Habitat suitability index model for the black-footed ferret- a me

Great Basin Naturalist Memoirs 5-1-1986 Habitat suitability index model for the black-footed ferret: a method to locate transplant sites B.. 1986 "Habitat suitability index model for

Great Basin Naturalist Memoirs

5-1-1986

Habitat suitability index model for the black-footed ferret: a

method to locate transplant sites

B R Houston

Department of Biological Sciences, Idaho State University, Pocatello, Idaho 83209

Tim W Clark

Department of Biological Sciences, Idaho State University, Pocatello, Idaho 83209

S C Minta

Department of Wildlife and Fisheries Biology, University of California, Davis, California 95616

Follow this and additional works at: https://scholarsarchive.byu.edu/gbnm

Recommended Citation

Houston, B R.; Clark, Tim W.; and Minta, S C (1986) "Habitat suitability index model for the black-footed ferret: a method to locate transplant sites," Great Basin Naturalist Memoirs: Vol 8 , Article 8

Available at: https://scholarsarchive.byu.edu/gbnm/vol8/iss1/8

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 scholarsarchive@byu.edu,

ellen_amatangelo@byu.edu

HABITAT SUITABILITY' INDEX MODEL FOR THE BLACK-FOOTED FERRET

A METHOD TO LOCATE TRANSPLANT SITES

B R.Houston',TimW.Clark',andS.CMinta"

Abstract.—AHabitatSuitabilityIndexModel(HSI), following the U.S FishandWildlife ServiceHSI Model Series, isdescribedfor the black-footed ferret.Theliteratureonwhichthemodelisbasedisreviewed, andmodel

assumptionsandstructure are discussed.Arealisticmodelis specifiedwithvariablesandtheir functions thatembody the critical spatialandresource heterogeneitycharacteristic of thebroad geographic environmentferretsoccupy.It

assumesthat ferretscanmeetyear-roundhabitatrecjuirements withinprairiedogcolonies providing:(1)prairiedog colonies are largeenough,(2)burrowsarenumerousenough, and(3)adequatenumbersof prairiedogsandalternate preyare available.Fivehabitat variables are identified:VIis thefrequencydistribution ofcolonysizes,V2is the total areaofcolonies,V3isburrowopeningdensity,V4is intercolony distance,andV5is prairiedogdensity Variables are compensatory.Asmoredatabecomeavailableandour understandingof ferretsexpands,the basicmodeldesign can readilyincorporateimprovementswithoutradical restructuring.

Habitatmodelsareanattempttodescribeand

quantify an animal's essential habitat

require-mentsor"life requisites" and are therefore a

useful tool in habitat evaluation The Habitat

SuitabilityIndex (HSI)ModelSeries,developed

bytheU.S.FishandWildlifeService(USFWS),

provideshabitatdescriptionsforseveral species

These modelsare useful forassessment of

im-pactsonwildlifeandhabitatmanagement

(US-FWS1980a, b)andmay proveespecially

valu-able inendangeredspeciesmanagement,where

determination ofhabitatqualityandsuitabilityis

oftencriticalformanagementandcontinuation

ofthespecies.HSI"modelsshouldbe viewedas

hypotheses ofspecies-habitat relationships

ra-therthan statements ofprovencauseandeffect

relationships"(Schambergeretal.1982:1)

ThispaperappliestheHSI Modelformatto

theMeeteetse,Wyomingenvironmentofthe

black-footedferret(Mustela nigripes;BFF)as

generallydescribedbyClarketal.

(Descrip-tionandhistoryA^^Q) and morespecifically

by Forrestetal. (1985)(Fig 1). Applications

anduses of themodelare:(1)tocompareother

areastoBFFhabitatatMeeteetse,(2)touse

those comparisons to select areas to be

searchedforBFFs, and(3)toselectsuitable

areasfortransplantsites.Ouruseof theHSI

formatcloselyfollowstheUSFWS(1981)and

parallels applications by Allen (1982a, b,

1983, 1984)forotherspecies

Ouruse of theHSI modelforBFFs

incorpo-rates several recent improvements on the

rolesof ecologicalmodels:(1)Westressmodel reality of a single species more than focus

upon model precision or generality (see Levins1966,Rosen1978,Kaiser 1979, Pielou

1981) (2) Fewhighly measurable variables dictatetheHSI,and,althoughsomeare colin-ear, togetherthey contain high explanatory

power,atthesametimeallowing comprehen-sibleresultsandsimplified sensitivity analy-sis. This reflects thegrowing consensus that

thereisno apparentrelationbetween model complexityandpredictiveutility inanyfield

of forecasting (e.g., Ascher 1978, K E F Wattpersonalcommunication).(3)Our model

uses nonlinear representations ofvariables,

ratherthanlinear, becausethosemore

accu-ratelyexpress thedynamicnature of biologi-calresponsesandrealisticspecies-habitat re-lations (Whittaker1975, Green 1979,

West-man1980,Johnson1981,Meentsetal. 1983) Nonlinearitypermitsustomimic more realis-tic biologicalprocesses that involve thresh-oldsandlimitsandthesmoothedtransitions between them(HoUing1985,J.R.Krebs per-sonal communication) (4) The model

vari-ablesandtheirfunctionsembodythecritical 'Department of Biological Sciences, Idaho State University, Pocatello, Idaho 83209.

^Department of Wildlife and Fisheries Biology, Universityof California, Davis, California 95616.

,/4.,

5aa#*^^ '

'*'^"

Fig 1 Ph()t()tirai)lis ()ll(laik-fboted ferret habitat (prairiedogcoloniesandprairie dogs) aiidtt rift predation.Photos

byTimClark.

A. White-tailedprairiedogcolonyoccupiedbyferrets.

CJ. Twowliite-tailed prairie clogs.

*sii?it*" ^R«.

ry

D

importanceofspatialandresource

heterogene-ity.Thestructuralsimplicityof the BFF-prairie

dog{Cynomijsspp.)communitypromotesa

de-signwhere allvariables directly assess spatial

patchiness and resource variability,

consider-ations that have pivotalimpact on population

dynamics and population viability (reviews in

Steele 1974, Wiens 1974, Southwood 1977,

Shugart1981)

Theoutcomeoftheabovefourfeaturesisonly

a slightincreaseinmodelcomplexity tradedfora

dramatic increasein ecologicalreality. Perhaps

ofequal benefitistheease ofmodelvalidation

Asmoredatabecomeavailableandour

under-standing ofBFFsexpands, thebasicmodel

de-signcanreadilyincorporateimprovements

with-out radical restructuring Data sets already

completed and cited below could likely be

reevaluatedwith futuremodelversions

This HSI application for the BFF draws on

Clarketal.{Descriptionandhistory,1986)and

Forrest et al. (1985), who describe the

Mee-teetse, Wyoming, BFFstudy area (1981-1985)

anditsusebyBFFsaswellasallthe datafrom

theMelletteCounty, SouthDakota,BFFstudy

(1964-1974).Becauseofthelocalizednatureand

hmited sizeofthesetwo studyareas, this HSI

modelwilllikelyrequireupdatingifBFFsare

foundinother areas indifferentecological

set-tings In the meantime, this HSI model can

serveasa useful tool inBFFrecovery planning

toevaluateproposedtransplant/relocationsites.

Background

Requestsforevaluation ofBFFhabitathave

beenfrequentlymentionedintheliterature

The Black-footed Ferret Recovery Team

(1978) requested research to define

compo-nentsof aprairiedogcolony necessaryto

sup-portBFFs The BFFRecoveryPlanalsonotes

the need to establish ideal habitat sites for

successfulintroduction of transplantedBFFs

(see Linderetal. 1972) . The South Dakota

BFFandPrairieDog Workshopin1973

sug-gestedseveral BFF managementneeds,

in-cluding a definition of habitat (Hillman and

Linder 1973, Stuart 1973, Erickson 1973)

Othershavediscussed theneedforBFF

pre-serves and habitat descriptions (Clark 1976,

1984, 1986) FlathandClark(1986)described

forthe period 1908-1914 ThisearlyMontana situationprobably representedahabitat setting

inwhichBFFsevolvedamongthecomplex in-terrelationships of species and environmental interactionsoftheprairiedogecosystem

Hillmanetal.(1979)describedprairiedog dis-tribution inthe areaoccupiedbyBFFsinSouth

Dakota Their descriptionwas widelyusedby managementagenciesasaguidetothenumber andspacing ofprairiedogcoloniestobeleftafter prairiedogeradicationprograms

Clark etal. {Descriptionand history, 1986)

providedadescriptiveandhistoricaloverviewof theMeeteetseBFFenvironment.Forrestetal. (1985)notedthatBFFsarerestricted toa prairie dog complex—agroup ofprairie dog colonies distributedso thatindividualBFFscan migrate

among them commonlyandfrequently.The37

colonies ofthe Meeteetsecomplex (total size

2995 ha)were described andtheiroccupation

historybyBFFsnoted Theaverage density of adult BFFswas 1 BFF/56.6ha Burrow open-ings,basedonliteraturereviews, are correlated with the number of prairie dogs present (r=0.71). High burrow densities are desirable

forBFFsinthatthey provideaddedprotection

from predatorsandshelterfromtheelements Colonies greater than 100ha supported more

thantworesident adultBFFs, whereascolonies

from12.5hato102.0hasupportedonlyoneBFF throughouttheyear BFFstraveledamongthe

colonies,buttoanunknownextent BFFs may

use burrows at low densities and colonies of small size in travels between larger colonies BFFsmoving betweencolonieshaveagreater

chanceoffindinganother colonyifthe colonies

are largeandclosetogether

Several bibliographies ofBFFs(Harvey1970,

Snow1972,HillmanandClark 1980,Caseyetal. 1986)andofprairiedogs (Clark1971, in prepara-tion, Hassien 1973) exist. Thesealso serve as backgroundforthis HSImodel General infor-mationonBFFsissunnnarizedinthe

bibliogra-phies listed above, in primary sources from SouthDakotastudies(e.g.,Hillman1968,

Hen-dersonetal. 1969,Fortenbery1972),and,more recently, from Meeteetse, Wyoming (e.g., Clark et al Description and history, 1986; Clarketal Descriptive ethology, 1986; Camp-bell etal. 1985, Richardsonetal. 1985; Forrest

HabitatUse Information

Overview

A member of the family Mustelidae, the

BFFistheonlyferretnativetoNorthAmerica

(Hall1981)andisperhapstherarestand most

endangered mammal species on this

conti-nent (Cahalane 1954, Hillman and Clark

1980) BFFsaresolitaryexceptduring

breed-ingand maternal careofyoungand are

pri-marily nocturnal Theypreyonprairiedogs,

whose burrows they also use for coverand

litterrearing

Food The BFFreliesonprairiedogsasitsprimary

food source, although other prey, both live

and dead, are taken in considerably lesser

amounts (Hillman 1968, Henderson et al.

1969, Sheetsand Under 1969, Sheetsetal.

1972, Clarket al. 1985) Sheetsetal. (1972)

found91%of82BFFscatsfrom SouthDakota

containedprairiedogremains,andCampbell

etal.(unpublisheddata)found87%of86BFF

scats from Meeteetse contained prairie dog

remains Prairie dogs,onthisbasis,compose

themajorBFFfood

Strombergetal. (1983)generateda

preda-tor-prey model ofmetabolizable energy

re-quirements that estimated: (1) annual prey

requirements for one reproductive female

BFFand herlitterof fourand(2)prairiedog

populationsizesneededperBFF. Powellet

al (in press) estimated BFFwinter energy

expenditure(about104kcal/day)and prey

re-quirements (about20prairiedogs from

De-cemberthroughMarch)atMeeteetse.A

lac-tatingfemale withfouryoungarepredictedto

need sixtimes thewinterestimate, orabout

oneprairiedogperdayinsummer.

Water

BFFsapparentlysatisfywater requirements

through prey consumption and have never

been observedinthewild drinkingfree

wa-ter. Henderson et al. (1969) reported that

captive BFFs drank water irregularly L

Richardson (unpublished data) watched a

BFFeatingsnowatMeeteetse

Cover CoverforBFFsisprovidedbyprairiedog

anceand thermal cover throughoutthe year (Clarketal. 1985, Richardsonetal. inpress)

Anyprairiedog burrowisassumedtobe suffi-cient to satisfy BFF cover requirements

Higher biurow densities provide greater cover

Reproduction Reproductive habitat re(|uirements for

BFFsareassumedtobeidentical tofoodand cover requirements described above because all BFFactivitiesare associated withprairie dog burrowsystems(Clarketal. Descriptive ethology, 1986; Richardson et al. in press;

Forrestetal. 1985).Large, mounded,

multi-entrancedburrowsmaybe importantforlitter rearingbecauseoftheirpresumedextensive tunnelnetwork

Interspersion

A picture ofBFF home range patterns is emergingfromresearchefforts atMeeteetse

Asingle adult male's range may encompass

homeranges of several females, whichshow

much smaller ranges (Richardson et al.

un-published data) Females remain with their littersuntil latesummer, when young become independent(Hendersonetal. 1969, Clarket

al. Descriptive ethology, 1986) BFFsappear

tohavea typicalmustelidspacing pattern de-scribedbyPowell(1979),Forrestetal.(1985), and Richardsonetal (inpress) More infor-mationis neededon BFF homerangesand movements,dispersalofyoungoradults,and inter-andintrasexualinteractions

InterspersioncharacteristicsofBFFs repre-sentatwo-dimensional management consid-eration—individual and populational

Indi-vidual interspersion patterns are better

known than populational interspersion

pat-ternsrequiredforminimumpopulationsizes

Aresidentfemale snow-tracked from

Decem-ber throughMarchused16.0haand was

over-lappedbya residentmalethatused136.6ha

(Forrestetal. 1985).Studies of radio-collared

BFFs showayoungfemale used 12.6hain OctoberandNovember(Bigginsetal.1985)

Population interspersionisdependentonthe

size, configuration, andintercolony distance

ofprairie dogcolonies making up the com-plex Datashowthat,ifcolonies are too small

andintercolony distances are toolarge,then BFF

104 GreatBasin NaturalistMemoirs No.8

Thesearchforfood (energetics)becomes

pro-hibitive,avoidanceof predatorsbecomes

dif-ficult or impossible, and adequate thermal

coverisrare or nonexistent,allreducing both

individualand populationsurvival

SpecialConsiderations

Successful management ofBFFs depends

on maintaining adequatenumbersandareas

ofprairiedogcolonies Minimumviable

pop-ulation(MVP)sizesandarearequirementsfor

BFFs were addressed by Groves andClark

(1986).Additional estimates of these variables

are undei-way by Shaffer et al (in

prepara-tion),whoaremodelingeffectsofboth

demo-graphic and environmental stochasticty on

BFFpopulations ofvaryingsizes The MVP

represents a threshold below which

popula-tionsarenotself-sustaining Populationsmay

persistforalong timebelow theMVP, but

probablyatalossof adaptability andahigh

susceptibilityto localextinction Grovesand

Clark(1986)notedthatthegeneticmethodof

determining MVP forthe Meeteetse BFFs

estimatedthatabout200 animalsareneeded

formaintenanceofshort-termfitness.The

es-timated 200 animals needed is about four

times the number of breeding adults

esti-matedtocurrentlyexist atMeeteetse (Clark

1986)

Poisoning and shooting of prairie dogs

shouldbeprohibitedfromareaswhere BFFs

occuraswellasfromother selected portions of

prairie dog range Hubbard and Schmitt

(1984)suggesteda "refugia"conceptof

man-aging prairie dogs in which relatively large

areas are omitted from poisoningand other

disturbance Theysuggested that refugiabe

large enough to support a BFF MVP and

based suchareaestimatesontheStromberget

al. (1983) predator-preymodel Clark (1986)

outlined a series ofmanagementguidelines

forBFFs.

Differences in black-tailed (C

Itidovi-cianus)andwhite-tailedprairiedogcolonies

have been noted (Tileston and Lechleitner

1966, Campbelland Clark 1981, Clarketal.

1982) Black-tailedcolonies oftenshow

great-erprairiedogandburrowopeningdensities

—

two importantvariablesofBFFhabitat

Satis-fying habitat recjuirements for BFFs on

white-tailedcoloniesasdescribedinourHSI

model isassumed also to satisfy hai)itat

re-quirements on black-tailed and Gunnison's (C.gunnisoni)prairiedogcolonies

ApplicationofHabitatSuitabilit\'Model

ModelApphcability

Geographic area.—Although this model wasdeveloped ondatafromtheonlytwoBFF

populations ever studied, it should apply throughoutthehistoricrangeof theBFFuntil

additional BFF populationsindifferent

eco-logicalsettingsarefound, studied,andresults showitdoesnot apply Eventhoughasingle

prairiedogcolonycannot supportaBFF MVP (unlessit isextremelylarge),itcanpotentially

supportoneor moreindividuals Therefore,

anyprairiedogcolonyshouldbe considered

potential BFFhabitat Historic andcurrent land use patterns affect the quality ofBFF habitat A constellation of prairie dog

colonies, describedbyClarketal.

(Descrip-tion and history, 1986) and Forrest et al. (1985)asaprairiedog"complex,'

isneededto supportaBFF MVP.

S<?fl.son.—This model has beendeveloped

tocompareyear-round BFFhabitatat

Mee-teetse to habitat in other areas Because

prairiedogsmay becometorpidorhibernate over winteratnorthernlatitudes,it is

recom-mended that evaluation take place when

prairiedogsare activeand when snowcoveris minimal or absent: late May to lateJune is

recommended.

Cover Types.—This model compares the

BFF habitatat Meeteetsetootherpotential

BFFhabitatinallcovertypeswhereprairie

dogsarefound

MinimumHabitat Area.— Minimum habi-tatarea, asdiscussedforBFFs byForrestet

al.(1985), isdefinedastheamount

ofcontigu-oushabitatthatisrequiredbeforeanareawill

be occupied by aspecies (Allen 1982a) We

recommend that a preliminary estimate of

4,000-6,000 ha ofprairie dogs is needed to support a MVP of100 BFFs (Forrestet al.

1985,Groves andClark1986)

ModelReview.—Drafts ofthis model were reviewedby ourcolleaguesintheIdahoState University/Biota Research and Consulting,

Inc.ferretstudy team-StevenForrest,Louise Richardson, Tom Campbell, and Denise Casey;ArthurAllen, Habitat Evaluation

Pro-cedures Group, USFWS; Wayne Brewester

1986 Houstonetal.:Habitat Suitabiuty 105

Habitat Variable

VI Frequencydistribution

ofcolonysizes

V2 Totalareaof all colonies

V3 Burrowopening density

V4 Intercolonvdistance

V5 Prairiedogdensity •

LlKERKgUISITE

Cover/Reproduction

Food

CoverType

Allcovertypes havingprairie_ dogcolonies

HSI

Fig 2. Therelationship of habitat variables, life requisites,and covertypes to theHSIfor the black-footed ferret.

cies, USFWS; DonaldStreubel,Department

of BiologicalSciences,IdahoStateUniversity;

CraigGroves, Idaho HeritageProgram, The

Nature Conservancy; Mark Stromberg, The

National Audubon Society; John Hubbard,

Endangered Species Program, New Mexico

GameandFish;JohnCadaand DennisFlath,

NongameProgram, Montana Departmentof

Fish,Wildlife,andParks;HarryHarju,

Wyo-ming Gameand Fish Department; and Sid

England and Dale Lott, Department of

WildlifeandFisheries Biology,University of

California-Davis Improvementsand

modifi-cationssuggestedbythesepersonsare

appre-ciatedandwereincoq3oratedinto thismodel

ModelDescription

Overview.— The BFF can meet its

year-round habitat refjuirements within prairie

dog colonies providing: (1) prairie dog

colonies are large enough, (2) burrows are

numerousenough,and(3)adequatenumbers

ofprairie dogs and alternateprey are

avail-able This model thereforeassumes that

re-producing populations of BFFs use only

prairie dog colonies, and habitat evaluation

based on this model considers only the life

requisitesprovided by such colonies BFFs

donotrely solelyonprairiedogsforfood,but

breedingpopulationsmay dependonprairie

dogcolonieswiththeirhost of associated

ver-tebrates, many of which are known food

items It assumes that these colonies will

provideasufficientprey base(including

alter-native prey) and sufficientburrowopenings

forpredatorevasionandas sitesoflitter

rear-ing, thus providing maximum potential for

maybe foundiffuture populations ofBFFs

arediscovered, orifBFFsarefoundonareas other thanprairiedogcolonies

Thefollowing sectiondocumentsthe logic and assumptions usedtotranslatehabitat

in-formation forthe BFF to the variablesand

equations used in the HSI model Specifi-cally, thissection covers: (1)identificationof variablesusedinthemodel,(2)definitionand justification ofthe suitability levels ofeach variable, and (3)description of theassumed

relationship between variables The BFF

habitat variableshavebeengroupedintotwo sets: (1) an aggregrated setof four variables

that assess cover/reproduction as life

requi-sitesand(2)asingleliferequisitevariablefor food Figure 2 illustratesthe relationship of habitat variables, life requisites, and cover

typefortheBFF Thefivehabitat variables identified underthetwo liferequisite

cate-goriesare:VIisthefrequencydistributionof

colonysizes,V2isthetotalarea ofcolonies,

V3isburrowopeningdensity:average num-berofburrowopenings per haof colony,V4is intercolony distance:meandistancebetween

colonies (these four variables aregrouped

un-derthe cover/reproductionliferequisite),and

V5 is prairie dog density: mean number of

prairiedogs per ha(thisvariableisthefoodlife requisite) The aggregrated variables are

viewedascompensatory(i.e., anincreasein onevariablewillincreasetheHSI, butnot the

suitability of other variables) and thus are

combinedtoproduceasingleHSI.The

limit-ingfactormethodissuggestedforevaluating resulting values of thetwovariablesets. Cover/reproductivecomponent.— BFFs

re-lyonprairiedog burrowsforcover andlitter

106 GreatBasinNaturalistMemoirs No.8 Variable 1 examines the relationship

be-tween the distribution ofprairie dog colony

sizes in a region and its Suitability Index

Prairiedogcoloniespresentattheturn of the

century represented extremelylargeareasof

contiguousprairie dog distribution (e.g., in

Montanasee FlathandClark1986) Such

ar-easrepresenteda100%prairiedogoccupancy

and were assumedto be optimalhabitatfor

BFFs By comparison more recently,

Mel-letteCounty, South Dakota, showed 1.7%of

itsareaoccupiedbyprairie dogs,withamean

colonysizeofabout9ha (Hillmanetal. 1979)

The BigHorn Basin ofWyomingcontaining

theMeeteetseBFFshasabout1 7%ofitsarea

occupiedbyprairiedogs inmanysmall,

low-density colonies,althoughafew exceed1,000

ha (Clark et al. Description and history,

1986) Clark et al. (1982) described several

sample areas in New Mexico that showed

about 1% in prairie dogs, with colony sizes

averaging 33 ha (range 10-61 ha); in Utah

about1.9%, with colonysizesaveraging33 ha

(range 2-73 ha); in Wyoming on Thunder

Basin National Grassland about 1.3% in

prairie dogs showingawiderange incolony

sizes; in southern Wyoming about 3.2% in

prairiedogs, with colonysizesrangingupto

2,500ha;andinanotherareainUtah,colonies

averaged 125ha(range0.2-958ha).Thetotal

sizesofthese areas varied,andthis factclearly

influenced the distribution of prairie dog

colonysizelocated Ifalineisdrawnaround

theprairie dog complex at Meeteetse (least

polygonenclosingall50+hacolonies)andthe

area occupied by prairie dogs inside this

polygoniscalculated(about130sqkm),then

about22%oftheareaisoccupiedbyprairie

dogs The 50 hafigure does not mean that

smaller colonies are notimportant to BFFs;

indeedthesmaller colonies areusedat

Mee-teetse(Forrestetal.1985) Colonysize

distri-bution within this area is listed in the

Ap-pendix(Table3).

VIisamultidimensional probability estimate

andisnotgraphableasaretheremaining

vari-ables TheAppendixdescribescomputation of

VI

Variable 2 is the total area ofprairie dog

colonies AssumingaBFF MVPconsistsof100

breedingadults(eventhoughGrovesandClark

then100 colonies of50 haeach (about5,000ha)

isrequiredtosupportthem.It isassumedthat greater colony area means greater sites for coverandreproductionforBFFs.

Variable3isburrowopening density: the

averagenumberofburrowopenings perhaof colony ColoniesatMeeteetseare character-izedby burrowopeningdensitiesaslowas10 openings/ha and up to 100+ openings/ha This compares with other areas ranging 21-135/hafor black-tails, 32-57/hafor

Gun-nisons, and 2-64/ha for other white-tails (Clark et al. 1982) It is assumed that the greater the burrow opening density, the greater thecoverandsitesforsuccessful

rear-ing ofyoung

Variable 4 is the mean of intercolony (nearestneighbor)distances This variableis essential for cover/reproductive

require-ments but is also essential for expansion of

BFF populations and dispersal In pristine times, BFFsinlargecolonies mayhave dis-persed from their natal areas to new areas

withoutever leavingthe single large prairie

dog colony Dispersal between colonies,

where escape coverisminimal orabsent, is thoughttoexposeBFFstohighratesof mor-tality. Intercolony distance at Meeteetse is about0.92km(range0.13to3.70). InSouth Dakotaintercolony distanceaveraged2.4km.

Intercolony distanceforasample of 11

Gun-nison's colonies in New Mexico, Colorado, and Utah was2.4km andfor33white-tailed coloniesinUtahandColoradowas4.9km.In

winter at Meeteetse BFFs in intracolony

movements often travel 2+ km per night hunting Movements upto 8km have been noted duringthe breeding season It is

as-sumed that the smaller the intercolony

dis-tance,thehigherthe quality ofBFFhabitat Food component — Foodisdescribedbya singlevariable

Variable 5isprairie dogdensity (number/ ha) High densities ofprairie dogs provide

increased opportunity for BFFs to

success-fullymeettheirenergyandnutrient

require-mentsaswellasprovidingalternateprey asso-ciated in prairie dog colonies colonies Additionally, a high density ofprairie dogs meansan increased densityofburrows,which

Table1. Equationsfordetermining year-roundlife recjuisites for the black-footed ferret (2.0 isincludedas a scaling factor).

Cover/Reproduction

Food

Allcover typeswhereprairiedog coloniesoccur

Sameasabove

(2xVIxV2xV.3xV4)''

V5

Variable Relationships

SuitabilityofBFFhabitatdependsentirely

onattributesofprairiedogeolonies.VI

con-vertsthe distribution ofcolonysizes (relative

tothetotalcolonyarea) intoa single SI

mea-sure.V2accountsforthetotalarea of colonies

relative to BFF requirements and is

espe-ciallydiscriminativeintherangeofMVParea

size.V3gaugesthevalue of coloniesinterms

ofcover (burrow opening density) and,

al-though it generally covaries with food (V5:

prairiedogdensity), anyparticularcasemay

becriticallyuncorrelated.V4(intercolony

dis-tance)appraisestheeffectofcolonydispersion

inreferencetoBFFmobilityandbehavior In

summary, VIreflectscolonysizedistribution,

V2thetotalcolonyarea thesizedistribution

represents,V3thecovervalue of the colonies,

V4 the spatial dispersion ofthose colonies,

andV5thefoodvalue of the colonies

SuitabilityIndex(SI)graphsandequations

forhabitat variables.—This section contains

suitability index graphs and equations that

illustrate the habitat relationshipsdescribed

intheprevioussection(Fig.2).

Equations.—Life requisite values for the

BFF can be obtained by combining the SI

valuesthroughtheuseofequations(USFWS

1981) A description and explanation of the

assumedrelationshipbetweenvariableswas

includedunderthe Model Description,and

thespecificequationsin thismodel were

cho-sentomimicthoseperceivedbiological

rela-tionshipsascloselyaspossible.Thesuggested

equationforobtainingyear-roundlife

requi-sitevaluesfortheBFFaregiveninTable 1.

The four cover/reproduction variables are

multipliedbytwo(ascalingfactor forVI)and

aggregrated by using the geometric mean,

GM Wenecessarilyuse theGMbecausethe

quantitiesinvolved are measured on aratio

scaleandthe variables are not arithmetic

se-Variable

VIDistribution ofcolonysizes,

P(AB|N„0^ ^."'^

t)

Withn,asthenumberof colonies ofsizei,the resulting probability increases nonlinearly

withincreaseincolonysizes(numerator) rela-tive to the size ofthe complex (denomina-tor)(seeAppendixforexamplecalculationsof

thisequation)

VARIABLE:

V2Totalarea of colonies, 1.0

0.0

2500 5000 7500 10000 TOTALAREAOF COLONIES (HA)

V3 Burrowopeningdensity(meannumberof

burrowopenings/haof colony)

25 50 75 KX)