Wolves make a rapid recovery in Europe

 

Peer-Reviewed Publication

PLOS

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Wolf on agricultural plains in northern Greece

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Credit: Seryios Papaioannou, CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

 Wolf populations in Europe increased by nearly 60% in a decade, according to a study led by Cecilia Di Bernardi and Guillaume Chapron at the Swedish University of Agricultural Sciences, published in the open-access journal PLOS Sustainability and Transformation.

Large carnivore populations are declining worldwide. However, in Europe, conservation policies have supported the recovery of wolves (Canis lupus) in recent decades. To understand current trends in their populations, researchers collated data on wolf numbers in 34 countries across Europe. They found that by 2022, at least 21,500 wolves lived in Europe — an increase of 58% compared to the estimated population of 12,000 a decade earlier. In most countries analyzed, wolf populations were increasing, with only three countries reporting declines over the previous decade. The researchers also investigated sources of conflict between humans and wolves, such as livestock deaths. They estimated that in the European Union, wolves killed 56,000 domestic animals per year, out of a total population of 279 million livestock. Although the risk varied between countries, on average, livestock faced a 0.02% chance of being killed by wolves each year. Compensating farmers for these losses cost European countries 17 million euros annually. Still, wolves can also have positive economic impacts, such as reducing traffic accidents and damage to forestry plantations by controlling wild deer populations. However, there wasn’t enough data available to quantify these benefits.

Considering Europe’s large human population and the widespread alteration of landscapes for agriculture, industry and urbanization, the rapid recovery of wolves over the last decade highlights their extraordinary adaptability. However, as conservationists transition from saving endangered populations to sustaining a successful recovery, the challenge will be to adapt national and international policies to ensure that humans and wolves can coexist sustainably in the long term, the authors say. 

The authors add: “The recovery of wolves across human-dominated landscapes of Europe has been continuing during the past decade, with their population growing to over 21,500 individuals by 2022 – a 58% increase in a decade. Ongoing and future challenges include damages directly caused by wolves and broader socio-political issues.”

The freely available article in PLOS Sustainability and Transformation:  https://plos.io/41wNLjq

Yellowstone wolves and other carnivores drive strong trophic cascade

 

Peer-Reviewed Publication

Conservation Biology Institute

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Downstream view of the East Fork of Blacktail Deer Creek in 2004 and 2021, northern range of Yellowstone National Park, USA.

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Credit: R.L. Beschta

Corvallis, OR — February 6, 2025 — A new study reveals the profound ecological effects of wolves and other large carnivores in Yellowstone National Park, showcasing the cascading effects predators can have on ecosystems. In Yellowstone, this involves wolves and other large carnivores, elk, and willows. The research, which utilized previously published data from 25 riparian (streamside) sites and collected over a 20 year period, from 2001 to 2020, revealed a remarkable 1,500% increase in willow crown volume along riparian zones in northern Yellowstone National Park, driven by the effects on elk due to a restored large carnivore guild following the reintroduction of wolves in 1995–96, and other factors. The study was led by Dr. William J. Ripple of Oregon State University and the Conservation Biology Institute in Corvallis, OR, and published today in Global Ecology and Conservation.

Trophic cascades, the effects of predators on herbivores and plants, have long been a topic of ecological interest. The study quantifies the strength of this phenomenon for the first time using willow crown volume as a proxy for aboveground biomass, demonstrating a significant three-dimensional recovery of riparian vegetation represented by the growth in both crown area and height of established willows. The strength of the Yellowstone trophic cascade observed in this study surpasses 82% of strengths presented in a synthesis of global trophic cascade studies, underscoring the strength of Yellowstone’s willow recovery process. The authors note that there is considerable variability in the degree of recovery and not all sites are recovering.

Even though riparian areas in the western United States comprise a small portion of the landscape, the study has particular relevance since these areas provide important food resources and habitat for more wildlife species than any other habitat type. These areas also connect upland and aquatic ecosystems and are widely known for their high diversity in species composition, structure, and productivity.

“Our findings emphasize the power of predators as ecosystem architects,” said William Ripple. “The restoration of wolves and other large predators has transformed parts of Yellowstone, benefiting not only willows but other woody species such as aspen, alder, and berry-producing shrubs. It’s a compelling reminder of how predators, prey, and plants are interconnected in nature.”

Wolves were eradicated and cougars driven to low numbers from Yellowstone National Park by the 1920s. Browsing by elk soon increased, severely damaging the park’s woody vegetation, especially in riparian areas. Similar effects were seen in places like Olympic National Park in Washington, and Banff and Jasper National Parks in Canada after wolves were lost. While it’s well understood that removing predators can harm ecosystems, less is known about how strongly woody plants and ecosystems recover when predators are restored. Yellowstone offers a rare opportunity to study this effect since few studies worldwide have quantified how much plant life rebounds after large carnivores are restored.

“Our analysis of a long-term data set simply confirmed that ecosystem recovery takes time. In the early years of this trophic cascade, plants were only beginning to grow taller after decades of suppression by elk. But the strength of this recovery, as shown by the dramatic increases in willow crown volume, became increasingly apparent in subsequent years,” said Dr. Robert Beschta, an emeritus professor at Oregon State University. “These improving conditions have created vital habitats for birds and other species, while also enhancing other stream-side conditions.”

The research points to the utility of using crown volume of stream-side shrubs as a key metric for evaluating trophic cascade strength, potentially advancing methods for riparian studies in other locations. It also contextualizes the value of predator restoration in fostering biodiversity and ecosystem resilience.

Protected areas provide habitat for threatened lynx, but wildfire poses risks

Future wildfire may be the greatest threat to lynx habitat in the southern Rocky Mountains

Peer-Reviewed Publication

USDA Forest Service - Rocky Mountain Research Station

 

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A lynx prowls through thick snow with trees in the background. USDA Forest Service Rocky Mountain Research Station-Canada Lynx of the Rockies Research Program. 

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Credit: John Squires USDA Forest Service Rocky Mountain Research Station-Canada Lynx of the Rockies Research Program.

FORT COLLINS, Colo., January 6, 2025 — Canada lynx are specialized hunters, able to travel in deep snow and spot prey in the darkness from 250 feet away. Keen hearing and vision make them excellent trackers, but what do we learn by turning the tables and tracking them? Scientists are using GPS data and advanced modeling to refine maps and identify important habitat characteristics, particularly in the forests of western Colorado, southern Wyoming, and northern New Mexico, the southernmost extent of its range.

New research and updated maps show that more than half of lynx habitat in the southern Rocky Mountains overlaps protected areas like wilderness and national parks. The maps also show that lynx habitat is sparse, patchy, and poorly connected, existing only in narrow bands due to Colorado’s complex mountainous terrain. These maps can identify corridors where habitat loss affects animal movement, which is important for healthy populations. Updated maps help managers focus conservation efforts and pinpoint places to promote new habitat, especially given the frequency of human development, fire, and forest insect outbreaks near the edges of lynx range.

Lynx in the western US prefer high elevation spruce-fir forests with tree branches hanging close to the snow or ground surface, providing dense horizontal cover. This forest structure supports the cat’s favorite prey, snowshoe hares, which can make up 90 percent of their diet in winter. Lynx were reintroduced to the southern Rockies over two decades ago, after populations had fallen below a self-sustaining level. Several generations of females and kittens have successfully established here, allowing these populations to bounce back.

Researchers used GPS collars to track lynx within the study area, focused on the southern extent of its range. They also considered over 40 habitat or environmental characteristics, such as the amount of precipitation as snow versus rain, the month with the coldest temperature, road density, vegetation, and slope position. Combining GPS and habitat data and using state-of-the-art statistical tools and modeling methods, these scientists identified which characteristics best predicted where lynx were found and used this information to map “likely” lynx habitat where cats have a high probability of living, breeding, and successfully raising young.

Then they took this information one step further, comparing “likely” habitat with disturbances like insect outbreaks, wildfire, timber harvest, or human-footprint impacts such as urbanization or ski resorts. The scientists found that around one-third of likely habitat overlapped with disturbance, including forest insect outbreaks (31 percent), wildfire (5 percent), and forest management activities like tree harvest and prescribed burning (3 percent). Despite the greater overlap of lynx habitat with insect outbreaks, the scientists are more concerned by the impacts of severe wildfires.

“We know that lynx continue to occupy areas after insects kill the overstory trees during spruce-beetle outbreaks. Forests impacted by spruce beetles still have enough young conifer trees in the understory to support hares,” said Dr. John Squires, the study's principal investigator and a Rocky Mountain Research Station research wildlife biologist. “Also, some tree species like subalpine fir often survive spruce beetle attacks. That means that forest insects represent a more benign threat to lynx compared to broad-scale, high-severity wildfire.”

The researchers found that lynx tend to avoid fire-impacted areas until the understory has regrown–a process that takes upwards of 50 years in subalpine forests in the southern Rockies, where fires tend to burn less frequently than lower elevation forests and are often stand-replacing.

“Although fire disturbance from 1990-2022 overlapped only 5 percent of likely lynx habitat in this area, we believe that frequent, high-severity fire is the main risk to lynx in high-elevation forests moving forward,” continued Squires.

“We were surprised at how little lynx habitat overlapped areas of managed forest and human development,” said Dr. Lucretia Olson, study coauthor and a Rocky Mountain Research Station ecologist. “While forest management mainly causes temporary habitat changes, activities like urbanization and developing or expanding ski areas often lead to permanent losses. We hope our work will reach managers who make complex decisions about land uses.”

Squires stressed the value of partnerships for conducting this research: “Close collaboration across state and federal agencies in terms of data sharing, logistical support, and added expertise made this research possible.”

Authors of this research include John Squires and Lucretia Olson of the Forest Service Rocky Mountain Research Station, Jacob Ivan of Colorado Parks and Wildlife, Peter McDonald of the Forest Service Rocky Mountain Region, and Joseph Holbrook of the University of Wyoming. For more information about this research, please refer to the scientific publication or visit the webpage for the Canada Lynx of the Rockies Research Program.

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Journal

Biodiversity and Conservation

DOI

10.1007/s10531-024-02978-8 

Conserving high-elevation grasslands in Peru is key to protect Andean bears

 

Bears forage for young bromeliad plants in Peru’s puna grasslands, but prefer to avoid cattle

Peer-Reviewed Publication

PLOS

 

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Examples of foraging sign left by Andean bears on terrestrial bromeliads in Peru. Clockwise from the top right: (a) juvenile Andean bear consuming a bromeliad at a camera station inside MNP; (b) characteristic observation of a vegetative P. leptostachya foraged by Andean bear; (c) a close up of the basal meristematic tissue that Andean bears feed on.

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Credit: Pilfold et al., 2024, PLOS ONE, CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

Andean bears carefully select the best foraging locations and plants to maximize nutrition and avoid livestock, according to a study publishing December 18, 2024, in the open-access journal PLOS ONE by Nicholas Pilfold at the San Diego Zoo Wildlife Alliance in California, U.S., and colleagues.

Andean bears, also known as ‘spectacled bears’ because of the white markings encircling their eyes, are endemic to the Andes. They are listed as vulnerable by the IUCN and are threatened by habitat loss, climate change, and conflict with humans. Flowering plants called bromeliads form a major part of their diet, but little is known about their foraging behavior and feeding preferences across the mixture of cloud forest and grassland habitats within their range.

Researchers conducted surveys of two species of bromeliad (Puya leptostachya and Puya membranacea) in high-altitude grasslands, called ‘puna’, in and around Manu National Park in Peru. They recorded the location of each plant and whether there was evidence of consumption by Andean bears (Tremarctos ornatus), through observations of dug up, partially eaten stalks, a characteristic feeding sign of the bears.

Trail cameras confirmed that Andean bears were present at the survey locations. However, the surveys showed that the bears were foraging in just 16.7% of available bromeliad patches. Andean bears were more likely to forage for bromeliads in the dry season when there were young, tender plants available, which are likely easier for them to digest and more nutritious. The bears preferred to eat P. leptostachya plants growing on east-facing, steep slopes of puna grassland at the forest’s edge. They rarely foraged for bromeliads outside the national park, where livestock like cattle are grazed.

The results suggest that Andean bears actively seek out bromeliads in locations where they feel safe from human disturbance. Although the bears avoided areas with livestock, they foraged in locations that had been grazed by livestock only a few decades ago. This behavioral flexibility may help them to regain lost territory quickly with help from targeted conservation measures. High-altitude grasslands bordering cloud forest are key habitats for Andean bears and conservation managers should consider how livestock impact this important ecosystem, the authors say.

The authors add: “Using the largest collection ever of field data on the feeding behavior of Andean bears in high elevation grasslands, we found that the bears actively selected for specific food resources within the grasslands, indicating that these areas are of nutritional importance to the bears. We also found Andean bears strongly avoided areas with livestock impacts to the grasslands, but that the cessation of livestock keeping restored the grasslands into areas Andean bears prefer within a short timeframe.”

The freely available article in PLOS ONE: https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0314547

High survival rates explain 20 years of rapid expansion of wolves in Germany

 

 

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Wolf (Canis lupus) at the forest edge, photographed in the Bavarian Forest National Park, Germany

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Credit: Photo by Jan Zwilling/Leibniz-IZW

Since wolves returned to Germany 20 years ago, they have spread quickly in many parts of the country. The rapid increase in the number of wolves was due to high survival and reproduction rates in areas with favourable environmental conditions. This is the result of an analysis carried out by the Leibniz Institute for Zoo and Wildlife Research (Leibniz-IZW) in collaboration with the LUPUS Institute, the Federal Agency for Nature Conservation (BfN), and the Senckenberg Center for Wildlife Genetics. The probability of survival for wolves during the period analysed was higher than anywhere else in the world. However, the expansion phase will end as soon as the carrying capacity of suitable German landscapes are reached ­– at which point survival rates can be expected to fall, according to the scientific team in a new paper in the scientific journal Wildlife Biology.

Young wolves in Germany had an annual survival probability of 75 percent during the first two decades since the beginning of the recolonisation of Germany; for adult wolves it was as high as 88 percent. For young wolves up to the age of two years, the variation in survival rate depended on the suitability of the habitat ­– the less suitable it was for wolves, the lower their survival; for adult wolves, such a relationship could not be established by the scientific team led by the Department of Ecological Dynamics at the Leibniz-IZW. “Using survival analysis, we were able to determine the median survival time of a wolf in Germany to be 146 weeks, which is around three years”, says Prof Dr Stephanie Kramer-Schadt, head of the Leibniz-IZW department and professor at the Technische Universität Berlin. The highest recorded longevity of a wolf in the study dataset was almost 13 years.

“The survival rates of the German wolf population were very high compared to other regions, in fact they were among the highest in the world”, Kramer-Schadt continued. “This indicates that the wolves settled in habitats that were very suitable for them during the 20 years which we analysed in this paper. Strict legal protection has also contributed to high survival rates.” Landscapes that qualify as suitable are those that offer sufficient cover – for example by forest cover – and areas that are as far away from roads as possible. These areas allow wolves to avoid humans and can serve as refuge for the animals. If wolves settle in less suitable habitats, this reduces their survival and reproduction. “While adult animals can still survive and establish territories in less suitable areas, the lower survival rate of young and sub-adult wolves and the lower number of young per litter slows down population growth and thus the expansion of the species.” As soon as the optimal areas are occupied, the growth of the population will slow down, according to the scientists.

The scientific investigation was based on a long-term dataset provided by the Federal Documentation and Consultation Centre on Wolves in Germany (DBBW) and included the surveys and findings of the comprehensive wolf monitoring in the German Federal States. It furthermore considered the results from tens of thousands of DNA analyses by the Senckenberg Center for Wildlife Genetics, which made this scientific investigation possible in the first place. The team also determined the reproductive performance of a total of 201 breeding females from 165 territories which were part of the analysed long-term dataset from 2000 to 2020. “We were able to analyse data from those female wolves over the years, that had offspring – at on average for 2.8 years”, says IZW scientist and first author of the paper Dr Aimara Planillo. “The analyses also show higher reproductive success of females with more experience and in more suitable habitats, with up to five reproductive years.” Summarising, the models show that a well-suited habitat and the reproduction experience of the female have a positive effect also on litter size, which in Germany averages at least four offspring.

Similar scientific analyses in other countries and regions illustrate how high the survival rate for adult wolves in Germany actually is. Other non-hunted populations also have high adult wolf survival rates of 78 per cent in the USA or 82 per cent in the Alpine regions in central Europe, although these do not come close to the 88 per cent in Germany. The reason for this is that the German population is still expanding.

The Department of Ecological Dynamics at the Leibniz-IZW investigates (among other things) the population development of large carnivores such as the wolf or Eurasian lynx using individual-based, spatial models, ranging from analysing past processes – such as the recolonisation of Germany by the wolf – to predicting future developments. For the paper recently published in “Wildlife Biology”, the scientists used demographic data on the wolf population (age, sex, year and place of birth, dates and places of resighting as well as cause of death) from the years 2000 to 2020 and related them to environmental variables such as habitat suitability (accounts for the different land use types and human disturbance effects, e.g. forested areas, distance to roads or human density), wolf population density (annual density of wolf territories around the focal territory) and the season of the year.

A significant proportion of the data comes from the ongoing molecular genetic analysis of samples collected in the field, which are sent to Senckenberg as part of the wolf monitoring programme of the German Federal States. Based on the genetic profiles created, numerous wolf individuals are collected several times over the years and can be assigned to wolf packs by analysing their relatedness. The Leibniz-IZW team then developed spatio-statistical models to determine the influence of environmental variables on the probability of survival of the population, the annual survival rates of different age classes, the probability of reproduction and reproductive performance. This revealed how well a variable – such as high habitat quality – can predict each population parameter and how substantial its influence is on the number and spatial distribution of wolves in Germany.

Previous scientific investigations by the Leibniz-IZW showed that

• the recolonisation of Germany by wolves is not a homogeneous, continuous process, but characterised by changing conditions. This means, for example, that wolves show different behaviours during different phases with regard to the suitability of habitats: In early phases, wolves liked “cherry-picking” when establishing new territories, while they were much less selective during later phases when the population was close to habitat saturation. The lower survival rate of young animals identified in the current paper and the lower number of offspring in less suitable areas provide an explanation for these preferences of wolves. Additional information: https://www.izw-berlin.de/en/press-release/wolves-like-cherry-picking-modelling-shows-how-they-recolonised-germany-and-where-they-could-live-in-the-future.html (Leibniz-IZW press release from November 16, 2023)
• the wolf population in Germany is essentially healthy and that human-caused deaths such as traffic accidents and illegal killings are responsible for the overwhelming majority of wolves found dead. Data from the 1,000 wolves dissected at the Leibniz-IZW in Germany show that around three quarters of dead wolves die in traffic collisions – mostly with cars on country roads or motorways. In 13.5 per cent of all wolves examined, evidence of a criminal offence such as illegal gun wounds was found, although the animals did not always die as a result. Additional information: https://www.izw-berlin.de/en/press-release/systematic-monitoring-leibniz-izw-carries-out-1000th-wolf-autopsy-since-the-species-comeback-to-germany.html (Leibniz-IZW press release from July 23, 2024)

The scientific investigation was funded by the Federal Agency for Nature Conservation (BfN) with funds from the Federal Ministry for the Environment, Nature Conservation, Nuclear Safety and Consumer Protection (BMUV) under grant number 3521 83 1300.

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Journal

Wildlife Biology

DOI

10.1002/wlb3.01246 

Large herbivores have lived in Yellowstone National Park for more than 2,000 years

 

Chemicals from dung buried in lake sediments reveal their presence and ecological impacts

Peer-Reviewed Publication

PLOS

 

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Bison cows and calves congregate in a meadow beside the Lamar River, Yellowstone National Park.

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Credit: John Wendt, CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

Large herbivores like bison or elk have continuously lived in the Yellowstone National Park region for about 2,300 years according to a new analysis of chemicals preserved in lake sediments. John Wendt of Oklahoma State University, U.S., and colleagues present these findings in the open-access journal PLOS ONE on October 30, 2024.

The near-extinction of bison in North America in the 19th and 20th centuries was a major ecological catastrophe and little is known about where and how these animals lived before European colonization. In the new study, researchers attempted to determine the dominant large herbivores that lived in the northern Yellowstone National Park area by analyzing steroids from animal dung in lake sediments dating from about 238 B.C. to the present time.

To perform this analysis, researchers first determined which types of steroids occur in the dung of several large herbivores believed to have lived in the region, including bison, elk, moose, mule deer and pronghorn. They found they could identify moose, pronghorn and mule deer based on the steroids in their dung alone, but that bison and elk were harder to differentiate from each other. When the researchers analyzed steroids from different layers of lake sediments, they saw that either bison, elk or a combination of the two, were the primary large herbivore species in the watershed for the last 2,300 years.

The analysis also showed high steroid levels in the 20th century, a time when hunting was banned, bison and elk were discouraged from migrating in winter, and their natural predators were eliminated. Based on the levels of plant pollen, microalgae or plankton detected in these sediments, the researchers concluded that the expanded animal populations likely ate up local forage plants, like willow and Idaho fescue, and that their dung may have fertilized the growth of diatoms in the lake, changing the local ecosystem. Provisions of winter hay in nearby meadows, provided by park managers, also kept animals in the area for longer, resulting in impacts to the watershed.

The new research demonstrates that the analysis of steroids from lake sediments is a promising tool that can help wildlife managers and conservationists understand how communities of hoofed animals and their impacts have shifted over time. While the results shed light on historical changes within a single watershed, researchers expect that extending this approach to a network of sites could provide much-needed information on past grazing animal communities at Yellowstone National Park and beyond.

The authors add: “We developed a 2,300-year record of wild herbivore activity in northern Yellowstone National Park with fossil biomarkers found in lake sediments. This information is critical for understanding long-term dynamics of ecologically and culturally important herbivores such as bison and elk.”

The freely available article in PLOS ONE: https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0311950

Citation: Wendt JAF, Argiriadis E, Whitlock C, Bortolini M, Battistel D, McWethy DB (2024) A 2000-year record of fecal biomarkers reveals past herbivore presence and impacts in a catchment in northern Yellowstone National Park, USA. PLoS ONE 19(10): e0311950. https://doi.org/10.1371/journal.pone.0311950

Journal

PLOS ONE

DOI

10.1371/journal.pone.0311950 

How do coexisting animals find enough to eat?

  

 

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Bison are seen grazing in a meadow in early winter at Yellowstone National Park. 

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Credit: Photo courtesy of Bethan Littleford-Colquhoun

PROVID Ecologists have long sought clarity on the dietary habits of different animal species. For scientists at Brown University and the National Park Service, it wasn’t obvious how herbivores in Yellowstone National Park, who subsist on grasses, wildflowers and trees, could compete for enough of those foods to survive the winter.

Over two years, with the aid of cutting-edge molecular biology tools and GPS tracking data, the researchers were able to determine not only what herbivores in Yellowstone eat, but also what strategies the animals use to find food throughout the year. The team published its findings in Royal Society Open Science.

“In Yellowstone, we know vegetation changes across seasons, but until now, we didn’t know how these seasonal changes influenced what animals eat or how they sustained themselves when options were limited,” said lead study author Bethan Littleford-Colquhoun, a postdoctoral research associate at Brown. “It turns out that while species eat similar categories of food, their diets differ from one another in cryptic and nuanced ways. And an animal’s body size plays an important role in how this is achieved.”

For decades, ecologists have debated how wildlife should confront challenges with their food supplies, said co-author Tyler Kartzinel, an associate professor of ecology, evolution and organismal biology at Brown.

Some experts argue that animals should diversify their diets to satisfy their taste preferences when they have the most freedom to select their favorite foods in summer, Kartzinel said. Others have posited that animals should diversify what they eat when they’re forced to accept whatever happens to be available — such as in a hard winter when they may have to compete for even undesirable foods to survive.

“These opposing predictions couldn’t both be true, so it wasn’t at all clear how Yellowstone's assemblage of herbivore species — with such a diversity of foraging behaviors — could succeed in finding enough food throughout the year,” Kartzinel said.

Seasonal specialization

For the study, the researchers used two years of GPS tracking and dietary DNA data to elucidate dietary variation across times of resource limitation and resource abundance for five of Yellowstone’s best-known species: bison, elk, deer, bighorn sheep and pronghorn antelope.

Scientists and staff at Yellowstone tracked the animals. Researchers at Brown, many of them undergraduate students overseen by Littleford-Colquhoun, analyzed fecal samples using a sophisticated molecular technique called metabarcoding, which helped to identify what foods the animals had consumed.

They found that all species capitalized on the seasonal abundance of wildflowers in summer, and that each species consolidated its foraging efforts around the subset of plant types that it was best prepared to compete for in winter. But the researchers discovered that feeding behaviors depended on the animal’s body size.

Members of the smallest species, such as deer and sheep, tended to fan out across summer meadows and dramatically expanded their diets before gathering in protected valleys where they survived the winter on leftover plants, according to the study. Larger animals like bison tended to do the opposite: In the winter, they were large enough to avoid competing for dwindling resources, so they instead ventured out into deep snow to find unique food reserves inaccessible to smaller deer and sheep.

“The study showed that these species can feed far more adaptably than anyone had previously assumed,” Littleford-Colquhoun said. “All species switch the ways they search for food, but the opportunities an individual bison has to fuel its migration or survive a hard winter might only work for it because it’s big. Meanwhile, other species might need to group together for protection in winter because they’re small.”

So when should animals search for unique foods to diversify their diets — summer or winter? Kartzinel said it depends on the kind of animal.

“Because of the variety of ways animals behaved in our study, we learned that both hypotheses about how animals fuel their migrations were right, but in different ways and at different times,” Kartzinel said. “So the question that biologists should have been bickering about for the past generation shouldn't have been, ‘Which foraging strategy is right?’ but rather, ‘When does each strategy work best for a given group of animals?’”

Kartzinel hopes the more nuanced insights about foraging behavior will help scientists take a more customized approach to wildlife conservation.

“If we want to help wildlife populations thrive,” Kartzinel said, “we should be maintaining a diversity of habitats and plant resources across their migratory corridors so that many animals, each with their own preferences, personalities and needs, can find what's best to fuel their journey.”