What does it take to conserve a species whose original range once stretched uninterrupted across an entire continent, from Mexico to northern Canada and Alaska, from the eastern seaboard across the Great Plains to Nevada and California?

The story of the American bison (Bison bison), a species turned symbol of the of the wild frontier roots of the American West, is a story of near total destruction followed by slow recovery; a story that has been told and retold as a troubling reminder of the how the earliest Americans interacted with the landscapes they inhabited. Though the considerable recovery of bison populations might be considered a conservation success, the fate of the American bison as a wild species remains unsettled. The landscapes that bison populations of today inhabit are radically different than those of the past. Today, the once-great grasslands of North America play host to increasingly dense human populations, highways, subdivisions and land converted to agricultural use. Bison were just one thread in a delicate web of life, simplified and restructured with plow and axe to make a place for human society.

Bison once roamed freely in massive herds across the Great Plains of North America. At its peak, the total population of wild bison on the continent is estimated to have been between 30 and 60 million. As Anglo-American pioneers and settlers began moving across the country, bringing with them railroads and fences, horses and cattle, guns and diseases, the once-great range of the bison became increasingly fractured and limited. By the mid-1880s, the herds of tens of million once inhabiting the United States had been reduced to a total of fewer than 2000 animals, driven nearly to extinction by unrestricted commercial hunting and indiscriminate slaughter. Today, thanks to collaborative conservation efforts that began all but too late, several hundred thousand exist in North America, mostly in private herds that are treated as livestock. Only a small handful–roughly 20,000–American bison today live as truly free-ranging wild animals. Even so, historical attempts at crossbreeding have created longstanding genetic issues for bison populations, including the introduction of domestic cattle DNA into the bison genome, a concern that still plagues many conservation herds. Even at a molecular level, humans–perhaps unknowingly–left an irreversible stain upon the evolutionary future of the wild bison.

What scientists have only recently begun to realize following the near-extinction of wild bison from North America is the role bison played as keystone herbivores in prairie ecosystems of the past. Referred to as “ecosystem engineers,” bison shape the structure, composition, and diversity of the ecosystems they inhabit through different aspects of their behavior: how they eat and digest food, where they move, and even how their bodies decompose. For example, bison graze in distinct patches, favoring certain areas over others, in addition to relying more heavily on certain forage species. These grazing patterns create a patchy mosaic of grazed and ungrazed areas across the landscape, which not only allows less dominant plant species to thrive but also reduces fuel loads and diversifies the effects of fire, if and when it occurs. Bison urine is also an extremely important source of nitrogen, which limits plant growth in most grasslands. Through another behavior unique to American bison, wallows (shallow depressions of exposed soil that bison repeatedly paw at and roll in) create patches with increased water retention and unique vegetation. These are not the only ways that bison shape their habitat, but it has become clear that through their unique ecology, bison increase diversity and resilience in prairie ecosystems, and at an especially large scale.

It has been the very realization of these keystone profound effects of bison that has inspired conservation efforts to return wild bison to the landscapes they once called home. Conservation initiatives to preserve and protect the largest land mammal in North America, however, are no small feat, requiring cooperation and careful management across fence lines, across political borders, across ownership boundaries, across whole ecosystems, and across generations. Although the future of the American plains bison as a wild species may be uncertain, what remains constant are these animals, the land that sustains them, and the evolutionary relationship that binds them to one another.