Black-Footed Ferret Prey Requirements: An Energy Balance Estimate

Abstract

A model based on calculations of metabolizable energy requirements estimated the annual prey requirements for 1 reproductive female black-footed ferret (Mustela nigripes) and her young. Both whitetailed and black-tailed prairie dogs (Cynomys leucurus and C. ludovicianus) were considered as prey. Gestation, lactation, and ferret pup growth were modeled. A general regression of mustelid prey biomass requirements helped verify metabolizable energy calculations. Populations of between 474 and 1,421 blacktailed prairie dogs and between 412 and 1,236 white-tailed prairie dogs are predicted as necessary to sustain the annual predation by the female and young ferrets. Minimum ferret densities expected to occur and sizes of ferret preserve areas were calculated. These results can be used by administrators and managers charged with black-footed ferret surveys and conservationists planning preserves for this endangered species. J. WILDL. MANAGE. 47(1):67-73 The endangered black-footed ferret is one of North America's rarest mammals (Cahalane 1954, Snow 1972, Hillman and Clark 1980). Recent ferret surveys have generated increasing evidence that low densities of ferrets still exist in parts of their former range. For example, in Wyoming over 30 seemingly credible ferret reports have been recorded since 1970 (Clark 1973, 1978, 1980). Other evidence includes ferret skeletal remains in whitetailed prairie dog towns (Clark and Campbell 1981). Black-footed ferret management is severely hampered by the lack of knowledge of the ferret's behavioral ecology (Henderson et al. 1969, Linder et al. 1972). Ferrets are largely obligatory predators on prairie dogs (Linder et al. 1972, Hillman et al. 1979, Campbell and Clark 1981). This paper examines the question of how many prairie dogs are needed annually to support a ferret raising a litter. We assume each adult ferret occupies 1 prairie dog colony, each young ferret disperses to a new colony when mature, and prairie dogs are the only prey. The energetics model we used follows the additive model discussed by Grodzinski and Wunder (1975). In addition to the limited data on ferrets, we used estimates derived from literature on mustelids and other carnivores. The basic procedure estimates the total biomass of prey required by a female ferret during gestation, lactation, growth of young, and for the adult alone after litter dispersal. The model (available from authors) was writt n in BASIC and for programmable calculators to simulate ferret energetics on a daily basis and to estimate total annual consumption of prairie dogs. Energy requirements of the female, or female and litter, vary through time involving a number of parameters. We estimated values for many of these parameters and our estimated annual consumption includes substitution of a range of values for lesser known energetic processes. High, medium, and low values were set in our model for several parameters. Annual simulations were run to include over 2,000 possible combinations of values. We acknowledge contributors including M. Demment, C. Hillman, G. Innis, R. Powell, J. Weaver, and W. Welch. A review by B. Wunder was most helpful. Financial and computing assistance from J. Wildl. Manage. 47(1):1983 67 This content downloaded from 207.46.13.149 on Mon, 03 Oct 2016 05:44:48 UTC All use subject to http://about.jstor.org/terms 68 FERRET PREY REQUIREMENTS * Stromberg et al. PRAIRIE DOG KILL (K) WASTE (W) CONSUMPTION (C)