Impact of Restoring Large Predators Is Complex

Large predators can’t be added or subtracted to an ecosystem like simple arithmetic—many factors shift simultaneously.

By Helen Petre

During the past 50 years, large predators, such as wolves, pumas, and bears, have made a remarkable recovery across North America. Predators are a critical component of ecosystems, and it is commonly accepted that reintroduction of large carnivores reduces populations of large mammals with hooves, or ungulates, such as deer and elk. This in turn results in increased growth of plants, the food that ungulates eat, reestablishing trophic levels to what they were before predators were removed from the ecosystem, usually by overhunting and habitat destruction. 

A recent article, written by scientists at the University of California – Santa Cruz, questions this conclusion. The research group, led by Chris Wilmers, a wildlife ecologist who studies large carnivores, reviewed 170 studies from 1940 to the present. They found that the relationship is not what we think. It is more complex and the result of many other factors, including hunting regulations, land development, and human-animal interactions. The researchers concluded that anthropogenic factors may be more influential in establishing trophic cascades than reintroducing large predators. 

History of carnivores in America

Gray wolves (Canis lupus), pumas (Puma concolor), grizzly bears (Ursus arctos), and black bears (Ursus americanus) are the four most widespread carnivores in North America. Since European colonization of America, the populations of large carnivores have increased and decreased depending on human conceptions of their value, due to legislation, eradication programs, and hunting practices. Decisions were seldom based on ecological value. 

People are afraid of big, meat-eating mammals and complain about livestock losses. Predators were important sources of income in the fur trade, and, as with everything that is abundant, humans did not consider the results of destroying something that seemed to be in endless supply. We decimated our large carnivores, and their populations declined precipitously. Once it became apparent that maybe killing all the large predators was a bad idea, we tried to remediate this error. Over the past 50 years, efforts to restore predators to ecosystems has resulted in relatively high populations of predators. This provides the opportunity to compare the effects of reintroduction of predators to historical ecosystems. Are we actually restoring ecosystems or are we establishing new ecosystems? 

Reintroduction of gray wolves in Yellowstone National Park

By 1994, there were only two small populations of wolves in the continental United States: one in the northern Great Lakes region, and another around Glacier National Park. In 1995, 66 wolves were introduced into Yellowstone National Park. As of 2023, there were 2,800 individuals in six states, all descendants of this population. Success? Yes. 

The reintroduction of gray wolves to Yellowstone National Park is an example of how reestablishing the trophic cascade results in a beneficial system for the natural environment. Reintroduction of predators results in decreased populations of ungulates, increased plant growth, and increased diversity of prey species, establishing a top down control system for the ecosystem. At first, people did not support the introduction of scary big things that eat meat, but after much convincing, people agreed that it seemed to be a good idea. After 50 years or so, people are again thinking maybe not.

Besides wolves, populations of other large carnivores are increasing. Black bears have experienced recovery, mostly due to restrictive hunting regulations. Pumas have recovered in the western United States, mostly due to the elimination of bounties, and establishment of hunting regulations. Grizzly bears have recovered somewhat. They are listed as federally threatened and only occupy 6 percent of their historical range in the continental United States, mostly around Yellowstone, but their populations have increased in Alaska and Canada.

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Results of decline and recovery 

Recovery is great, but recovery after steep decline may not actually recover the historical ecological relationships. Large carnivore decline began in the early 1900s, when Aldo Leopold encouraged predator removal. He then realized, by the 1930s, that removal of predators was causing overgrazing, and resulting in habitat destruction. He also noticed aspen decline in the Kaibab Plateau, due to, again, predator removal. He realized the error of removing predators and tried to fix it, but at the same time, other factors were influencing trophic levels, ecology, and species success. In the 1930s we were logging, suppressing fire, overhunting, and grazing cattle, all of which had at least as much effect on trophic levels and species success as removal of predators. 

It took the rest of us a little longer than Aldo Leopold to realize the error of our ways, but by the 1960s, we decided that reintroducing predators would reduce ungulate populations and bring back historic vegetation and ecosystems. Maybe. Not everyone agreed. Not everyone agrees today. Some ecologists noted that removing cattle from the land would reduce grazing and increase deer populations by providing more food for the deer. Maybe the deer population increased because of removing cattle, not removing predators. Some scientists argue that aspen and willow were recovering at Yellowstone because of beaver, or bison populations, and not due to reintroduction of wolves which theoretically scare the elk away from the waterways where aspen and willow grow. All of these questions led Dr. Wilmers, and his research group at UCSC, to ask: Can predator restoration restore ecosystems, or is the system beyond restoration? 

That is a difficult question to answer. Science, and well designed-experiments, can tell us what happened, but not why, or how. The studies Dr. Wilmers and his team reviewed concluded that a population increased, or decreased, but they could not conclude that it was due to reintroduction of predators, because, well, it could have rained a lot, or not, or maybe there was a disease in the deer population, or something changed that we might have missed. 

Results of large carnivore restoration

Large carnivores eat meat, therefore it is logical to think that introducing them into a system will result in fewer prey, or ungulates, in the Yellowstone case. But other factors influence deer density, including hunting regulations, availability of food, disease, and parasites. Large carnivores usually kill calves. Hunters kill larger animals, usually those capable of breeding, so overall, the animals killed by hunters reduce the population more than the number killed by wolves. 

It is important to consider that after reintroduction of wolves at Yellowstone, the population of pumas, grizzly, and black bears also increased. Did the wolves kill more elk, or did the other predators? At the same time, bison populations increased and bison competed with elk for available vegetation, which could reduce the elk population. So maybe the decrease in elk population is a result of multiple predators, competition with other herbivores, and hunting preferences, not just reintroduction of predators. 

The researchers reviewed different studies with different combinations of predators and prey, and different vegetation, competitors, and climates. They found vastly different results. In some situations, hunters killed more herbivores than predators, in other situations, wolves killed more, or bears killed more, or wolves killed more bison and elk increased in population. The answer is not based on one factor alone and the results are not linear. 

What about fear? 

Maybe it is not the actual kill that changes the number of organisms in a specific habitat. Besides actually being eaten by a predator, is it possible that herbivores fear the reintroduced predators and change their feeding behavior to avoid them? For example, maybe the willows are growing along the waterways in Yellowstone because the elk are afraid of going there because they think the wolves are hanging out waiting for them to come to drink. That is what happens with giraffes and lions on the Serengeti. Maybe herbivores change their birthing locations out of fear, or maybe they move to an area where the food is not as adequate, resulting in fewer successful pregnancies. All of these are possible, but very difficult to measure. Science can measure what happened, but not why. 

It is impossible to determine if adding or removing predators is the cause of changes in populations because of multiple other factors, including changes in hunting regulations. Usually when predators are introduced, hunting is allowed, and when predators are removed, hunting is not allowed. In fact, two of the studies the researchers reviewed concluded that hunting had more of an impact on ungulate populations than reintroduction of wolves. 

Complex prey interactions 

Sometimes having a different available prey item provides the predator diet with an alternative food source. During an extremely harsh winter, the reintroduced wolves at Yellowstone fed on bison that died of starvation, and the elk populations increased due to less pressure. 

Large predators do not always kill herbivores. Sometimes they kill each other. 

Sometimes wolves kill bears and pumas. The review of available published literature showed a large variation in prey species for all large carnivores in different geographical regions. Wolves, bears, and pumas kill herbivores, but they also kill coyotes, foxes, and other smaller predators. On the other hand, wolves, bears, and pumas provide carrion for coyotes and smaller predators, so it is difficult to ascertain from published literature if reintroduction of large carnivores suppresses or enhances populations of small predators. It is probably safe to say both occur in different situations. 

Other studies showed that reintroduction of large carnivores decreased human vehicle collisions by reducing roadkill. It is well known that vehicle deer collisions are frequent during the rut and result in high losses both physically and financially. It is logical that if there are fewer deer due to predation, there will be fewer collisions, and this is a good thing for humans and vehicles. Reintroduction of wolves in Wisconsin resulted in a 24 percent reduction in deer vehicle collisions and the economic benefits far outweigh the cost of livestock losses, not to mention saving human lives. Vultures, on the other hand, probably lose out on their common roadkill diet. 

To complicate the predator choice situation, wolves steal puma kills and grizzlies steal wolf kills, perhaps encouraging pumas, and maybe wolves, to focus on smaller prey, such as coyotes and foxes. 

Public and private land as a consideration

Most of the studies the team reviewed concerned public land, but actually most predator prey interactions that humans are aware of occur on private lands. That most likely changes the interactions, as small predators, such as bobcats and coyotes, move closer to humans when pursued by larger carnivores on private lands. That may seem counterproductive, but it makes sense. If a bobcat is living in an area with an increased number of wolves, it will move to an area where there are fewer wolves, and perhaps more farm animals. 

Introduction of wolves in an area with a large population of coyotes, will naturally result in a decrease in the coyote population. That will result in a decrease of coyote kills of ungulates, especially fawns, which can indirectly increase the tick population. That is a potential benefit for humans, but is this a restoration of the historical ecosystem? Maybe. Maybe not. 

Drawing a conclusion on large predators 

After all of these considerations, it remains unclear if reintroducing predators increases vegetation, or if the increase in vegetation is the result of other factors. In the Yellowstone case, it is unclear if the reduction of the elk population is due to wolves, humans, or competition with bison, water availability, or climate change. Without assurance that wolves are the cause of the elk population reduction, no real conclusions can be made about the results of carnivore reintroduction. Again in the Yellowstone example, it is certain that reducing herbivores increases vegetation, but only in the presence of beavers, which build dams and increase water availability. Then, wolves can prey on the beavers, too. And, then, large carnivores tend to eat young and sick organisms, so it is safe to say that they reduce disease, such as Chronic Wasting Disease, by taking the sick ungulates out of the population. Thus, there are many factors that influence the populations of predators, prey and vegetation. 

After a thorough review of literature, the research team conclude that herbivores impact plant composition and abundance, but how and why is dependent on many abiotic and biotic factors. They conclude, logically, that restoring historical systems is difficult, if not impossible, and the best alternative is not removing native large predators in the first place. Reintroduction of predators alters ecosystems in ways that we cannot readily predict. Multiple predators, targeting multiple prey, with multiple competitors, with multiple food and water sources, will result in multiple conclusions. 

This study was published in the peer-reviewed journal Annual Review of Ecology, Evolution, and Systematics.

References

Wilmers, C., Levi, T., Prugh, L., Ruprecht, J., & Stahler, D. The ecological impacts of large-carnivore recovery in North America. (2025). Annual Review of Ecology, Evolution, and Systematics, 46, 337–363. https://doi.org/10.1146/annurev-ecolsys-102722-021139

Featured image “Aspen, Male Gray Wolf, Wolf Park” by Raed Mansour on Flickr, licensed under CC BY 2.0.