When the last wild wolves were killed in Yellowstone National Park in 1926, the decision was widely approved. Farmers had long seen wolves as competitors, hunters as a threat to game, and park rangers as a species that undermined the idea of a safe wilderness. For seventy years the park was wolf-free. Then, in 1995 and 1996, thirty-one wolves were captured in Canada and released into Yellowstone in the face of considerable local resistance. The reintroduction has since become one of the most closely studied experiments in ecology.

The rationale for bringing the wolves back had less to do with wolves themselves than with elk, the large deer that had multiplied in their absence. Without their main predator, the elk population had expanded to levels that the park's vegetation could not sustain. Young willows, aspens and cottonwoods were being browsed almost as soon as they sprouted, especially along rivers and streams. Beavers, which depend on willow for food and dam-building, had almost disappeared. Without beaver dams, streams ran faster and cut deeper channels into the earth, removing habitat for fish, amphibians and insects. The hope was that the reintroduction of wolves would not simply reduce elk numbers but would also change elk behaviour, making them warier of open, exposed areas near water.

This hope has been largely, though not entirely, borne out. Within a decade, elk numbers in the park had fallen by roughly a third, and the elk that remained behaved differently: they tended to avoid narrow valleys and stream sides, where wolves could corner them. Wildlife ecologist Dr. Ella Whiteford at Montana State University has documented a recovery of streamside vegetation in several of the most-studied valleys, with willow densities rising and aspens producing new growth for the first time in decades. Beaver colonies have returned in modest numbers, followed by a range of smaller species that depend on the habitats beavers create.

The phrase often used to describe this chain of effects is 'trophic cascade'. A change at the top of the food chain propagates downwards: wolves affect elk, elk affect trees, trees affect rivers, rivers affect countless other species. The elegance of the cascade has made Yellowstone an influential case study, cited in policy debates about predators in other regions, from wolves in central Europe to sea otters on the Pacific coast. Dr. Whiteford warns, however, that the Yellowstone result is partly a reflection of the park's particular circumstances, including its large area, its legally protected status, and the absence of year-round agriculture. Applying the model to smaller, more fragmented landscapes has produced mixed results.

The reintroduction has not been universally popular. Ranchers outside the park have lost livestock to wolves, and some communities have seen hunting pressure on elk decline enough to affect local tourism. Compensation schemes, paid for partly by conservation charities and partly by state government, cover documented livestock losses but rarely match the emotional weight of the event. Political debate has shifted several times. Wolves in the region were removed from the federal endangered-species list in 2011, added back by court order, and then partially removed again. Management is now shared among several state agencies, each with its own rules on hunting seasons and protected zones.

Several related debates have arisen as a result. One concerns 'landscape of fear' effects: the idea that the presence of a predator alters prey behaviour even in places where predation is rare. Critics argue that some of the elk-behaviour changes in Yellowstone may be the result of other factors, such as climate and human hunting, rather than of wolves directly. A 2022 analysis by the Rocky Mountain Research Network attempted to separate these influences statistically and concluded that wolves accounted for perhaps half of the observed effect, with climate and other predators responsible for the rest. The analysis did not dispute the basic story but complicated it.

Conservation biologists generally treat the Yellowstone reintroduction as a partial success. It has demonstrated that the removal of a top predator can be reversed and that ecosystems can recover more quickly than was once thought. At the same time, it has shown that reintroduction requires careful design, long-term political support, and honest discussion with communities whose livelihoods may be affected. 'The wolves are not a magic solution,' Dr. Whiteford has said. 'They are a reminder that restoring a landscape is as political as it is biological.'