GREAT FALLS, Mont. (AP) — Wildlife officials say the prairie dog population at a Montana wildlife refuge has been decimated by a bacterial disease, causing the number of black-footed ferrets at the site to plummet.

U.S. Fish and Wildlife Service biologist Randy Matchett says 70 percent of prairie dogs at the UL Bend National Wildlife Refuge have died since a sylvatic plague event in the winter of 2017 and 2018.

The Great Falls Tribune reports that researchers counted only one black-footed ferret on the refuge this spring. The ferrets are considered an endangered species and depend on the prairie dog population for food.

The central Montana refuge is not the only place where ferrets have been reintroduced to the wild. Researchers say there are a total of 24 reintroduction sites in the western U.S.