Black-footed ferrets were reduced to a remnant population of 10 in 1985 due to diseases (plague, canine distemper), but successful captive breeding and releases have improved the prospects for ferret recovery. Comparisons between black-footed ferrets and Siberian polecats, close relatives that can interbreed and produce fertile offspring, allow the following evolutionary speculation. Predation on ferrets and polecats tends to narrow their niches and promote specialization due to requirements for escape habitats. In Asia, that influence is countered by the larger and more diverse area of steppe and alpine meadow habitats for polecats, and by plague which causes large variation in prey abundance. In North America, the selective pressure favoring specialization in ferrets on prairie dog prey and burrows had no strong counter-force before plague invaded. Plague is an immense challenge to black-footed ferret recovery, and several management tools including vaccines and vector control may be necessary to conserve the species.