Researchers uncover hidden hunting tactics of wolves in Minnesota's Northwoods

 

In first systematic analysis of their ambushing behavior, U of M researchers find wolves' hunting strategies are highly-flexible depending on their prey.

UNIVERSITY OF MINNESOTA

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IMAGE: IN A NEW PAPER PUBLISHED IN THE JOURNAL BEHAVIORAL ECOLOGY, , RESEARCHERS FROM THE UNIVERSITY OF MINNESOTA AND THE VOYAGEURS WOLF PROJECT SHOW THAT WOLVES HAVE EVOLVED AMBUSH HUNTING TACTICS... view more 

CREDIT: VOYAGEURS WOLF PROJECT

Wolves are arguably the most well-studied large predators in the world, yet new research shows there is still a lot to learn about their hunting tactics. Typically, wolves hunt large mammals like moose, deer, and bison in packs by outrunning, outlasting, and exhausting their prey. However, throughout the dense boreal forests in North America and Eurasia, during the summer wolves often hunt beavers by themselves.

But how does a wolf catch a semi-aquatic prey that spends little time on land and never ventures far from the safety of its pond? Turns out with patience, and a lot of waiting.

In a new paper published in the journal Behavioral Ecology, researchers from the University of Minnesota and the Voyageurs Wolf Project--which studies wolves in the Greater Voyageurs Ecosystem in the northwoods of Minnesota--show that wolves have evolved ambush hunting tactics specifically tailored for catching and killing beavers.

"Over a five-year period, we estimate that our field research team collectively put in over 15,000 person-hours to search nearly 12,000 locations where wolves had spent time. Through this effort, we ended up documenting 748 locations where wolves waited to ambush beavers but were unsuccessful, and 214 instances where wolves killed beavers," said Sean Johnson-Bice, a co-author of the study. "This dedicated fieldwork has provided unprecedented insight into the hunting tactics wolves use in boreal environments."

Beavers have extremely poor eyesight, so they rely primarily on their well-developed sense of smell to detect predators on land--and wolves appear to have learned this through time. Researchers discovered wolves choose ambushing sites within a few feet of where beavers are active on land because wolves have learned beavers cannot visually detect them. When doing so, wolves almost always choose ambushing sites that are downwind to avoid being smelled by beavers.

"The results are very clear," said Tom Gable, the study's lead-author: "89-94% of the ambushing sites were downwind, where beavers were likely unable to smell wolves."

When staking out beavers, wolves appear to be surprisingly patient. They spend substantial periods of time waiting next to areas where beavers are active on land, such as near beaver dams and trails.

"Wolves waited an average of four hours during each stakeout. But they often waited eight-12 hours or more, and one wolf even waited-in-ambush for 30 hours," said study co-author Austin Homkes.

The researchers note that these behaviors were not unique to a few wolves. Instead, wolves from multiple packs across several years used the same ambushing tactics, indicating that this behavior is widespread throughout the Greater Voyageurs Ecosystem and likely other ecosystems where wolves hunt beavers. Notably, wolves and beavers co-occur across much of the Northern Hemisphere so the implications have wide applicability.

"Gathering data to demonstrate how wind direction influences the ambushing behavior of predators has been difficult for animal ecologists," said co-author Joseph Bump. "Scientists have long-thought that ambush predators are able to strategically choose ambush sites in areas where prey are unable to detect them via scent. Until now though, documenting these hunting tactics in exhaustive detail proved extremely challenging."

Ultimately, the study challenges the classic concept that wolves are solely cursorial predators (i.e., predators that kill their prey by outrunning and outlasting them). Instead, wolf-hunting strategies appear highly flexible, and they are able to switch between hunting modes (cursorial and ambush hunting) depending on their prey.

"It is the first systematic analysis of wolf ambushing behavior," said Gable. "It overturns the traditional notion that wolves rely solely on hunting strategies that involve pursuing, testing, and running down prey."

New challenges for wolf conservation

People view the wolf as either a threatening predator or a sign of a healthy natural habitat. Many proponents of nature and animal conservation welcome the spread of wolf populations in Germany. By contrast, farmers who graze herds directly impacted by the wolves' return are more critical. The team of Nicolas Schoof, Prof. Dr. Albert Reif of the Chair of Site Classification and Vegetation Science at the University of Freiburg, and Prof. Dr. Eckhard Jedicke of the Competence Center Cultural Landscape and the Department of Landscape Planning and Nature Conservation of Hochschule Geisenheim University has assessed the existing legal situation. On the basis of a range of environmental data they have determined the conflicts and drawn up possible solutions. In an article in the German specialist journal "Naturschutz und Landschaftsplanung" (Nature Conservation and Landscape Planning) the researchers present in detail how wider distribution of the predator could negatively affect legally binding nature conservation goals.

Schoof explains that experts - based on the assumption that the wolves were living mainly in isolation - initially ascribed those that have reappeared in Germany to the Central European Lowlands population. Nevertheless, newer genetic studies show the population is at least interacting with the Baltic wolf population, meaning that incest risk is low. The Freiburg researcher goes on to say that European law is very strict and does not allow - other than has been often maintained - any type of regulatory culling. "As a result," says Schoof, "there is a high growth and survival rate for young animals, allowing the wolf to spread to many regions."

The species itself primarily indicates there has been growth in the native fauna. Schoof's team is analyzing the consequences the growing wolf population - and with it the increasing attacks on livestock and resulting demands to protect herds in Germany - will have for maintaining biodiversity. "There's the threat of a partial abandonment of livestock grazing, especially in locations that are crucial to conserving nature," explains Schoof. Areas affected could, for example, be meadows or grasslands on steep slopes as well as those with stony soils. These habitats are - like the wolf itself - the focus of legally binding conservation measures and are dependent on the continuation of grazing. Unlike the wolf habitats, these environments are so rare that they are at risk.

In many cases, new fences can offer herds much better protection. But this isn't possible on steep slopes, for example. Dependent on the size and condition of the pastures, sheepdogs could be introduced. But the researchers say that is a markedly labor and cost intensive option which would only come into question for a few livestock owners. But sheepdogs cannot be effectively used above all in partially open pasture lands that are an essential component for conserving biodiversity. Yet it is precisely in these areas that the problems caused by wolves could increase, says Schoof, meaning that only fixed, wolf-resistant fences are a solution, but these would limit the spatial efficacy of large grazing projects. What is more, the expected erection of fencing systems will generally form a massive impediment to other wildlife that have found an optimal habitat on grazing land.

Based on these conflicts, the researchers say they have no doubt that the permissible legal regulatory measures to cull problematic individuals through hunting must be simplified significantly and stringently implemented. Taken in perspective, comprehensive active management of the wolf population must be considered and the necessary legal regulatory changes undertaken. The researchers argue that the wolf population is not endangered because of the individual numbers that have been reached, the generally low incest risk, and the current exponential growth in population. Schoof points out that simpler solutions are not in sight or legally possible as yet.

Additionally, the researchers propose in their study that on the one hand, all the required measures to protect herds should be comprehensively supported. "On the other hand, better financial support for the frequently economically unattractive farming of grazing animals could contribute to moderating the existing conflicts," says Schoof. "In that way, owners of grazing livestock would clearly be shown that they are key partners when it comes to implementing practical conservation efforts," he adds.

Wolves affect wetland ecosystems by killing beavers leaving their colonies to create new ponds.

 

Beavers are some of the world's most prolific ecosystem engineers, creating, maintaining and radically altering wetlands almost everywhere they live. But what, if anything, might control this engineering by beavers and influence the formation of North America's wetlands?

In a paper to be published Friday in the journal Science Advances, researchers with the University of Minnesota's Voyageurs Wolf Project and Voyageurs National Park observed and demonstrated that wolves affect wetland ecosystems by killing beavers leaving their colonies to create new ponds.

Beavers are important ecosystem engineers that create wetlands around the world, storing water and creating habitat for numerous other species. This study documents that wolves alter wetland creation when they kill beavers that have left home and created their own dams and ponds.

Juvenile beavers disperse alone and often create new ponds or fix up and recolonize existing, old ponds. By studying pond creation and recolonization patterns along with wolf predation on beavers, project biologists and co-authors Tom Gable and Austin Homkes found that 84% of newly-created and recolonized beaver ponds remained occupied by beavers for more than one year. But when a wolf kills the beaver that settles in a pond, no such ponds remain active.

This relationship between wolves and dispersing beavers shows how wolves are intimately connected to wetland creation across the boreal ecosystem and all the ecological processes that come from wetlands.

"How large predators impact ecosystems has been a matter of interest among scientists and the public for decades," said Gable, project lead of the Voyageurs Wolf Project. "Because wolves are the apex predators in northern Minnesota and beavers are ecosystem engineers, we knew there was potential for wolves to affect ecosystems by killing beavers."

Researchers found that wolves can have this impact on wetlands without necessarily changing the abundance or behavior of beavers. This newly supported link between wolf predation, dispersing beavers and wetlands may have long-lasting impacts for boreal landscapes and habitat for other species. The Voyageurs Wolf Project is investigating the long-term effects of this relationship.

"In 2015, we documented a wolf killing a dispersing beaver in a newly-created pond," said Homkes, a field biologist with the Voyageurs Wolf Project. "Within days of the wolf killing the beaver, the dam failed because there was no beaver left to maintain it. The wolf appeared to have prevented the beaver from turning this forested area into a pond. This initial observation was fascinating and we realized we needed to figure out how wolves were connected to wetland creation in the Greater Voyageurs Ecosystem."

After five years of intensive fieldwork, Gable, Homkes and colleagues have estimated that wolves altered the establishment of about 88 ponds per year and the storage of over 51 million gallons of water annually in the Greater Voyageurs Ecosystem. The pond observed in 2015 still has not been colonized by another beaver, said Gable, who visited the site in mid-September 2020.

"Our work hints at the possibility that wolves might have a longer-term impact on wetland creation and generate habitat patchiness that supports many other species across the landscape, but we need to study this mechanism further," said Joseph Bump, co-author and associate professor in the University's Department of Fisheries, Wildlife, and Conservation Biology.

Wolves and large predators are usually thought to have outsized ecological effects primarily by reducing the abundance of their prey or by altering the behavior of their prey through fear of predation, both of which allow predators to indirectly impact lower parts of the food chain such as vegetation, songbirds and other wildlife. Some research has even claimed wolves impact river ecosystems through trophic cascades, but this has been met with substantial skepticism and remains hotly debated among scientists.

"The fact that we have convincingly shown wolves can impact wetlands without necessarily changing the abundance or behavior of beavers is a really exciting finding," said co-author Sean Johnson-Bice, a project collaborator from the University of Manitoba.

"The Greater Voyageurs Ecosystem sits within a flat landscape that is dominated by water and trees, creating the perfect conditions that currently support some of the highest beaver densities in North America," said Voyageurs National Park wildlife researcher and co-author Steve Windels.

"Though we don't have evidence that wolves are limiting the size of the beaver population in Voyageurs, understanding the nuanced and complex ways that predators and prey affect one another and their environment is critical to fulfilling the National Park Service's mission to protect and preserve our resources for future generations."

This study identified a novel and unique way by which predators influence ecological processes, which ultimately enriches our understanding of the diverse roles predators play in ecosystems.

"There are a number of good reasons to maintain and restore healthy predator populations and this study should be helpful for understanding the full role and therefore value of predators, especially when they eat ecosystem engineers," Bump said.

What does the fox say to a puma?

VIRGINIA TECH

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IMAGE: PUMAS ARE THE TOP PREDATORS IN THE STUDY RESEARCH AREA IN THE CHILEAN ANDES. CAMERA TRAP PHOTO BY CHRISTIAN OSORIO. view more 

CREDIT: CHRISTIAN OSORIO

In the high plains of the central Chilean Andes, an ecosystem consisting of only a few animal species is providing researchers with new insights into how predators coexist in the wild.

"The puma and the culpeo fox are the only top predators on the landscape in the Chilean Andes," said Professor Marcella Kelly, of the College of Natural Resources and Environment. "And there isn't a wide range of prey species, in part because the guanacos [closely related to llamas] aren't typically found in these areas anymore due to over-hunting. With such a simplified ecosystem, we thought we could really nail down how two rival predators interact."

Kelly worked with Christian Osorio, a doctoral student in the Department of Fish and Wildlife Conservation, and researchers from the Pontifical Catholic University of Chile to chart the locations of and potential interactions between pumas and foxes in central Chile. They focused on three axes of interaction: spatial (where the animals are on the landscape), temporal (the timing of specific activities on a given landscape), and dietary (what each species is eating).

To understand the interplay between pumas and foxes, researchers deployed 50 camera stations across two sites in central Chile, one in the Rio Los Cipreses National Reserve and another on private land where cattle and horses are raised. They also collected scat samples at both locations to analyze the diets of pumas and foxes.

The team's findings, published in the journal Diversity, showed that while pumas and foxes overlapped significantly where they lived and what time they were active, there was little overlap in what they were eating, with the puma diet consisting primarily of a large hare species introduced from Europe, while the culpeo foxes favored smaller rabbits, rodents, and seeds. The two predator species can successfully share a landscape and hunt for food over the same nighttime hours because they are, in essence, ordering from different menus.

"It is likely that foxes have realized that when they try to hunt hares, they might run into trouble with pumas," Osorio explained. "If they are hunting smaller mammals, the pumas don't care, but if the foxes start targeting larger prey, the pumas will react."

How predator species interact is a crucial question for ecologists trying to understand the dynamics that inform ecosystem balances. And while the puma has been designated a species of least concern, the animal's populations are declining and continue to be monitored by conservationists.

"Least concern does not mean no concern," Osorio noted. "We have laws in Chile that protect the species, but the data we have to make a conservation designation are very scattered. As we accumulate more consistent and reliable data, the puma may be reclassified as vulnerable or even endangered."

The hares that comprise approximately 70 percent of the biomass in the puma's diet are a nonnative species, introduced to the area by European settlers. With guanacos absent from the landscape, the puma has had to adapt its diet to survive.

With some land managers and conservationists campaigning for the removal of the introduced hare species as a way to restore the area's native ecosystem, Kelly and Osorio note that it is important to understand that pumas would be significantly impacted by a reduction in their primary food source.

A further concern, which the two are currently researching, is the interplay between wildlife and humans. The national reserve increasingly sees visitors eager to witness big cats and foxes in their natural environment, while the sheep and cattle industries are increasingly using remote terrain for livestock cultivation.

"Pumas do occasionally kill livestock, which is a challenge we're looking into right now," said Kelly, an affiliate of Virginia Tech's Fralin Life Sciences Institute. "The government would like to preserve the puma, but there are competing challenges of what kind of threat they pose to livestock and what kind of threat cattle or sheep farming poses to them."

Understanding how two predatory species can come to coexist has the potential to provide conservationists and ecologists with better ideas for how humans and wild animals can share a landscape.

Controversial delisting of the US gray wolf

 

IMAGE: A GRAY WOLF; THE SPECIES WAS RECENTLY DELISTED AS RECOVERED UNDER THE ENDANGERED SPECIES ACT. view more 

CREDIT: MICHAEL LAROSA ON UNSPLASH

On 29 October 2020, the US Fish and Wildlife Service (USFWS) announced the "successful recovery" of the US gray wolf population, with US Secretary of the Interior Secretary David Bernhardt stating that the species had "exceeded all conservation goals for recovery." These claims have been rebutted by numerous experts, who argue that the delisting decision is premature. Writing in BioScience, independent ecologist Carlos Carroll and colleagues argue that the declarations of recovery should be based on a more ambitious definition of recovery than one requiring the existence of a single secure population. Instead, they propose a framework for the "conservation of adaptive potential," which builds on existing agency practice to enhance the effectiveness of the Act. The authors argue that such an approach is particularly crucial in light of climate change and other ongoing threats to species. On this episode of BioScience Talks, Dr. Carroll is joined by coauthors Adrian Treves, Bridgett vonHoldt, and Dan Rohlf to discuss the recent USFWS action as well as prospects for gray wolf conservation.

Light pollution alters predator-prey interactions between cougars and mule deer in western US

A new study provides strong evidence that exposure to light pollution alters predator-prey dynamics between mule deer and cougars across the intermountain West, a rapidly growing region where nighttime skyglow is an increasing environmental disturbance.

The University of Michigan-led study, published online Oct. 18 in the journal Ecography, is the first to assess the impacts of light pollution on predator-prey interactions at a regional scale. It combines satellite-derived estimates of artificial nighttime lights with GPS location data from hundreds of radio-collared mule deer and cougars across the intermountain West.

The study found that:

• Mule deer living in light-polluted areas are drawn to artificial nighttime lighting, which is associated with green vegetation around homes.
• Cougars, also known as mountain lions and pumas, are able to successfully hunt within light-polluted areas by selecting the darkest spots on the landscape to make their kill.
• While mule deer that live in dark wildland locations are most active around dawn and dusk, those living around artificial night light forage throughout the day and are more active at night than wildland deer--especially during the summer.

The animal data used in the study were collected by state and federal wildlife agencies across the region. Collation of those records by the study authors yielded what is believed to be the largest dataset on interactions between cougars and mule deer, two of the most ecologically and economically important large-mammal species in the West.

"Our findings illuminate some of the ways that changes in land use are creating a brighter world that impacts the biology and ecology of highly mobile mammalian species, including an apex carnivore," said study lead author Mark Ditmer, formerly a postdoctoral researcher at the University of Michigan School for Environment and Sustainability, now at Colorado State University.

The intermountain West spans nearly 400,000 square miles and is an ideal place to assess how varying light-pollution exposures influence the behavior of mule deer and cougars and their predator-prey dynamics. Both species are widely distributed throughout the region--the mule deer is the cougar's primary prey species--and the region presents a wide range of nighttime lighting conditions.

The intermountain West is home to some of the darkest night skies in the continental United States, as well as some of the fastest-growing metropolitan areas, including Las Vegas and Salt Lake City. Between the dark wildlands and the brightly illuminated cities is the wildland-urban interface, the rapidly expanding zone where homes and associated structures are built within forests and other types of undeveloped wildland vegetation.

For their study, the researchers obtained detailed estimates of nighttime lighting sources from the NASA-NOAA Suomi polar-orbiting satellite. They collected GPS location data for 117 cougars and 486 mule deer from four states: Utah, Arizona, Nevada and California. In addition, wildlife agencies provided locations of 1,562 sites where cougars successfully killed mule deer.

"This paper represents a massive undertaking, and to our knowledge this dataset is the largest ever compiled for these two species," said study senior author Neil Carter, a conservation ecologist at the U-M School for Environment and Sustainability.

Deer in the arid West are attracted to the greenery in the backyards and parks of the wildland-urban interface. Predators follow them there, despite increased nighttime light levels that they would normally shun. Going into the study, the researchers suspected that light pollution within the wildland-urban interface could alter cougar-mule deer interactions in one of two ways.

Perhaps artificial nighttime light would create a shield that protects deer from predators and allows them to forage freely. Alternatively, cougars might exploit elevated deer densities within the wildland-urban interface, feasting on easy prey inside what scientists call an ecological trap.

Data from the study provides support for both the predator shield and ecological trap hypotheses, according to the researchers. At certain times and locations within the wildland-urban interface, there is simply too much artificial light and/or human activity for cougars, creating a protective shield for deer.

An ecological trap occurs when an animal is misled, or trapped, into settling for apparently attractive but in fact low-quality habitat. In this particular case, mule deer are drawn to the greenery of the wildland-urban interface and may mistakenly perceive that the enhanced nighttime lighting creates a predator-free zone.

But the cougars are able to successfully hunt within the wildland-urban interface by carefully selecting the darkest spots on the landscape to make their kill, according to the study. In contrast, cougars living in dark wildland locations hunt in places where nighttime light levels are slightly higher than the surroundings, the researchers found.

"The intermountain West is the fastest-growing region of the U.S., and we anticipate that night light levels will dramatically increase in magnitude and across space," said U-M's Carter. "These elevated levels of night light are likely to fundamentally alter a predator-prey system of ecological and management significance--both species are hunted extensively in this region and are economically and culturally important."

Carnivores living near people feast on human food, threatening ecosystems

 Ecologists at the University of Wisconsin-Madison have found that carnivores living near people can get more than half of their diets from human food sources, a major lifestyle disruption that could put North America's carnivore-dominated ecosystems at risk. The researchers studied the diets of seven predator species across the Great Lakes region of the U.S. They gathered bone and fur samples for chemical analysis from areas as remote as national parks to major metropolitan regions like Albany, New York. They found that the closer carnivores lived to cities and farms, the more human food they ate.

While evolution has shaped these species to compete for different resources, their newfound reliance on a common food source could put them in conflict with one another. That conflict could be reordering the relationship between different carnivores and between predators and prey, with an unknown but likely detrimental impact on ecosystems that evolved under significant influence of strong predators.

Jon Pauli, a UW-Madison professor of forest and wildlife ecology, and his former graduate student Phil Manlick, published their findings this week in the Proceedings of the National Academy of Sciences. The study is the most comprehensive look yet at how most of the region's major carnivores -- like gray wolves, coyotes, and bobcats -- have changed their diets in response to people.

How much human food they ate varied considerably by location. On average, more than 25 percent of the carnivores' diets came from human sources in the most human-altered habitats.

It also varied by species. For instance, committed carnivores like bobcats ate a relatively small amount of human food. "But what you see is that the sort of generalist species that you might expect -- coyotes, foxes, fishers, martens -- in human-dominated landscapes, they're getting upwards of 50 percent of their diet from human foods," says Manlick, the lead author of the study who is now a postdoctoral researcher at the University of New Mexico. "That's a relatively shocking number, I think."

Pauli and Manlick found that relying on human food sources increased how much carnivores overlapped one another in their competition for food. Compared to when these predators vie for distinct prey, this increased competition could lead to more conflicts between animals. Their reliance on human food could also make the carnivores vulnerable to human attacks near towns, or even change how and when they hunt traditional prey, with potentially harmful ecological consequences.

The researchers studied the diets of almost 700 carnivores, including red and gray foxes, fishers, and American martens. They gathered bone and fur samples from Minnesota, Wisconsin, New York and the Upper Peninsula of Michigan with the help of state and federal researchers and citizen-science trappers. The researchers compared the carnivores' diets to the extent of human development in the region, which varied from essentially pristine wilderness to urban sprawl.

Thanks to quirks in how plants incorporate carbon as they grow, a sample of bone or fur is enough to get a snapshot of an animal's diet. Different weights, or isotopes, of carbon are common in different plants -- and in the animals who ultimately eat them.

"Isotopes are relatively intuitive: You are what you eat," says Manlick. "If you look at humans, we look like corn."

Human foods, heavy in corn and sugar, lend them distinctive carbon signatures. In contrast, the diets of prey species in the wild confer their own carbon signatures. The ratio of these two isotope fingerprints in a predator's bone can tell scientists what proportion of their diet came from human sources, either directly or from their prey that ate human food first.

The geographic extent of the study and the large number of species the ecologists examined demonstrate that the trend of human food subsidies in carnivore diets is not limited to a single location or species. The ultimate outcome of such widespread disruptions remains unclear.

"When you change the landscape so dramatically in terms of one of the most important attributes of a species -- their food -- that has unknown consequences for the overall community structure," says Pauli. "And so I think the onus is now on us as ecologists and conservation biologists to begin to understand these novel ecosystems and begin to predict who are the winners and who are the losers."