Across North America, mountain lions, bears, and gray wolves have made a remarkable comeback over the last 50 years. Once nearly exterminated, these animals have been recovering their populations and returning to the landscapes they historically roamed, thanks to protections like the Endangered Species Act, hunting limits, and reintroduction programs.
The ecological impact of restoring these large carnivores is potentially huge, in part because of the way they could help to balance ecosystems by keeping prey populations under control.
One famous example is a study from Yellowstone National Park in the early 2000s, which seemed to indicate that the restoration of gray wolves helped forests recover by scaring elk away from habitats where they might otherwise eat vulnerable tree saplings. The study garnered a frenzy of international media attention as an illustration of an ecology concept called "trophic cascade," where the introduction or removal of an animal at one level of the food chain creates a series of effects throughout.
However, further research in Yellowstone National Park and elsewhere has since presented a murkier picture of whether, when, and how such impacts have occurred to-date across North America. UC Santa Cruz Professor Chris Wilmers, a wildlife ecologist who studies large carnivores, has been contributing to and carefully tracking these developments.
"The Yellowstone trophic cascade example has really been oversimplified in the media," he said. "The scientific evidence today shows that there are many factors at play, so the effects we're seeing can't neatly be attributed solely to the reintroduction of wolves. That's important to understand because, if the goal of large carnivore restoration in other parts of the world is to initiate a trophic cascade, it's going to be a lot more complicated than what people might expect."
To help address the issue, Wilmers wanted to get to the bottom of what we can confidently say about the ecological impacts of large carnivores in North America at this stage in their recovery and what further research and clarification is still needed. So he led a team of scientists in developing a new paper that analyzes findings of more than 170 studies from the 1940s to the modern era. This comprehensive approach allowed the team to evaluate the weight of evidence in a way that can help to guide the future direction of both research and wildlife management.
Dynamics between predators and prey
One clear trend that emerged from the team's research is that there are often more important forces at play in North American ecosystems than the dynamics between wolves, bears, and mountain lions and their preferred prey.
Human impacts like hunting and land-use changes ultimately have a much greater impact than large carnivores on the population size, distribution, and behaviors of animals like deer, elk, and moose. Environmental constraints related to habitat and food are also more influential in limiting population size for these prey animals than predation.
That's not to say that large carnivores can't still meaningfully control populations of prey species. But those effects are more likely under a very specific set of conditions. For example, predators have more impact in spatially constrained systems, like islands, where their prey have nowhere else to go, and in instances where multiple predators are targeting the same species at different life stages.
A prey species is also more vulnerable to population impacts when it's competing with another more resilient species. That's because a growing population of a competitor species, often due to human-caused habitat changes, can elevate predator populations in a way that reduces the prey species not being bolstered by population growth. Population reductions of bighorn sheep and mountain caribou in Western Canada are a few examples of this effect.
Large predators also seem to suppress populations of smaller carnivores across North America by about 18% on average, according to a correlational analysis of species abundance conducted in the new paper. Those impacts can sometimes help traditional prey animals or other small carnivores. For example, pronghorns and red foxes have benefited from population reductions of coyotes, following the recovery of larger carnivores.
Broader ecosystem impacts
How these impacts ripple down to the very bottom rungs of the food chain is a bit less clear. But long-term research in Yellowstone National Park and a handful of other systems has helped build consensus around what key mechanisms are necessary for a true trophic cascade. In situations where browsing and grazing is suppressing plant growth, predators can have an indirect positive effect on plants if their presence reduces plant-eating by other animals.
Research shows that even the fear of large predators changes prey behavior, and this could theoretically reduce pressure on plants in some cases. But, in practice, evidence for behaviorally mediated trophic cascades has been inconsistent. So trophic cascades are more likely to be observed in situations where predators are truly limiting prey populations. Tailoring future research to these specific situations could help illuminate the process.
In Yellowstone National Park, large carnivore recovery has certainly triggered some ecological changes that are consistent with a trophic cascade. But key mysteries remain. Initial behavioral theories about what types of habitats elk would avoid in response to fear of wolves and how this might affect their access to browse on woody plants have not been supported by later studies. And it remains unclear to what extent observed elk population declines can be attributed specifically to wolves versus other predators, competitors, or environmental factors like drought.
Ecosystem recovery in the park also seems to be limited by a number of factors. Research across North America has shed new light on certain conditions that can dampen the effect of a trophic cascade. When multiple prey animals eat the same plants, but one is less vulnerable to predation, trophic cascade may be masked. For example, both bison and elk eat tree saplings in Yellowstone, but adult bison are too large for predators like wolves to take down, so grazing and browsing pressure from bison has remained largely unchecked.
The ability of plant communities to recover can also be limited by changes in underlying environmental conditions that have occurred since predators were first removed from an ecosystem. In Yellowstone, the loss of both wolves and beavers changed the park's hydrology so much that rivers became narrower over time with steeper banks, reducing suitable habitat for key tree species today.
Lessons for wildlife conservation
Overall, the new paper demonstrates how efforts to draw cause and effect connections between a particular large carnivore and ecosystem recovery are often obscured by complex interactions among and between different species of predators and prey, how they move and behave across landscapes, and how humans are transforming these systems. But that doesn't mean restoration efforts for mountain lions, bears, and wolves aren't ecologically beneficial.
"Restoring predators certainly will add to biodiversity and to the complexity of how your ecosystem works, and that is a good thing," Wilmers explained. "It's just that it's not going to have a simple effect that you can easily predict before restoring these species."
Rapidly improving technologies such as GPS telemetry, genetic sampling, camera traps, and bioacoustic monitoring may get us closer to understanding and predicting impacts in the near future, by enabling better tracking of predator and prey populations and their interactions.
In the meantime, though, the very fact that so much uncertainty remains about how best to restore the ecosystem functions of large predators is strong evidence of the need to protect threatened species before they disappear.
"One of the things the research points to most clearly now is that you want to avoid losing these species of large carnivores from systems in the first place," Wilmers said. "Because putting them back, while useful to do, could take 50 to 100 years or more to really restore what was lost."
