A four-minute mini documentary called “How Wolves Change Rivers” was probably the first ecology-related video that went completely viral. With over 44 million views, the short video emphasized the role of the wolf as a top predator with an immense effect on their environment, triggering the trophic cascade of Yellowstone National Park.
What is trophic cascade?
A trophic cascade is an ecological phenomenon where changes to the population, or introduction to an area, of a top predator cause a chain of effects on lower trophic levels, often leading to significant changes in ecosystem structure and nutrient cycling. Trophic cascade is used to explain how changes at one level of the food chain can ripple through the entire ecosystem.
How the wolf reintroduction to Yellowstone changed the ecosystem
Previously extinct to the area, wildlife conservationists reintroduced wolves to Yellowstone after an absence of 70 years. The wolves through hunting, started regulating the number of large herbivores in the ecosystem. Naturally, they reduced the number of elk, but perhaps more importantly, the prey’s behavior changed. The grazing animals started avoiding certain open areas where they were more likely to get eaten, including the river banks.
In turn, the trees (priorly eaten by herbivores) started regenerating, leading to reforestation. The process resulted in stabilizing riverbanks because now larger, intact trees prevented bank erosion. The reintroduction of the wolves, therefore, not only changed the biodiversity of Yellowstone, but also its physical geography.
In the video, beavers also get an honorable mention – as ecosystem engineers whose populations also recovered thanks to wolves. The reason is obvious – more woody vegetation (for feeding and dam building) became available to beavers once deer became skittish and avoiding areas near river banks, allowing trees to repopulate.
While this presumption is true, the beaver-wolf relationship is far more complex than once thought, according to the study published in Proceedings of the Royal Society B this November.
The research by Dr. Thomas D. Gable and his team showed that, while wolves indeed clear competition for the beavers, they also change beaver behavior – because beavers are also wolf’s prey. Consequently, just like in the case of elk, beavers change their behavior to survive, which, in turn, affects the surrounding vegetation and the entire ecosystem.
Beavers Also Shape Ecosystems
We commonly see the phrase “ecosystem engineer” to describe the “beaver.” Ecosystem engineers are species that change their environment in a way that creates new habitats used by other organisms. Truly influential ecosystem engineers like beavers change both abiotic (e.g., river flow) and biotic (e.g., vegetation composition) of their habitats. Other ecosystem engineers include woodpeckers (that create nesting holes in trees), corals (that create entire coral reefs), and others.
Beavers have a unique behavior of cutting down trees and creating dams in small waterways and waterbodies. This leads to the formation of beaver ponds. Although beavers do these waterway modifications for their own sake, their actions also provide novel habitats for many other species. Also, the dams and ponds created by beavers play a significant role in many ecosystem processes such as nutrient cycling.
Without beavers, the beaver ponds would not exist, with a negative effect on multiple pond-dependent species. This is not a mere hypothesis but was tested in places where beavers became extinct. These habitats saw a drastic change in the landscape and the loss of many important wetland habitats.
Their influence positions beavers as a keystone species. In fact, the National Park Service openly calls beavers “the ultimate keystone species.” A keystone species is “a unique organism that supports the entire biological community.” Or, if you want it more poetically written, take it from Ben Goldfarb, an environmental journalist and self-professed “Beaver Believer.”
“To acknowledge that beaver create environments that store water and help sustain other creatures is insufficient. Beaver are nothing less than continent-scale forces of nature and in part responsible for sculpting the land upon which Americans their communities.” (in ‘Eager: The Surprising Secret Life of Beaver,’ 2018)
However, as hinted in the beginning, the wolf is another critically important keystone species. To sum it up – as apex predators, wolves greatly influence populations of their prey species, both in terms of number and behavior. In turn, the tweaks of their prey’s activity change the ecosystem and its landscape.
Yet, what happens when the two keystone species clash? That’s precisely what this study within the Voyageurs Wolf Project is trying to find out.
The Voyageurs Wolf Project Looks at the Beaver-Wolf dynamic’s impact on the ecosystem
The project, led by Dr. Gable, studies the ecology of wolves and their prey in and around Voyageurs National Park, Minnesota. One of the questions the research team was interested in was the wolf-beaver predator-prey dynamic and its consequences.
In their previous work, they found that wolves may hinder or stop the creation of new wetlands by removing beavers. This time, researchers were interested in how wolf predation affected forest composition. They went to great lengths to track beaver hunting by wolves in the Voyageurs.
Beavers venture out at night to the forest to cut fresh wood and then drag it back to their colonies. All the hauling back to the beaver’s home slash dam creates logging trails that extend from the pond and into the forests. Beavers tend to use these paths consistently.
The predictability of the beaver movement has led the intelligent and ever-opportunistic wolves to use these beaver outings as a chance for an easy meal. During spring and summer, the wolves ambush beavers along the logging trials. Back in 2018, Dr. Gable first noticed that wolves were more keen on hunting beavers on trails farther from the water and wondered if there was a pattern with wider implications.
Because wolves did not leave much evidence left at the “crime scene,” discovering successful beaver hunts required tedious forensic work. Over the years, the team sampled beaver tissue and hair in spots where GPS-tracked wolves had spent 20 minutes or more (implying they were ambushing and/or feeding). This way, they ended up documenting hundreds of both successful and unsuccessful ambushes and recorded the length of the respective trails.
The comparison of their data with the area’s well-recorded beaver trail network led them to conclude that wolves truly hunted more beavers on the longer logging trails. The median trail that secured a successful hunt was 57 feet (17.4m) longer than an average beaver trail.
Wolves influence the growth of forests because they make beavers cautious about how far they go from ponds or rivers to find food. Wolves often catch beavers when they wander far from water on long feeding paths. Beavers tend to travel more and stay out longer on these longer paths, making them more likely to be caught by wolves. To avoid predation, many beavers stay closer to water to avoid wolves, which limits how much of the forest they change.
The wolf-imposed effect on beaver logging has a visible effect on tree species composition, especially close to the ponds that have beavers. Since the distance from the pond for their tree felling is limited by wolf predation, beavers seem to cut down more deciduous trees near the water. In an example given in a study, they removed almost every aspen tree within 65 to 98 feet (20-30m) of the pond, but left standing the conifer trees.
Consequently, “forests around beaver ponds become dominated by conifers or less preferred deciduous species,” as the study reports. Visually, it is easy to notice the trademark conifer halo around the pond’s shores from aerial footage. In the same manner, many deciduous trees farther away from the beaver pond – where wolves lurked – were spared.
However, things may not be as simple. Clive Jones, an emeritus senior scientist at the Cary Institute of Ecosystem Studies in Millbrook, New York, who first introduced the concept of ecosystem engineers and wasn’t involved in the study, said that this research alone is not enough for definitive conclusions on how wolves or beavers shape forest dynamics – there are numerous other influences playing their parts and need to be accounted for, too.
Still, he added that the Voyageurs study raises interesting possibilities and hoped it would catalyze more studies.
Gable, T. D., Johnson-Bice, S. M., Homkes, A. T., Fieberg, J., & Bump, J. K. (2023). Wolves alter the trajectory of forests by shaping the central place foraging behaviour of an ecosystem engineer. Proceedings of the Royal Society B, 290(2010), 20231377. http://doi.org/10.1098/rspb.2023.1377
Gable, T. D., Johnson-Bice, S. M., Homkes, A. T., Windels, S. K., & Bump, J. K. (2020). Outsized effect of predation: Wolves alter wetland creation and recolonization by killing ecosystem engineers. Science Advances, 6(46), eabc5439. DOI: 10.1126/sciadv.abc5439
Acadia’s North American Beaver: The Ultimate Keystone Species. National Park Service.
Chandler, Houston. “Ecosystem Engineers: Creating Habitat for Others.” The Oriane Society, 12 December 2018
Farquhar, Peter. “The scientists behind the viral ‘How Wolves Change Rivers’ video are back and have made a small update to their claim.” Yahoo! News, 8 November 2018
Giaimo, Cara. “Leave It to Beavers? Not if You’re a Wolf.” The New York Times, 7 November 2023