Somewhere in the 20th and 19th europeans in Canada  wanted to move westward because the land there was fertile. This was the prairies. They took up most of the land where prairie dogs and ferrets lived. This was the first decline of ferrets.

 

Majority of a black-footed ferret's diet is Prairie dog. This was a problem because in the 20th century, people started killing prairie dogs because they were considered “pests.” Aswell, what already was a low population for the ferrets, coyotes and domestic dogs had a disease called distemper. This disease then spread to prairie dogs and the ferrets, it usually resulted in death. If the ferrets were not already infected, there would be nothing to left to eat because the Prairie dogs would be dead.