Other studies have found that prairie dog towns hold twice the density of wildlife and nearly double the number of species as prairie without the rodents. Vertebrate species associated with the towns number as high as 170, ranging from mice, to swift foxes, to raptors. Some animals, such as burrowing owls and prairie rattlesnakes, rely on the towns' underground architecture as shelter. A few, such as pronghorns, seek out the broadleafed herbs that grow in the heart of town. The best known of prairie dogs' dependents is the critically endangered black-footed ferret.
Ferrets live only in prairie dog burrows and eat their hosts, almost exclusively.
Biggins has estimated that towns covering fewer than 1,000 acres are too small to support even a minimal ferret population, yet few towns larger than this still exist on the plains. Captive breeding programs have brought that number up to a few hundred today, and ferrets have been reintroduced into the wild in a few large prairie dog colonies in Montana and South Dakota. Ever since ranchers first settled the West a century or more ago, they have viewed prairie dogs as pests. The rodents prefer to establish towns in the low grasses of heavily grazed areas. The heavier you graze the pasture, the better the prairie dogs do.