Great Basin Naturalist Memoirs Volume 8 The Black-footed Ferret Article 13 5-1-1986 Determing minimum population size for recovery of the black-footed ferret Craig R. Groves Department of Biological Sciences, Idaho State University, Pocatello, Idaho 83209 Tim W. Clark Department of Biological Sciences, Idaho State University, Pocatello, Idaho 83209 Follow this and additional works at: https://scholarsarchive.byu.edu/gbnm Recommended Citation Groves, Craig R. and Clark, Tim W. (1986) "Determing minimum population size for recovery of the black-footed ferret," Great Basin Naturalist Memoirs: Vol. 8 , Article 13. Available at: https://scholarsarchive.byu.edu/gbnm/vol8/iss1/13 This Article is brought to you for free and open access by the Western North American Naturalist Publications at BYU ScholarsArchive. It has been accepted for inclusion in Great Basin Naturalist Memoirs by an authorized editor of BYU ScholarsArchive. For more information, please contact [email protected], [email protected]. DETERMINING MINIMUM POPULATION SIZE FOR RECOVERY OF THE BLACK-FOOTED FERRET Craifj; R. Groves' ~ and Tim W. Clark' Abstract. —A minimum viable population (MVP) size is estimated for the critically endangered black-footed ferret by examining five basic methods: experiments, biogeographic jiatterns, theoretical models, simulation models, and genetic considerations. Each method is evaluated for its appiical)ilit\ to the ferret and endangered species in general with two criteria in mind: (1) potential research impacts to target species and (2) the value of scientific accuracy and precision in relation to short-term conservation needs. For the black-footed ferret, the genetic method proved to be the most useful, resulting in an MVP estimate of about 200 ferrets for maintenance of short-term fitness. For most endangered species, a combination of the simulation and genetic methods will probably yield the best estimate of MVP size. MVP estimates have direct implications for future research, management, and recovery efforts of endangered species. Following the 1981 discovery of the critically (Lacava and Hughes 1984). In this paper, we endangered black-footed ferret (Miistela ni- estimate minimum ferret population numbers gripes) in northwestern Wyoming, the only pop- and, as a corollary, minimum area require- ulation of the species known, two primary goals ments. Our purpose is to establish guidelines for of a ferret study were established: (1) conserva- future ferret research and management and to tion of the population and (2) recovery of the provide impetus, direction, and encouragement species (Clark 1984a). Estimation of population to other endangered species programs to use the parameters such as distribution, birth rate, concept of minimum population size. death rate, and immigration/emigration was a major objective. In addition to providing valu- Concept OF Minimum Viable able life history data for the ferret, estimating Population Size these parameters would enable us to address a key question: What is the minimum ferret popu- The notion of a required minimum population lation size necessary for the species to persist size for species conservation was first embodied (i.e., avoid extinction)? of the apparent Because by Shaffer (1981) in his concept of minimum small size of the population in 1981, Wyoming viable population (MVP) size. He defined the the answer was paramount for successful conser- MVP for a species as the smallest isolated popu- vation and, ultimately, species recovery. lation having a 99% chance of remaining extant The minimum population size concept for spe- for 1000 years despite foreseeable effects of four cies conservation was recently introduced by types of stochastic events: (1) demographic Shaffer (1981), with potentially significant impli- stochasticity, or the chance events in the sur- cations for, endangered species programs. Al- vival and reproductive fitness of a finite number though many endangered species studies have of individuals; (2) environmental stochasticity, been conducted, few have tried to estimate min- or perturbations due to habitat parameters, imum population sizes (Shaffer 1978, 1981, La- competitors, predators, and disease; (3) natural cava and Hughes 1984, Salwasser et al. 1984). catastrophes, such as randomly occurring floods, Although US DA Forest Service regulations re- fires, droughts, etc.; and (4) genetic perturba- quire that minimum viable populations of en- tions residting from changes in gene frequency. dangered species be maintained on Forest Ser- Shaffer (1981) stressed the tentative nature ofhis vice lands, these population numbers are not definition and emphasized the importance of estimated but are instead routinely established defining MVP with explicit but flc^xible criteria as those levels specified in recovery plans such as time frame and survival probability. For Department of Biological Sciences, Idaho State University, Pocatello, Idaho 83209. Present address: The Nature Conservancy, 4696 Overland Road, Suite 51H. Boise, Idaho 8.370.5. 150 1986 Groves, Clark: Minimum Population 151 example, the survival probability could be set at logically simple, this approach is not plausible 95% or auy other level, aud the time frame could for many species, particularly endangered ones. be shortened or lengthened as appropriate for First, for many endangered species, there exists planning needs. no possibility of studying several isolated popu- Shaffer (1981) proposed five basic methods for lations. In the case of the ferret, there is only one determining MVP size: experiments, biogeo- known population, and consequently there is no graphic patterns, theoretical models, simulation way to measure variability in persistence of dif- models, and genetic considerations. In the next ferent-sized, isolated populations. Second, time section, we elaborate on the application and util- is not an unlimited resource in most endangered ity of each method for estimating MVP size for species studies. The years necessary to monitor ferrets in particular and endangered species in persistence of populations for MVP estimates general. The usefulness of each method is prag- are simply not available for most endangered matically evaluated relative to: (1) acceptable species. Lastly, many wildlife species cannot be levels of potential research impacts on target censused sufficiently without some type of mark/ species and (2) the real value of scientific accu- recapture program. These techniques can have racy and precision in relation to short-term con- negative research impacts (e.g., trap mortality) servation needs. In endangered species recov- that some endangered species projects may not ery work, these two concerns must be addressed. be willing to risk, particularly in the early stages. Because endangered species programs in gen- The MVP concept is a rapidly evolving one. In eral possess high levels uncertainty, late 1984, the Forest Service and other agencies of there may be hesitancy to increase uncertainty sponsored a workshop to provide a state-of-the- to even greater levels (Clark 1984b). Such has been the art review of the MVP issue (M. L. Shaffer, case in the personal communication). A major purpose of Wyoming ferret project, where one research team chose initially not to the workshop was to consider revisions of Forest employ mark/recapture and radio telemetry procedures Service procedures for estimating MVP size. until other methods proved the population large The primary focus of the workshop was a discus- enough to withstand some stress (Clark sion of the relative importance of demographic, 1981). The experimental method could prove useful environmental, and genetic variability in deter- when extensive population data are available mining minimum population numbers as well as prior to a species becoming endangered. For the development of new simulation and analyti- example, mountain goat (Oreamnos ameri- cal models for estimating MVP size. Proceedings canus) and bighorn sheep {Ovis canadensis) of this workshop, which will be published in populations have been reintroduced to several 1986, may provide additional methods not dis- sites in the western United States in recent years cussed in this paper and/or may suggest revi- (Rideout 1978, Wishart 1978). If wildlife re- sions of methods outlined and discussed below searchers monitor the dynamics of these intro- for estimating MVP sizes of endangered species. duced populations, the data should eventually allow them to estimate MVP sizes for these spe- Determining MVP Size cies. Such information would be useful not only for future translocations of these species' popula- Experiments tions but also for management if, for example, an isolated population showed a decline in num- This method estabhshes isolated populations bers. of species and monitors their population dynam- ics through time. If censusing can be accom- Biogeographic Patterns plished without stressful manipulation of indi- viduals (as can occur in a capture/recapture By studying the distribution patterns of spe- program), this approach requires only low levels cies occupying patchy or insular habitats, an esti- of research activities. The only necessary data mate of MVP size and minimum area require- are population numbers and the area inhabited ments can be obtained. However, populations over a specific time period (i.e., time frame in examined should be in equilibrium and their MVP definition). approximate period of isolation known (Shaffer Determining MVP via experimental