Black-footed Ferrets

Black-footed Ferrets
		by *Dean E. Biggins* National Biological Service *Jerry Godbey* National Biological Service

The black-footed ferret (Mustela nigripes) was a charter member of endangered species lists for North America, recognized as rare long before the passage of the Endangered Species Act of 1973. This member of the weasel family is closely associated with prairie dogs (Cynomys spp.) of three species, a specialization that contributed to its downfall. Prairie dogs make up 90% of the ferret diet; in addition, ferrets dwell in prairie dog burrows during daylight, venturing out mostly during darkness. Trappers captured black-footed ferrets during their quests for other species of furbearers. Although the species received increased attention as it became increasingly rare, the number of documented ferrets fell steadily after 1940 (Fig. 1), and little was learned about the animals before large habitat declines made studies of them difficult. These declines were brought about mainly by prairie dog control campaigns begun before 1900 and reaching high intensity by the 1920's and 1930's.

Fig. 1. Black-footed ferrets collected before 1980.

Black-footed ferrets, almost extinct by 1985, are being reintroduced from captive breeding but still lack genetic diversity. Courtesy D. Biggins, NBS

Much of what is known about black-footed ferret biology was learned from research during 1964-74 on a remnant population in South Dakota (Linder et al. 1972; Hillman and Linder 1973), and from 1981 to the present on a population found at Meeteetse, Wyoming, and later transferred to captivity (Biggins et al. 1985; Forrest et al. 1988; Williams et al. 1988). Nine ferrets from the sparse South Dakota population (only 11 ferret litters were located during 1964-72) were taken into captivity from 1971 to 1973, and captive breeding was undertaken at the U.S. Fish and Wildlife Service's Patuxent Wildlife Research Center in Maryland (Carpenter and Hillman 1978). Although litters were born there, no young were successfully raised. The last of the Patuxent captive ferrets died in 1978, and no animals were located in South Dakota after 1979.

Black-footed ferrets were "rediscovered" in prairie dog complexes at Meeteetse in 1981, giving conservationists what seemed a last chance to learn about the species and possibly save it from extinction. That population remained healthy (70 ferret litters were counted from 1982 to 1986) through 1984 (Fig. 2), a period when much was learned about ferret life history and behavior. In 1985, sylvatic plague, a disease deadly to prairie dogs, was confirmed in the prairie dogs at Meeteetse, creating fear that the prairie dog habitat vital for ferrets would be lost. In addition, field biologists were reporting a substantial decrease in the number of ferrets detected. The fear of plague was quickly overshadowed by the discovery of canine distemper in the ferrets themselves. It is a disease lethal to ferrets.

Fig. 2. Black-footed ferret population from Meeteetse, Wyoming, 1983-92 (all captive from 1986 to present).

In 1985 six ferrets were captured to begin captive breeding, but two of them brought the distemper virus into captivity, and all six died (Williams et al. 1988). A plan was formulated to place more animals from Meeteetse into captivity to protect them from distemper and to start the breeding program. By December 1985, only 10 ferrets were known to exist, 6 in captivity and 4 at Meeteetse. The following year, the surviving free-ranging ferrets at Meeteetse produced only two litters, a number thought too small to sustain the wild population. Because both the Meeteetse and captive populations were too small to sustain themselves, all remaining ferrets were removed from the wild, resulting in a captive population of 18 individuals by early 1987.
Captive breeding of ferrets eventually became successful (Fig. 2). Although the captive population is growing, researchers fear the consequences of low genetic diversity (already documented by O'Brien et al. 1989) and of inbreeding depression (see glossary). A goal of the breeding program is to retain as much genetic diversity as possible, but the only practical way to increase diversity is to find more wild ferrets. In spite of intensive searches of the remaining good ferret habitat and investigations of sighting reports, no wild ferrets have been found.
The captive breeding program now is producing sufficient surplus ferrets for reintroduction into the wild; 187 ferrets were released into prairie dog colonies in Shirley Basin, Wyoming, during 1991-93. Challenges facing the black-footed ferret reintroduction include low survivorship of released ferrets due to high dispersal and losses to other predators; unknown influence of low genetic diversity; canine distemper hazard; indirect effect of plague on prairie dogs and possible direct effect on ferrets; and low availability of suitable habitat for reintroduction. The scarcity of habitat reflects a much larger problem with the prairie dog ecosystem and needs increased attention.
At the turn of this century, prairie dogs reportedly occupied more than 40 million ha (100 million acres) of grasslands, but by 1960 that area had been reduced to about 607,500 ha (1.5 million acres; Marsh 1984). Much reduction was attributed to prairie dog control programs, which continue. For example, in South Dakota in the late 1980's, $6.2 million was spent to apply toxicants to prairie dog colonies on Pine Ridge Indian Reservation (Sharps 1988). At least two states (Nebraska and South Dakota) have laws prohibiting landowners from allowing prairie dogs to flourish on their properties; if the land manager does not "control" the "infestation," the state can do so and bill expenses to the owner (Clarke 1988).

Sylvatic plague also has been devastating to prairie dogs and was the likely cause of the dramatic decline in prairie dogs at Meeteetse. Although the Meeteetse complex recently supported the densest and most vigorous population of black-footed ferrets ever known, it cannot be considered as ferret habitat now because of plagued-induced losses of prairie dogs. Plague is present in most of the monitored white-tailed prairie dog (Cynomys leucurus) complexes, including the Shirley Basin ferret reintroduction site (Table). The plague's persistence could be responsible for the generally lower densities of white-tailed prairie dogs (averaging fewer than seven prairie dogs per hectare or fewer than three per acre).

Table. Prairie dog complexes evaluated for black-footed ferret reintroductions. (Some data from Black-footed Ferret Interstate Coordinating Committee.)

State	Site	Prairie dog species*	Complex size(ha)	Hectares of prairie dogs	Prairie dogs estimate	Prairie dogs/ha colony
United States
Arizona	Aubrey Valley	Gunnison's	44,167	7,390	34,067	4.61
Colorado	Little Snake	White-tailed	252,075	31,624	36,875	1.17
	Wolf Creek	White-tailed	65,607	3,174	20,009	6.30
	Sterling	Black-tailed	57,824	2,366	16,786	7.10
Montana	Custer Creek	Black-tailed	38,879	425	16,750	39.39
	Malta Bureau of Land Management	Black-tailed	583,430	7,600	167,299	22.01
	Charles M. Russell Refuge	Black-tailed	28,508	896	22,371	25.00
North Dakota	Roosevelt National Park	Black-tailed	14,126	594	39,270	66.11
	Marmarth National Park	Black-tailed	7,257	548	21,208	38.70
	Fort Yates	Black-tailed	6,739	579	20,823	35.96
South Dakota	Badlands National Park	Black-tailed	17,016	1,669	74,081	44.39
Wyoming	Meeteetse	White-tailed	53,846	5,111	1,299	0.25
	Shirley Basin	White-tailed	48,987	20,612	75,155	3.65
	Medicine Bow	White-tailed	74,958	27,235	24,492	0.90
	Recluse	Black-tailed	98,802	7,181	59,895	8.34
	Bolton Ranch	White-tailed	28,068	4,420	7,858	1.78
	Kinney Rim	White-tailed	43,509	7,220	608	0.08
Mexico
Chihuahua	Nuevos Casas Grandes	Black-tailed	87,866	54,541	994,986	18.24

^{*Gunnison's prairie dog (Cynomys gunnisoni), white-tailed prairie dog (C. leucurus), and black-tailed prairie dog (C. ludovicianus).}

Several prairie dog complexes have been eval-uated as sites for reintroduction of black-footed ferrets (Table). The evaluation involves grouping clusters of colonies separated by fewer than 7 km (4.3 mi) into complexes, based on movement capabilities of ferrets (Biggins et al. 1993); these areas include some of the best prairie dog complexes remaining in the states. Nevertheless, other extensive prairie dog complexes were not considered for ferret reintroduction.

Most of the original range of the black-footed ferret was associated with black-tailed prairie dog (Cynomys ludovicianus) complexes, which now exhibit the highest population densities of all prairie dogs (Table). Black-footed ferret reintroductions recently began at black-tailed prairie dog complexes near Malta, Montana, and Badlands National Park, South Dakota (Table). At present, the best example of a large complex of black-tailed prairie dogs is near Nuevos Casas Grandes, Chihuahua, Mexico (Table). It supports an impressive associated fauna and is a potential reintroduction site for black-footed ferrets.

Three species of prairie dogs make up 90% of the black-footed ferret's diet; prairie dog burrows are also used by the ferrets during the day. Courtesy D. Biggins, NBS

Ramifications of a healthy prairie dog ecosystem extend well beyond black-footed ferrets. The prairie dog is a keystone species of the North American prairies. It is an important primary consumer, converting plants to animal biomass at a higher rate than other vertebrate herbivores of the short-grass prairies, and its burrowing provides homes for many other species of animals and increases nutrients in surface soil. This animal also provides food for many predators. We estimated it takes 700-800 prairie dogs to annually support a reproducing pair of black-footed ferrets and a similar biomass of associated predators (Biggins et al. 1993), suggesting that large complexes of prairie dog colonies are necessary to support self-sustaining populations of these second-order consumers.
The 98% loss of the productive prairie dog ecosystem has not yet motivated legal protection or plans for management. There is no federal legislation directly promoting the welfare of the prairie dog ecosystem (even on public lands), and the only existing state legislation promotes poisoning.

To develop a plan for remedial action, several immediate research needs are apparent in the prairie dog ecosystem: determine the relative diversity and abundance of invertebrates and small- to medium-sized vertebrates on prairie dog complexes, as well as the degree of dependence on prairie dogs of selected associated species; examine the effect of complex size, as well as constituent colony sizes, numbers, and juxtaposition on diversity and abundance of associated species; investigate the recent history of plague on selected complexes to determine the relation between complex size (and morphology) and resistance to decimation by plague; and develop methods for reestablishing prairie dog colonies and reconstructing complexes in suitable areas where prairie dogs have been extirpated.

Prairie dog control campaigns, like this one in Arizona, circa 1913, contributed to the decline of the black-footed ferret. Courtesy B. Miller

The black-footed ferret cannot be reestablished on the grasslands of North America in viable self-sustaining populations without large complexes of prairie dog colonies. The importance of this system to other species is not completely understood, but large declines in some of its species should serve as a warning. The case of the black-footed ferret provides ample evidence that timely preventive action would be preferable to the inefficient "salvage" operations. Furthermore, there is considerable risk of irreversible damage (e.g., genetic impoverishment) with such rescue efforts.
		For further information: Dean E. Biggins National Biological Service Midcontinent Ecological Science Center 4512 McMurry Ave. Fort Collins, CO 80525

References
Biggins, D.E., B.J. Miller, L. Hanebury, R. Oakleaf, A. Farmer, R. Crete, and A. Dood. 1993. A technique for evaluating black-footed ferret habitat. Pages 73-88 in J.L. Oldemeyer, D.E. Biggins, B.J. Miller, and R. Crete, eds. Management of prairie dog complexes for the reintroduction of the black-footed ferret. U.S. Fish and Wildlife Service Biological Rep. 13. Biggins, D.E., M.H. Schroeder, S.C. Forrest, and L. Richardson. 1985. Movements and habitat relationships of radio-tagged black-footed ferrets. Pages 11.1-11.17 in S.H. Anderson and D.B. Inkley, eds. Proceedings of the Black-footed Ferret Workshop. Wyoming Game and Fish Department, Cheyenne. Carpenter, J.W., and C.N. Hillman. 1978. Husbandry, reproduction, and veterinary care of captive ferrets. Proceedings of the American Association of Zoo Veterinarians Workshop, Knoxville, TN. 1979:36-47. Clarke, D.C. 1988. Prairie dog control--a regulatory viewpoint. Pages 119-120 in D.W. Uresk and G. Schenbeck, eds. Eighth Great Plains Wildlife Damage Control Workshop Proceedings. U.S. Forest Service Gen. Tech. Rep. RM-154. Forrest, S.C., D.E. Biggins, L. Richardson, T.W. Clark, T.M. Campbell III, K.A. Fagerstone, and E.T. Thorne. 1988. Population attributes for the black-footed ferret (Mustela nigripes) at Meeteetse, Wyoming, 1981-1985. Journal of Mammalogy 69(2):261-273.	Hillman, C.N., and R.L. Linder. 1973. The black-footed ferret. Pages 10-20 in R.L. Linder and C.N. Hillman, eds. Proceedings of the Black-footed Ferret and Prairie Dog Workshop. South Dakota State University Publications, Brookings. Linder, R.L., R.B. Dahlgren, and C.N. Hillman. 1972. Black-footed ferret-prairie dog interrelationships. Pages 22-37 in Proceedings of the Symposium on Rare and Endangered Wildlife of the Southwestern U.S. New Mexico Department of Game and Fish, Santa Fe. Marsh, R.E. 1984. Ground squirrels, prairie dogs and marmots as pests on rangeland. Pages 195-208 in Proceedings of the Conference for Organization and Practice of Vertebrate Pest Control. ICI Plant Protection Division, Fernherst, England. O'Brien, S.J., J.S. Martenson, M.A. Eichelberger, E.T. Thorne, and F. Wright. 1989. Biochemical genetic variation and molecular systematics of the black-footed ferret, Mustela nigripes. Pages 21-33 in Conservation biology and the black-footed ferret. Yale University Press, New Haven, CT. Sharps, J. 1988. Politics, prairie dogs, and the sportsman. Pages 117-118 in D.W. Uresk and G. Schenbeck, eds. Eighth Great Plains Wildlife Damage Control Workshop Proceedings. U.S. Forest Service Gen. Tech. Rep. RM-154. Williams, E.S., E.T. Thorne, M.J.G. Appel, and D.W. Belitsky. 1988. Canine distemper in black-footed ferrets (Mustela nigripes) from Wyoming. Journal of Wildlife Diseases 24:385-398.

References

Biggins, D.E., B.J. Miller, L. Hanebury, R. Oakleaf, A. Farmer, R. Crete, and A. Dood. 1993. A technique for evaluating black-footed ferret habitat. Pages 73-88 in J.L. Oldemeyer, D.E. Biggins, B.J. Miller, and R. Crete, eds. Management of prairie dog complexes for the reintroduction of the black-footed ferret. U.S. Fish and Wildlife Service Biological Rep. 13.

Biggins, D.E., M.H. Schroeder, S.C. Forrest, and L. Richardson. 1985. Movements and habitat relationships of radio-tagged black-footed ferrets. Pages 11.1-11.17 in S.H. Anderson and D.B. Inkley, eds. Proceedings of the Black-footed Ferret Workshop. Wyoming Game and Fish Department, Cheyenne.

Carpenter, J.W., and C.N. Hillman. 1978. Husbandry, reproduction, and veterinary care of captive ferrets. Proceedings of the American Association of Zoo Veterinarians Workshop, Knoxville, TN. 1979:36-47.

Clarke, D.C. 1988. Prairie dog control--a regulatory viewpoint. Pages 119-120 in D.W. Uresk and G. Schenbeck, eds. Eighth Great Plains Wildlife Damage Control Workshop Proceedings. U.S. Forest Service Gen. Tech. Rep. RM-154.

Forrest, S.C., D.E. Biggins, L. Richardson, T.W. Clark, T.M. Campbell III, K.A. Fagerstone, and E.T. Thorne. 1988. Population attributes for the black-footed ferret (Mustela nigripes) at Meeteetse, Wyoming, 1981-1985. Journal of Mammalogy 69(2):261-273.

Hillman, C.N., and R.L. Linder. 1973. The black-footed ferret. Pages 10-20 in R.L. Linder and C.N. Hillman, eds. Proceedings of the Black-footed Ferret and Prairie Dog Workshop. South Dakota State University Publications, Brookings.

Linder, R.L., R.B. Dahlgren, and C.N. Hillman. 1972. Black-footed ferret-prairie dog interrelationships. Pages 22-37 in Proceedings of the Symposium on Rare and Endangered Wildlife of the Southwestern U.S. New Mexico Department of Game and Fish, Santa Fe.

Marsh, R.E. 1984. Ground squirrels, prairie dogs and marmots as pests on rangeland. Pages 195-208 in Proceedings of the Conference for Organization and Practice of Vertebrate Pest Control. ICI Plant Protection Division, Fernherst, England.

O'Brien, S.J., J.S. Martenson, M.A. Eichelberger, E.T. Thorne, and F. Wright. 1989. Biochemical genetic variation and molecular systematics of the black-footed ferret, Mustela nigripes. Pages 21-33 in Conservation biology and the black-footed ferret. Yale University Press, New Haven, CT.

Sharps, J. 1988. Politics, prairie dogs, and the sportsman. Pages 117-118 in D.W. Uresk and G. Schenbeck, eds. Eighth Great Plains Wildlife Damage Control Workshop Proceedings. U.S. Forest Service Gen. Tech. Rep. RM-154.

Williams, E.S., E.T. Thorne, M.J.G. Appel, and D.W. Belitsky. 1988. Canine distemper in black-footed ferrets (Mustela nigripes) from Wyoming. Journal of Wildlife Diseases 24:385-398.