Because black-footed ferrets eat prairie dogs and live in their burrows, they are completely dependent upon large prairie dog colonies for survival. With the dramatic loss of large prairie dog colonies came the loss of almost all black-footed ferrets as well; by 1986 only 18 black-footed ferrets remained.
Black-footed ferrets live only on large prairie dog colonies, and these; prairie dog colonies occur only within central and western grasslands  and shrublands. Much of the original grassland habitat in North America has been plowed, and very little of these grasslands are protected even today. Even worse for black-footed ferrets, many prairie dog colonies within the remaining grasslands and shrublands have been destroyed by poisoning and exotic disease, leaving ferrets with few remaining habitats.
Loss of Prey
Prairie dog  colonies have been reduced to less than 5% of the area they originally occupied. In addition to habitat loss, humans pose a great threat to prairie dogs, which they often consider vermin. Prairie dogs are lost to poisoning and shooting by those wishing to eradicate them from their land. Sylvatic plague – an exotic disease to which prairie dogs have no known immunity – has also decimated these animals. The remaining colonies of prairie dogs today are relatively small and fragmented, often separated by great distances. Without sustainable populations of their main food source, black-footed ferrets cannot survive.