CAN TOWNSEND'S GROUND SQUIRRELS SURVIVE ON A DIET OF EXOTIC ANNUALS?

9 Pages·1992·3.7 MB·English

by  Eric Yensen

Creat Basin \atiiralist52io),pp.269-277
CAN TOWNSEND'S GROUND SQUIRRELS SURMVE
ON A DIET OF EXOTIC ANNUALS?
EricYensen ami Dana I,. OiiiniicN"
—
Abstrvct. Southwesteni Idahodcsrrtsliruh-bunchgras.sraiiiiclandisliciii",;;iiuadedIn liir-proiicexoticaniiiudsthat
pcnnanentlvdominatethelandscapeiollowingwildfires.ThisstiuK \\asluulertiikentodescribediet.soiTownsiMid'sliround
s(juirrels (Spennophilus totcii.sciulii idtiJiocnsi.s) at four stiuK sites with xaning degrees ol' exotic annual in\asion to
determineiftlieS([uirrelscouldutilizehighproportionsofexoticannualsintheirdiets.Townsend'sgrounds(juirrelswere
collectedin .March and Ma\ of 19(S7and 1988,andstomachcontentswereanal\7.edusingamicrohistologicaltechniciue.
Grasses comprised 37-87% ofTownsend's ground squirrel diets at the four sites. N'ati\e species. especialK' Sandberg's
bluegrass [Poasecunch),winterfat(Ceratoklcslanata),bigsagebrush(Artemisiatridentatu^andsix-weeks fescue(Viilpia
octojlora) constituted 7-96% (x = 47.2%) ofthe diet, whereas exotic species. especi;ill\'cheatgrass (Broinns tedontm),
tunibleweediSalsolaihcrica).iuidtans\miLStards[Descuminiaspp.) madeup4-68% (\ = 48.0%)ofthediet. Ateachsite
2-Aspeciescomprised>90%ofthediet.Therewasnoapparentcorrelationbetweentheimportance\aluesofexoticspecies
atasiteandtheirimportanceinTownsend'sgroundsquirreldiets.
Kvijuonl.s: .Spermophilust(n\nsendii,fixxlhabits,dictanianali/sis. Idaho,urotuidstjiiirrcls.
The Snake Ri\er Birds of Pre)' Area i.s a bunchgrasses, and shrubs apparentK' proxide a
243,()()()-ha tract of multiple-u.se shrub-steppe more constant, stable food soinx-e than exotic
rangeland achuinistered bv the U.S. Biu-eau of annual species that ma\- xan- in producti\1t>'
Land Management. Towaisends ground squir- between xxet and dn" xears b\- sexeral orders of
rels (Speniu)pliihis townsendii idaltoensis) are magnitude (Younget al. 1987).
important prey ofraptors, and continued exis- Like other ground squirrels of subgenus
tence ofthe area's dense breedingpopulations Spenuopluhis. Townsend's groimd s(juirrels eat
of raptors depends upon dense Townisends green xegetation earK' in their fom- to iixc-
ground scjuirrel populations (U.S. Department month actixe .season, then eat seeds of gras.ses
ofInterior 1979). and forbs to fatten up for hibernation Hoxxell
(
hnasion of southwestern Idaho rangeland 1938, Rickart 1982). In southwestern Idaho,
by e.xotic annuals such as cheatgrass (Broinus Toxx'usend's groimd squirrels are in (^stixa-
tectonun). tumblemustard {SisijDihhuiii aliis- tion/hibeniation from |une or (ulx' until the fol-
siiiiuni\ pinnate tan.symustard {Descuraiiiia lowing Januan or Februarx with loxv sunixal
piniuila). and tunil)I(n\-e(Hl (Salsola iheiica) has rates (ca. 289f; Smith and lohnson 1985). Food
resulted in trecjueut and destructive wildfires quantitx and cjualitx' could influence oxerwin-
thatkill natixeshrubsandweakennatixebunch- teiing surxixal as xx'ell as reproductixe success
grasses. Oxer time, fires ha\e n\sulted in the the folloxxingspring.
permanent replacenu'iit ot luitiw slirul)- and Toxxnsend's ground s(|uirrels are known to
bunchgrass-dominated comnumities b\ e.xotic eatnatixeforbs{Sphdcnilcca:Daxis1939),bunch-
annual-dominated comnumities (YenscMi 1980. grasses {Pod sp.; fune grass, Kiwlciia sp.; Daxis
Kochert and Pellant 1986). 1939), and desert shnibs (big sagebrush, Arte-
Townsend's gromid S(juirrel populations are uiisiaIridcntata:budsage,Artemisiaspinescens;
niucli less stable in exotic annual-dominated shadscale, Atriplex coufeiiifolia; Daxis 1939,
connnnumities than in natixe shrub communi- lohnson 1961), as xvell as in.sects such as grass-
ties(Yensenetal. 1992). Nativeperennialf()d)s, hoppers and cicadas, and occasional!)' carrion
,'MBuurseeauumofofLaNnatdurMaaluaInIeisntiorev-n.t..Mb.e39r4t8.sUouevCeollloepgmee,uCtalAdvwee.l.l,BoIidsea.hoId8,a3h6o0.58.370.5.Presentaildress:Idaho.Xnii\NationalCinard.DepartmentofEn\iroinnent.
Box45,GowenField.Boise,Idaho8370.5.
269
270 GreatBasin Natuhaljst [W^lume 52
Tabi.K 1.\e2;etation importancevalues (% relativecoverplus % fVeciiiencv) in May 1987and 1988at fourstudysites
nearCo\ote ButteintheSnakeRiverBirdsofPrevArea,southwestern Idaho.
19921 TowxsKXDs (iHorxn Soiikhki, Dikts 271
some introduced tuiiil)I('\\('('d. tlicatgrass, and Stomachs xx'ere remoxed from the animals
other (Aotic- animals present lTal)le 1V iimn(xliat(^lx-postmortem andpresenedin70%
fclXOTIc AXXIALS—This site (TIS. HIW, cthaiiol. In the lab, stomach contents xvere
Sec. 13; ele\'. 850 ni) is adjacent to the natixe remoxed from ethanol, diluted50% xxith xxater,
grass site andwas similar to it prior to the 1983 and homogenized 1 min in aWaringblentlerto
burn (13. L. Quinnex; unpnbHshed data). Both producefragmentsofuniformsize.Thehomog-
sites were bunied b\" the same fire. However, enate xvas xvashed through a l-mm siex-e
sincethefire, theexoticannualscheatgrassand (Hansen1978)andcollectedinaO.l-mmscreen
tumble mustard, with some remnant natixe to remoxe tiny, unidentifiable fragments. The
glasses, especiallx Sandbergs bluegrass (Table material xvas then mounted on microsco])e
1), haxe dominatedthe site. — slides using Hertxvigs and Hoxers media
RKHABILITATION seeding. This site is (Sparks and Malechek 1968).
locatedfikmeast,2.5kmsouth(TIS, RIE, Sec. Plant .species in the diet x\er(^ identified bx'
27: elex'. 885 m) ofthe unburnedbigsagebnisli compari.sons to a reference collection ofmicro-
site. The area burned in 1981, xxas reseeded scope slides using microhistological characters.
withdeseitx\iu\itgrass(Ag^ropi/roiidesertoniin) All referenceslidesxx'eremade fromcatalogued
in 1982, but burnedagain in 1983. In 1987 and specimens in the Albertson College Harold M.
1988, tlie area xx'as dominated bx' Sandbergs TuckerHerbariumandxx'erepreparedusingthe
bluegrass, desert xxheatsrass, tumblexxeed, and technicjue describedaboxe.
other natixe andexotic forbs (Table 1). Forfoodhabitsanalysis,oneslidexvasexam-
ined per stomach. Occurrence offood catego-
Methods ries (frequencx') xx^as recorded from each of20
microscope fields per slide using a phase-
contrast microscope at lOOX. Frequencv/20
To determine the degree of exotic annual
fields xx'as tlien converted to percent relatixe
inxasiou at each site, vegetation analxsis xxas
density' (Sparks and Malechek 1968) using a
conductedinearl)'June 1987andlate Max' 1988
table dex'ek)ped for fre(juencx-to-densitx con-
x\hile Towiisend's ground squirrels xvere being xersion (Frackerand Brischle 1944).
collected. At each site xve used a transect xxith
Theimportanceofeachdietarxcategorxxxas
foitx" 1-m' quadrats spaced at lO-m intenals
(Daubenmire 1959). Percentcoxerofeach spe- calculated in three xvays: (1) percent relatixe
densitv; a standarddn-xveight conxersion from
cies xx'as estimated using a 1-nr (jiiadrat frame
dixided into tiMiths to facilitate estimation. To frecpiencydata(Spark'sand'Malechek 1968);(2)
percent frecjnencx in stomaclis. the percentage
gixeabetterapproximationoftheaxailabilitx of
each plant species, percent relatixe coxer and of stomachs from a site xxith the item; and (3)
percent fre(jnencx' in micr()sco[)ic fields, the
jiercent relative frequencv xx^ere conxcMted to
importancevalues (Cox 1990). percentage of all microscopic fields from a site
xxith the item.
Squirrels xvere collected bx tia])ping and
shooting at all four sites in Max and June 1987 Txx'entx' microscopic fields xxere examined
in = 75) and in March and Max 1988 (// = 42) afnrodmferaecqhuesnlcixd'eoufsioncgcaurprreendceeteorfmienaecdhpsapttt^ecrine,s
eMxacyep1t98f8r.oSmqutihrererleshxaxb'ielreitaagteiodninsetheedifnigeldsiutseinign xcxaatsegroenc/o2r(d)edf.ielTdhseonfroecnjeuesnlciydeofxxaesacchomdpiaetraerxd
pelage and bodx xxeight ciiteria (Bureau oi
Land Management, unpublislied data). Hepre- xxithotherslides(orreplicatecountsofthesame
slide) using the Kulcvznski Index (Oo.sting
sdsekanertldaettiosxt-niesids[xipie=scki3im)n,esnosrxx(xi3xt)ehrseksukplurllselspoannl(/xy;l(/;=as=(12125))),staa(nn2d-) l1a9ri5t6x)i(nadlesoxx[xBerlalxk-naoxnxdiiC>ausrttihse1B9r5a7x]-)Curtis simi-
deposited in the Albert.son College Museum of 2w/(a +b)
Natural Histon. Tooth-xx-ear patterns (Yenseii Theindexxxascalculatedasadissimilaritx index,
f1o9r91al)lxxs'peerecicmonesniss.teBnatsxexidthotnhethaesseeascsrii"t;eirmiia(,Miatlsl 1 - [2w/{a + h)]
1987 specimensxx'erejuxeniles since thex xxere using a BASK' microcomputer program pro-
collected late in the active season xx'hile the xidedbv Ludxxigand Rexiiolds (1988).
adults xx'ere entering seasonal torpor: all 1988 \\'eather data xxere from the National Oce-
specimensxx^ereeitheryearlings oradults. anic and .Atmospheric Administration monthly
272 CiHEAT Basin Nathhalist [N'olunie52
Tablk2.Lateseason(25Mav-19June) 1987Townsend'sgroundstjuineldiets.DataarefromstomachsofjuvenileTGS
atfoursitesintlieSnakeRi\erBirdsofPrevArea.Adultswereenteringtoiporandnonewerecollectedduringthisperiod.
Dietar\-compositionisgiven aspercent relativedensitv'(RD). percentfrequencyin microscopefields (MF),andpercent
frequencyinstomachs(PS)foreachdietarycategor\-.Others\nil)()ls: + = <17f,- = aiiscut,and// = numberofstomachs.
Unbumed Native Exotic
bigsagebrush grasses annuals
Dietar)'categor>- RD MF PS RD MF PS RD MF PS
21 20
GlUSSES
Bromu.stcctonim 22 41 71 62 93 100
Poasc'ciinda 24 35 Sfi
Sitanionliystrix + 2 10
Oryzopsishijmcnoides - +
Grassseed + 5 19
Grassroot? 2 9 10
Totalgrasses 49
Siihi;bs
Ccratokh'slanala 3 9 52
Artemisiathdciitata + 2 14
Atiiplexniitttillii - -
Clinfsotlunniitislisciiliflmus - -
Totalshrubs 3
FOHHS
Sdlsolaihciiai 39 69 91
Sisi/inhiiiiui—(iltissiimiin + + 5
Di'scitniiuia 2spp. + 2 24
Lcpiditiiiipcrfoliatum - -
C.njptdttthd intcmtpta + + 5
Rdnitunihistcsticuldln.s + 1 5
Ldctucdscrriold
Gheno[5odiaceae - -
Uuidentiliedlorb - -
Totalfod)s 40
MlSCKLI-ANKOrS
Insects S 17 62 19 44
Fungi - - 1 4
Unknown + +
Unidentifiedseed +
I
Totiil mi.scelliuieous S 20
19921 TOWNSKXDS (;iU)lM) S()L lUKliL DlKTS 273
(lilfci"iMit iiiiiiu'i'ica] results, the rank orders duced annuals. BristK cnptantha (Cn/pfantha
among ealegories were generalK consistent inl('rnij)fa) was the onK nati\(^ forb found in
(Tables 2—1). Ho\\e\er, percent frecjuencx in Townsendsgn)unds(|uirrelstomachs. Altliough
stomachs was\er\-sensitixe to sample sizes. 1988samplesizesweresmall, theimportanceof
There were 1-9 food cate<2;ories per stom- forbs in the diet increased in the samples
ach. Sitemeansvariedfrom3.Sto4.4categories between March and Mav 1988, while the per-
per stomach. The total numher ol food catego- centageofgrassesandshnibsdecreased(Tables
ries usetl 1)\ all Townsentl's ground stjuirreis 3-4), thus .suggestinglargeseasonal differences
sampledatasite\arietlfrom4to ITonthethree between March and Nhivdiets.
sampling occasions (Ma\-|nne 1987, March A sm"j)rising number of insects were eaten,
1988, May 1988). However, ifspecies used in especially in Ma\-june 1987 (3-19%; Table2).
trace amounts (<5% relative densits) are elim- However, insects wen^ not important in 1988
inated, onlv3-6 (x = 4.0) categories were used (trace amounts at the big sagebrush site onlv).
per site and onK-2-4 species comprised >10% Insect remainsweresofragmentarv that identi-
otthediet. Speciescomprising>l()'yfoftliediet fication was not usuallv possible. However,
at one or more stud\' sites included Sandberg's abundant Lepidoptera lanae could be recog-
bluegrass, cheatgrass, six-weeks fescue, winter- nized bv the soft e.xoskeleton and prolegs, and
fat, bio; sagebrush, tumbleweed, Descunibiui fragments recognizable as beetle antennae and
spp., seeds of bur-buttercup [Rdiitiiiciiltts tcs- t4\trawere found.
ficiilatus), andinsects. The importance values of exotic species
Grasses were important constituents of the were lowest at the unburnedbigsagebrush site
dietinboth 1987and 1988andoftencomprised in both vears and highest in the exotic annual
o\er oiWc of tlie diet (37-889f relative density. site in 1987 and at the native grass site in 1988.
Tables 2—4). Sandberg's bluegrass and However, therewasnocorrelation between the
cheatgrasswerebothheavih'utilized,especialK importance values of all exotic annuals at a site
in March 1988 (55-87% of diet). Late in the andtheirimportance in thedietatthat site (r =
Townsends ground scjuirrel active season (Ma\ -.454; Tables 1-4).
and June) use ofgrasses declined (except at the
exotic annual site in 1988). Most of the grass Di.sc.'us.siox
eatenin .\hi\-|uneconsistedofseeds,especiallv
of cheatgrass. Sandberg's bluegrass leaveswere Th(^ data show tliat lor sites with varviug
utilized slightlv more than cheatgrass leaves degrees ofexotic annual invasion sampled over
(Tables 2-4), and the tvvo together were far atAvo-v(\u'period, Tovvnsend's ground scjuirrels
more important than all other grasses com- can and do utilize introduced species in their
bined. S(juirreltail was little used, altliough it diets, and that cheatgrass. tumbleweed. and
was the third most abundant grass. tumblenuistardarethemost impoitantofthe.se.
Winteifat (0-43% relative density ) and big Both thevegetation atasite andTmnsend's
sagebrush (0-21%) were both eaten, and ground squirrel diets varied considerably
winterfat was especiallvimportant at tiie exotic between years and among sites. Differences in
site where it was least abundant. Winterfat was amount ofprecipitation most likelv account lor
utilized at all sites in 1987, even though it was the differencesinvegetation importancevalues
not abundant enough to be sampled 1)\ the betA\-e(Mi vears.Therewas less September-Muv
vegetation analvsis at the exotic annual site. In pi-ecipitation 192 nun in 1986-87 and 170 nun
(
1988 it was eaten onlv at the unburned big in 1987-88 at Kuna ca. 20 km \). The
sagebnish-winterfat site, and its use declined Daubenmire (juadrats were taken on the same
between March and Ma\' 1988 (Table 2). Big transect in botli vx^ars bv the same technicians.
sagebnish was u.sedin March at all sites in both The substantial annual differences in
vears but was less important in Max. Townsends ground scjuirrel diets may be the
Tumbleweed and tumblemustard were the result of (1) vegetation differences between
most important forb species cc^nsumed. Tans\- vears. (2) the fact that juveniles were sampled
mu.stards [Descurainia sophia and D. pimiaia). in 1987 andadults andvearlingswere collected
peppergrass, seedsofbur-buttercup, andleaves in 1988, (3) differences in collecting dates (25
of pricklv lettuce {Lactuca seniola) were of Mav-19June 1987versus 16-19 Mav 1988). or
secondan importance. All of these are intro- (4) small .sample sizes.
274 CheatBasin Naturalist [Volume52
TA151.F.3. EarK'season (March) 1988Towjiscnd'sgroundsciiiirrel diets. Dataare from stomachsofachilt andyearling
TCS atfoursites in tlie Snake River BirdsofPreyArea. (Ju\enileswere nota\ailahlein March.) Dietaiycompositionis
givenaspercentrelatixeden.sitv'(RD),percentfre(iuencyinmicroscopefields(MF),andpercentfrequencx ofstomachs
(PS)containingeachdietaiTcategoiy.Otlier.symbols: + = <!%,- = ab.sent,n = numberofstomachs.
1992] Tow\si:\ns C.Horxn Soiihukl Diets 275
Txni I 4 I,ateseason(Ma\) 19cSSTowiisciurslironiulscjiiirreldiets. Dataaretroinstoinaclisofadultandxcarlin^TCIS
at loursitesin theSnake Ri\erBirdsof Pre\-Area. Dietar\'composition isgi\enaspercent relativedensit\ (HD).percent
fre(juenc\' in microscope fields (MF), and percent frecjuency of stomaciis (PS) containinsjeach dietan cateiiorx. Other
sNUibols: + = <1%,-= absent,ii = nimiberofstomachs. Site4Witsnotsampledin 19SS.
276 GreatBasin Naturalist [\'olume 52
WVominggroundsquirrelsinColorado. Hansen herbarium specimens, andweespecialK"appre-
andJohnson (1976:750) concludedthat ciate M. P. Luscher'sassistanceinpreparingthe
slides. R. G. Anthony, D. R. Johnson, S. Knick,
aRivcahraireddsoonfp[U=u\i\tVsomaisiit<hie]ygfirlloutlnideirsqsutirormealcshgsrarzaethoenr E. .\. Rickait. K. Steenhof, and B. Van Home
thanselectingonlvpreferredfoodswhentheirstom- made helpful comments on an earlier draft of
achsarenearl^•enipt). Tliismaybeane\olutionar\- the manuscript.
strateg)- de\eloped to allow them to consume
vetches. The dilution of toxic foods b\- non-toxic
foodsdecreiisestheprobability-ofplantpoisoning. Literature Cited
Frecland andJanzen (1974) re\ie\\ed strat-
egies ofherbi\()r\ h\ mammals in response to ^^LCORN.J. R. 1940. Liiehistor\'notesonthePiuteground
tsheactoandgaenne-raplliasntthecrobmip\oourendssh.oulTdhef\e-edsupgrgeedsotme-d Bray.suqpJu.liaRnn.ed.l.afnJoodruersJ.tnaTlcooCnfunnMruatnimismtaiel1s9o.g5o7\f. 2\s1on:ut1oh6re0dri-nn1a7t0Wi.iosncoonfsitnh.e
inanth' on one or hvo foods, but continue to EcologicalMonographs27:32.5—349.
sampleotherfoodspresent.Whenanherbixore Cook.C.W. 1977. Effectsofseasonandintensit\-ofuseon
experiences a nutritional deficiency. :t should desert\egetation. Utah Agricultural Experiment Sta-
tionBulletin483,UtahStateUni\ersit\',Logan.57pp.
sample all a\ailable foods until it finds some- Cox.G.W. 1990.Laboraton manualofgeneralecolog\.6th
thingwhich supplies that nutrient. ed.W'm.C.BrownPublishers.Dubuque,Iowa.251pp.
ThefeedingstrategiesproposedbyFreeland D.\LBENMIRE. R. F. 1959. A canopv-coverage method of
and Janzen (1974) and Hansen and Johnson \egetationanalvsis.NorthwestScience.33:4.3-66.
(1976) appear to occur in sexeral members of DwisP.ri\n\"t.erBs..1L9t3d9..,TCahledwReelcle,nItdamhaom.m4a0l0sppo.fIdalio.Caxton
the subgenus Spennophilus. The data indicate FiTCU.H.S.194S.EeologNoftheCaliforniagroundsquirrel
that ground squirrels specialize on 2-4 highly on grazing lands. American Midland Naturalist 90:
'
nutritionalspecies,butsupplementthemwith a .334^:340.
wide\ariet\'of(jtherspecies,apparentK as"poi- FRAt.KF.R. S. B.. and |. A. Brisciilk 1944. Measuringthe
loc;xldistributionn{'Ribcs. Ecologv^25:28;3-303.
soning insurance." In this study, Townsends Freeland. W. and D. H. Janzen. 1974. Strategies in
ground squirrels similarly depended on onl\ a herbi\'or\- bJ\.', mamniiils: die role of plant secondary-
few species forthe bulk ofthe diet, but a wide compounds..American Natin-alist lOS:269-289.
\ariet\'oftrace specieswas not axailable. Ifany IIansen.R.M.1978.Shastagroundslothfoodhabits,Ram-
piU'tCa\-e,.\rizona. Paleobiology 4:.302-319.
of these species should proxide insufficient Hansen R. M.. and M. K.Johnson 1976. Stomachcon-
(juantities of a key nutrient (e.g., linoleic acid tentweightand food selection bv Richardson ground
necessaiy for hibernation), then the limited squirrels.JournalofMamnialog\-57: 749-751.
selection of food species could ha\e negati\e Hansen. R. M., and D. N. Ueckert. 1970. Dietarysimi-
larity ofsome priman- consumers. Ecology- 51: 640-
population consequences.
648.'
ThequestionofwhetherTow-nsendsground Howell A. H. 1938. Re\ision of the North .\mericaii
squirrels can utilize exotic annuals as dietai-y groundsquirrels,withaclassificationofNorthAmeri-
staples is an.swered in the affirmatixe b\ this CimSciuridae. NorthAmericanFaima56: 1-256.
study. Nativeforbspecieswereofminorimpor- Johnson. D. R. 1961. The food habits of rodents on
rangelandsofsouthernIdalio. Ecology-42:407—110.
tance in the diet, but this does not necessarily Johnson. D. R., G.\V. Smith. R. M. Olson 1977. Popu-
reflectpreference. Nati\e forbs arenowso rare lationecology-andhabitat requirementsofToyvnsend
atthe foin-sitesthatnonewere recordedb\ the groundsquirrels. Pages203-225in Snake RiverBirds
\egetation anal\sis, andthus they maynothaye ofPrey ResearchProject,Annual Report, 1976.
[oHNsoN M. K. 1980. FoodofToyviLsendgroundsquirrels
beena\ailableforconsumption.Onlyonenative on the .\rid Land Ecology Reserve (Washington).
forb iCnjptantlia) was found in the stomaciis. GreatBasin Naturalist37: 128.
The consequences oflimited dietai-yyariet}'on KocHERT. M. N.. and M. Pellant 1986. Multipleusein
tlie long-tenn nutrition ofTownsend's ground the Snake Ri\cr Birds of Prev .Area. Riuigelands 8:
217-220.
squirrel are unknowni.
Li-nwogky;..JJo.lAm.,WainldeyJ.6cF.SoRnesy,nNoeldwsY.or1k9.88. Statisticalecol-
.VlKXOWLEDCMKXTS OosTINC;. H. 19.56.Thestudyofplantcommunities.2nd
J.
ed.W.W. Freeman,San Francisco.
We RicKART E. A. 1982. .\nnual cycles ofactivity and body
tliaiik j. \\'ea\cr and other Bureau of compositioninSprnnophilnstnwnsendiimollis. Cana-
Land .Managementpc^rsonnelwhocollectedthe dianJournalofZoology 60:329.8-.3306.
scjuirrels and the \egetation data, B. DMii and Ro(;ehs. L. E., and K. A. Gano 1980. Toyvnsend ground
T Foppe fordiscussion ofthe microhistological \sc\jaisiihrirnelgtodnie.ts|()iunrnatlheofsRlainmbg-estMeapnpaegeomfesnotut3h3c:en4t6r.a3l-
analysis technique, P. L. Packard for ac'cess to 465.
1992] TowxsKXDs C.RorxD SoriHREL Diets
S\ii1II C\\..andD. R.Johnson 19<S5. DemogiaplnoCa Area. Progress Report. U.S. Department n\ inU-nor,
Towiisciu!groundsquirrelpopulationinsoutliwrsteni BureauofLand Management, Boise, hlaho. S2pp.
Idaho. EtologN'66: 171-178. Yensen, E. 1991. Taxonomy and distribution oftlie Idaho
SoK.\WL. RH.. RF.r.e;eumidanF.&J.CoR.o,hSlafnFrUaJnSlc.iscBoi.onSu.5-9tnp.p.2nd cd. gMraonmuinadlosgqu)i-rr7e2l:,58S.f3H-T6H0i0o.philns bnnuicus. journal of
Sl'\RKS. D.R.,andJ.C. 1VL\LECIIEK 1968. Estimatingper- Yensen. E.,D.L.Quinnev K.Johnson K. ri\i\ii:H\i.\N.
centage dr\'weightin diets usinga microscopictech- and K. Steenhof 1992. Towiisend's ground.stjuirrel
nique.JoumidofRange Management21: 264—265. population fluctuationsin southwestern Idaho, .\mer-
U.S.Dep.vhtment of Intekior. 1979. SnakeRiverBirds ican MidlandNaturalist. Inpress.
ofPrev.SpecialResearchReport.U.S.BureauofLand YouNo.J.A.,R.A. E\ANs R.E.EckEin |i; antl B. L. k\v
McUiagement, BoiseDistrict, Boise, Idaho. 142pp. 1987.Cheatgrass. Rangelands9:266-270.
Yensen D. L. 1980. A grazing histor\- of southwestern
IdahowithemphasisontheSnaki' Ri\erBirtlsof Pre\' Rcceiied5Maij 1991
Acccjilcd1 September1992