The Reintroduction of Wolves in Yellowstone: A Cascade of Events

Beginning in 1994, wolves relocated from Montana and Canada began their reintroduction to Yellowstone National Park in Wyoming. This was initially an attempt to correct the extermination of native wolves over the last two hundred years, but something unexpected happened as a result. What followed from this reintroduction is termed a “trophic cascade,” or a set of reactions radiating down the food chain, all stemming from the addition or subtraction of the system’s top predators. These interactions start out obvious, such as changes in the populations of prey animals, and get more indirect and abstract, even altering the landscape itself. Observation of this phenomenon can both expand our understanding of the natural world and inform our conservation efforts toward it. The example presented by wolf reintroduction in Yellowstone makes a useful illustration of both the obvious and surprising effects that predator populations have on the systems of which they are a part.

Humans caused the original elimination of wolves from Yellowstone. In the early 1800s, western expansion into the United States radically changed the landscape and animal populations of North America. During expansion, animals such as buffalo were hunted to near extinction, and predators dangerous to humans and their food supply were routinely exterminated as pests. Wyoming was no exception, and even after Yellowstone was made a national park the exterminations continued. At the time, policymakers simply didn’t see wolves as valuable, much less integral to the ecosystem, and thus excluded them from protection. This resulted in the near total elimination of wolves from the lower 48 states by the mid-1900s. That purge would set the stage for a restoration effort that would come decades later.

In 1973, the passage of the Endangered Species Act saw the first attempts to add wolves back into Yellowstone, but not without opposition. Human attitude toward predators may be more nuanced now, but wolves can still threaten human interests. Yellowstone’s surroundings are overwhelmingly agricultural, and the possibility of wolves threatening domestic animals or even humans remains an undesirable risk. Opposition came in the form of public comments and legal challenges to the proposed restoration, most commonly from ranchers and their advocates. Many locals of Yellowstone’s surroundings seemed to accept and endorse the policy of extermination that had reigned thus far, and saw attempts to reform it as a potential threat.

Not everyone felt so uneasy about the wolves though, and public support ultimately overwhelmed arguments against restoration. Thanks in part to National Park Service regulations mandating restoration of native species where possible, the decisions to reintroduce wolves won out. Legal challenges continued to be waged, but never successfully overturned the reintroduction plan. Finally in 1995 after a long acclimation period, the first relocated wolves were released into Yellowstone, and the changes began.

It took a surprisingly short time for the wolves to establish themselves as a viable population once introduced. One factor explaining this may be the near record number of elk in the park, who’s numbers were held back only by the exhaustion of the vegetation that sustained them. The original five year reintroduction plan was cut down to only two and that proved to be sufficient. Packs and breeding pairs were formed, and they began to have an effect on the rest of the park. First, the behavior of the prey animals started to change to accommodate a new peak predator. Instead of grazing freely, elk and deer began avoiding probable ambush sites, such as valleys and waterways, and being more cautious around water sources. Herd sizes contracted as sick or otherwise infirm prey animals were killed off leaving more adept and nimble packs skilled at living on the move. As elk and deer populations became more savvy and scarce, wolves expanded their hunting to bison, who adjusted their behaviors as well. In addition to prey animals, wolves also attacked and frequently killed coyotes in territory disputes. These initial changes constitute the most direct step in the trophic cascade.

The next wave of changes followed from the behavioral adaptations of animals reacting to the wolves. With the relative abundance of wolf kills, scavengers took immediate advantage. Populations of ravens and hawks increased to exploit the wealth of scraps left behind. With coyotes giving wolf packs a wide birth, rodent and hare populations rose to further sustain birds of prey, as well as badgers and foxes. Bears too experienced a change in routine. They found fewer frozen carcasses waiting for them after hibernation. As a result, they had to become more aggressive hunters, attacking fauns in the spring, and reinforcing the wolves’ pressure on prey species.

Figure 1. The relative growth (Primarily willows) around a river junction in Yellowstone [Credit: : Ripple W. J., and Beschta, Robert L. "Trophic Cascades in Yellowstone: The First 15 Years after Wolf Reintroduction." Biological Conservation 145.1 (2012)]

Figure 1. The relative growth (Primarily willows) around a river junction in Yellowstone [Credit: : Ripple W. J., and Beschta, Robert L. “Trophic Cascades in Yellowstone: The First 15 Years after Wolf Reintroduction.” Biological Conservation 145.1 (2012)]

As those adaptations unfolded, still more were occurring at the same time. As herbivores became less numerous and more conservative, plant life regenerated, particularly around water sources. Specifically studied in one article were aspen, willow, and cottonwood trees (Figure 1). Their growth was shown to significantly increase along with sapling survival as browsing by elk declined in the years following wolf introduction. This created habitats for various bird species and a supply of lumber to be used by beavers whose population started to rise in response to the vegetation returning. The dams built by the beavers created still waters and coves providing habitats for reptiles, amphibians, and various species of fish. This created ideal feeding grounds for ducks and other migratory birds. By this point, ecological diversity was on the rise as the habitats available expanded in scope and diversity.

The last piece in this cascade of events may be the most surprising, however. In a process detailed by Robert Beschta, the dynamics of water in the park began to change in response to the wolves. As old plants expanded their roots and new plants germinated and grew on the hillsides and banks of rivers, they stabilized the soil. Erosion slowed, and large gravely washouts on hillsides were focused into waterfalls and brooks. Likewise, the waterways narrowed and deepened, and floodplains were redefined. A process that began with the introduction of a predator at the very top of an ecosystem eventually changed the geography of the land on which they now dwell. Hence, the wolves changed the behavior of their prey, which in turn changed the behavior of the flora, which in turn changed the behavior of the rivers.

Evolutionary theory can help explain the processes observed here. Species differentiated into their particular forms over millions of years to better exploit the niches in their environment. When one animal is removed, specifically a powerful player in its ecosystem, not only does their niche become available, but many many others collapse. Every spot occupied in a system relies on every other. Waterfowl rely on frogs and tadpoles for food, those frogs spawn in still waters provided by beavers, and beavers rely on flora to eat and use as building materials. That fits the usual bottom-up model of ecosystems, but who protects the flora for beavers? In this case, it turned out to be wolves.

This all may seem like nothing more than a scientific curiosity, but it has significant impacts on both the natural world and the human species. Wolves are dangerous and prey on animals that humans own and consume, so it’s unfortunately easy to marginalize them as well as other predatory species. However, examples such as this make it clear that predators can keep a balance in an ecosystem that loses stability without them. They are, in that sense, curators of wild places. As beings that enjoy, rely on, and profit from such places, we benefit directly from their existence, and from understanding the processes by which trophic cascades operate.

It’s worth noting that at time of writing, wolf reintroduction occurred only 20 years ago, so the data is incomplete. Variables like climate and forest fires have to be accounted for, and that will take time. Only the early stages of this process have been observed, and much more observation will follow from this point. So far, the changes have been rapid and noticeable, and that rate of change must inevitably slow. In theory, wolf populations will continue to grow until they stabilize through either resource exhaustion, or by hitting the geographic limits imposed on them. Expansion into unfriendly territory will lead to their numbers being held at a relative constant. With that, Yellowstone will fall back into a rhythm probably very much like the one it had before the destruction of native wolf populations.

However, things could go very differently. The ecosystem could dramatically alter itself as the wolf population rises. They could become too successful as hunters, threatening the prey animal populations and stressing their resources into a state of famine. They might push on their boundaries too hard leading to a backlash from human populations in the area, resulting in a restoration of the old practice of extermination. The ecosystem could, in any number of ways, change and become inhospitable to any of the populations now flourishing in Yellowstone. Twenty years is a short time in ecology, and much has happened already. Furthermore, the observations themselves could have led to a hasty conclusion. As Andy Dobson puts it, “Although many strong patterns are observed, several of these may be correlation without causation. . . . .” Any prediction made now can only be a hypothesis to be tested.

As an experiment, the wolf reintroduction has been a tremendous learning opportunity with regards to ecology. It challenges the more familiar “bottom up” ideas typically employed to understand ecosystems, and demands we observe the “top down” forces just as closely. Creating a deep understanding of what forces create and maintain the natural world means expanding our perspectives in just such a way. With that in mind, we are best served by allowing this experiment to continue, and rigorously investigating the results. Only then can the predictions made today be tested and learned from. Hopefully, the next twenty years of watching the wolves of Yellowstone will prove just as informative and surprising as the last.


ClaytonClayton Hull-Crew is a Colorado Springs native, small business manager, nationally certified EMT, and member of the local music community. He now spends most of his time studying the biological sciences in college, and he intends to pursue medical school.