Bison once numbered in the tens of millions in the United States. Now, a nonprofit is working to restore the shortgrass prairie, where the American icons and their ecosystem can thrive again.

Around 200 chocolate-brown bison raise their heads, following the low growl of a pickup truck slowly motoring across the sagebrush-studded prairie.

Snorting and quietly bellowing, their feral odors riding the wind, they slowly trot across the prairie hills, eager to maintain distance from the truck.

This knot of bison — colloquially referred to as buffalo, though they are not the same species — is part of a project to rebuild a vast shortgrass prairie not only to return large numbers of bison here, but also to eventually restore the complex and productive grassland ecosystem the animals once engineered with their churning hooves, waste, grazing and even carcasses.

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