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