”

Shortly after wolves colonized Pleasant Island in 2013, the deer population on the island plummeted. With the wolves having consumed most of the deer, their main food source, Levi said he would have expected the wolves to leave the island or die off. Instead, the wolves remained and the pack grew to a density not previously seen with wolf populations, Levi said. The main reason, he believes, is the availability of sea otters as a food source.

The findings outlined in the PNAS paper build on research findings published in 2021 by the same researchers. In that paper they showed – in what they believe is a first – that wolves were eating sea otters. This was documented throughout the Alexander Archipelago, a group of Southeastern Alaskan islands which includes Pleasant Island.

The research has now expanded to study wolves and sea otters in Katmai National Park & Preserve, which is in southwest Alaska, about 700 miles from Pleasant Island. Early research by Ellen Dymit, a doctoral student in Levi’s lab, and Roffler indicates that wolves are also eating sea otters there. In fact, at that location Roffler and Dymit observed three wolves killing a sea otter near the shore

In addition to Levi and Roffler, co-authors of the PNAS paper are Charlotte Eriksson, a post-doctoral scholar in Levi’s lab, and Jennifer Allen, the environmental genetics lab manager in Levi’s lab. Levi is in the Department of Fisheries, Wildlife, and Conservation Sciences in the College of Agricultural Sciences.

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JOURNAL

Proceedings of the National Academy of Sciences

New research shows humans impact wolf packs in national parks

 

IMAGE: TRAIL CAMERA FOOTAGE OF THE SHOEPACK LAKE PACK WALKING ALONG A SANDY BEACH IN VOYAGEURS NATIONAL PARK IN THE FALL. view more 

CREDIT: CREDIT: VOYAGEURS WOLF PROJECT

New research shows how humans are a substantial source of mortality for wolves that live predominantly in national parks — and more importantly, that human-caused mortality triggers instability in wolf packs in national parks.

Published today in Frontiers in Ecology and the Environment, the study was led by Kira Cassidy, a research associate at Yellowstone National Park, and included co-authors at five national parks and University of Minnesota Voyageurs Wolf Project researchers Thomas Gable, Joseph Bump and Austin Homkes.

“For gray wolves, the biological unit is the pack or the family. We wanted to focus on the impacts of human-caused mortality to the pack, a finer-scale measure than population size or growth rate,” said Cassidy. “We found the odds a pack persists and reproduces drops with more human-caused mortalities.” 

While many studies have looked at how humans impact wolf populations, this study took a different approach and examined how human-caused mortality affects individual wolf packs. To do this, Cassidy and her team contrasted what happened to wolf packs after at least one pack member was killed by human-causes with packs where no members died of human-causes.

The researchers found that the chance a pack stayed together to the end of the year decreased by 27% when a pack member died of human causes, and whether or not that pack reproduced the next year decreased by 22%. When a pack leader died, the impact was more substantial, with the chance of the pack making it to the end of the year decreasing by 73% and reproduction by 49%.

Although the researchers did not examine whether human-caused mortality alters the size of wolf populations in national parks, this work shows that people are clearly altering certain aspects of wolf ecology in national parks even if they are not impacting overall population size.

One reason for this is that humans are a disproportionate cause of mortality for wolves that live predominantly in national parks. In other words, wolves die more often of human-causes than would be expected for the amount of time wolves spend outside of park boundaries. 

Of all national parks in the study, wolves in Voyageurs National Park spent the most time outside of park boundaries. In fact, wolves that had territories in or overlapping Voyageurs spent 46% of their time outside of the park. The result: 50% of all mortalities for these wolves came at the hands of people, with poaching being the most common cause of death.

“The unique shape of Voyageurs means that there are very few wolf packs that live entirely within the boundaries of the park. Instead, many wolf pack territories straddle the park border and when wolves leave the park, they are at an increased risk of being killed by people,” said Gable, a post-doctoral associate in the University of Minnesota’s College of Food, Agricultural and Natural Resource Sciences and project lead of the Voyageurs Wolf Project, which studies wolves in and around Voyageurs National Park.

However, Voyageurs was hardly unique as this pattern was similar across the other national parks in the study — Denali National Park and Preserve, Yellowstone National Park, Grand Teton National Park, and Yukon-Charley Rivers National Preserve — with human-caused mortality accounting for 36% of collared wolf mortality across all five parks.

Legal hunting and trapping of wolves outside of national park boundaries accounted for 53% of all human-caused mortality for wolves from national parks during hunting and trapping seasons.

These findings highlight why collaboration between different state and federal agencies is key when conserving and managing wildlife that go in and out of protected areas such as national parks. 

“Wildlife populations that cross hard boundaries from federal to state ownership are a challenge to manage. Wolves don’t know the park boundary lines,” said Bump, an associate professor in the U of M’s College of Food, Agricultural and Natural Resource Sciences.

The Voyageurs Wolf Project is funded by the Minnesota Environment and Natural Resources Trust Fund as recommended by the Legislative-Citizen Commission on Minnesota Resources (LCCMR).

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JOURNAL

Frontiers in Ecology and the Environment

DOI

10.1002/fee.2597