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Oirk Van Vuren, Departmentof Systernaticsand Ecology Un versityof Lawrence Kansas66045

Bison West of the RockyMountains: An AlternativeExplanation

Abstract Althoush abundant on the C.eat Plains, bison (Aison bison)were relatively scarcewesr of rhe Rocky Mountains in the .Four explanationshave been advancedto :rccountfor this low density:l) relalive inaccessibilit]ol the atea, coupled with Indian,causednortaliryi 2) low prorein content of forages;3) lack of slnchrony betweenforage plant phenologyand the bison reproducti'e .ycl€; 4) periodicallyh€arf winter snowsdeep enough 1o kill all colonizingbhon. None ofthese explanations finds lolid supporr in rbe available evidence.I propose that low bison nunbers resulted fron low overall forage produc!ion, and from discontinuousbabitar shich isolatedbison populationsand sloved recolonizationfollowing periodic Iocal extinctions.

Introduction authors. My purposeis to examinethe four ex- planationsin light of the availableevidence, then and extraordinaryabun- The former distribution proposea new explanation. danceof bison (Bison bison\ot the (Roe hasbeen chronicled in detail 1970),but the FormerDistribution of Bison occurrenceof bison further west is not well known and has been a matter of considerable Numerouseyewitness accounts attest lo the abun- debate.Although bison apparently were widely danceof bisonin southwesternWyoming and the distributed throughout much of the Pacific SnakeRiver Plain and adjacentvalleys of south- Northwest,they were abundantwest of the Con- easternIdaho (e.g.,Ogden 1910,Work 1913, tinentalDivide only in southwesternWyoming Davis 1935,Haines 1955).There are no eye" and southeasternIdaho. Low densityof bison witnessaccounts of bison further westand north- overmost of their rangewest of the Divide stands west,in the PacificNorthwest, but broaddistribu- in marked contrast to availabilityof tion is suggestedby recoveryof bisonskulls and habitatrvhich sppmcd capable of supporling otherbones from at least44localities in ea"t"rn much higher numbers.This disparitybetween ,eastern , and southnestern numbersand apparentcarying capacitywas (hereafter"PNW") (Schroedl 1973, Agen- notedby the earliestexplorers in the area,and broad l9?8, Van Vuren and Bray l9B5).Only one hasstimulated much discussion since (Kingston oftheselocalities yielded evidence ofmore than 1932,Haines 196?,Christman 1971, Schroedl a fewindividuals; hundreds of bisonskulls were 1973,Butler 1978,Mack and Thompson1982, exposedat MalheurLake in easternOregon, sug- Daubenmire1985). Resolurion of the problemhas gesting that bison may have been locally com- important implications for plant ecology and mon there(Van Vuren and Bray l9B5).Most of evolution(Daubenmire 1978, 210, Mack and the 44 localitieswere in areascharacterized by Thompsonl9B2), zoogeography (Lyman and Liv- steppevegetation \vhich produces an appreciable ingston1983), and ethnography(Schroedl 1973, biomassof graminoids,the principalforage of Butler l97B). bison(Meagher 1973, Peden 1976, Reynolds et Four explanationshave been proposed to ac- aL l978, Van Vuren l9B4a).Available evidence countfor the scarcityof bisonwest of the Rocky suggeststhat bison becameextinct for uncertain Mountains.Critical evaluation of theseexplana- reasonsin easternOregon and Washin5on about tions,based on availableliterature, has been lack- 1800(Schroedl 1973, Van Vuren and Bray 1985). ing. Becausethe problemis historicalin nature (bison were exterminatedwest of the Rocky PreviousExplanations Mountainsover 100years ago), the causeof low bison numbersmay never be identified con- Kingston(1932) argued that physiographicbar- clusively;yet a considerablebody of pertinent riers greatlyrestricted immigration of bison into literatureexists, largely overlooked by previous the PNW, and that the few successfulimmigrants

NorthvestScience, Vol. 61, No. 2, t9B7 65 were soon killed by Indians. Archeological shift accordingto environmentalconditions, evidencedoes not supportthis explanation.The Moreover, Daubenmire (1985) argued that presenceof bones of immature bison rn ar- grassesin more mesichabitats peripheral ro cheologicalsite" in soulheaqternWashington sug- steppe remain green throughout the summer, geststhat a breedingpopulation inhabited the providing sufficient forage to sustaina substan- area(Osborne 1953, Schroedl 1973). A bisonkill tial number of bison. The ability of bison to ex- site in southwesternIdaho, in usean estimated ploit seasonallyavailable forage is dernonstrated 7000years, incorporated 26B0 m of stonefences by the severalextant herds which are migratory (AgenbroadI9?8); the efforr for constructionand (Soper1941, Meagher 1973, Van Vuren and Bray the duration of usesuggest that bison in the area 1986). vere an establishedpopulation rather than an Daubenmire(1985) proposed that occasion- occasionalgroup that strayed west. ally heary snowfallwest of the RockyMountains, Johnson(1951), noting the apparentlylow held in place by shrubs,was sufficiently deepto densitiesof bisonon tallgrassprairie east of the kill any bison that crossed the Continental MississippiRiver compared with high densities Divide.Telfer and Kelsall(1984) reported that on shortgrassprairie further west,suggested that bison seemedless efficient than severalother bison nurnberswere limited by presumedlower native North Americanungulates at coping with proteincontent of tallgrassspecies. Daubenmire deepsnow, but it is unknownif this relativein- (1985)interpreted this suggestionas an explana- efficiencytranslated into an absoluteinability to tion of low bisonnumbers in the PNW as com- survivein the PNW. There is a crucialdifference paredwith high numberson the Great Plains. betweensnowfall sufficient to causelocal mor- Data on proteincontent of grassesdo not sup- tality in bison, which has been documented port this interpretation.Protein content of steppe (Meagher 1973), and snowfall periodically so grassesin the PNW,either live (ca. 6-15% of dry severethat a vast area was renderedentirely weight)or cured (co. 4-7%) (Mcllvanie 1942, uninhabitable.Daubenmire derives suppod for Rickard el aL 1975, Uresk and Cline 1976, his explanationlargely from historicalaccounts Willms er al. 1981),was similar to proteincon- of domesticlivestock encountering difficulty with tentoflive (co.6-15%) or cured(ca- 3€%) steppe deep . The underlying assumption, that grasseseast of the Rocky Mountains (Jefferies bison and livestockare ecologicallysimilar, is and Rice 1969,Sims et al t971, Willard and probably invalid (e.g., Peden et al. 1974; Schuster 1973, Coetz 1975, Cogswell and Christophersonet al. l97B; Schwartzand Ellis Kamstra1976). l98l; Van Vuren 1982,1984a; Van Vuren and Mack and Thompson(1982) suggested that Bray 1983).Moreover, there are numerousre- grassphenology in the PNW may havelimited liable descriptionsfrom rhe lB00'sof facrorssuch bison numbers.They noted that btson cows as drowning, fire, and quicksand killing large usuallycalve late April throughearly June and numbersof bison,but noneof massmortality due proposedthat the nutritionalstress of lactation to deepsnow (Roe l9?0). Indeed, historical ac- coincidedwell with grassphenology on the Great countsindicate bison were not greatlyaffected Plains, but very poorly with phenology in the by snow (Roe 1970,203). PNW,where steppe grasses are dormant during Any explanationfor low bisonnumbers in the much of the summer.The underlyingassump- PNW shouldalso account for the disappearance tion, that the bison reproductivecycle cannot of bisonin the areaabout 1800.Chdstman (197t) shift in responseto forage availability, may not suggestedthat this extinction,whicn occurred bp valid.The midpointof the breedingsea"on shortlyafter Iocal Indiansacquired the horse, of sevenbison herds(McHugh t958, Halloran resultedfrom the increasedefficiency and mobil- and Class 1959,Fuller 1962,Meagher 1973, ity of mountedhunters. The preferenceof In- Petersburg1973, Mahan 1978,Lott 1981,Lott diansfor bison is demonstratedby numerousac- and Galland1985, Shull l9B5)was correlated with countsof PNW tribesmaking trips of 500 km latitude(R" = 0.75,P ( 0.05).Breeding, and or more on horsebackto hunt bisoneast of the presumablycalving, occurred up to six weeks ContinentalDivide (e.g.,0gden 1910,Stewart earlierin southernherds than in northern herds, 1938).Daubenmire (1985), however, argued that suggestingthat the bison reproductivecycle can Christman'ssuggestion was inconsistentwith

66 Van Vuren Schroedl's(1973) conclusion that bisonnumbers mancyof steppevegetation may havefuither re- in easternWashington were seemingly in decline duced carrying capacity (Mack and Thompson long beforeintroduction of the horse.Schroedl's r9B2). conclusionwas based on a temporaldecline in Second,vegetation east of the RockyMoun- numbersof archeologicalsites in which bison tains consistedof nearly continuousgrassland, havebeen found and in numbersof bisonbone virtually all of it suitablebison habitat.Any area fragmentsper site.This evidenceis questionable. sufferinga localpopulation reduction or erlinc- The declinein number of sitescontaining bison, tion of bison could be recolonizedquickly by ad- from nine (2500-1500BP) to six (500-200BP), is jacentherds. In contrast,much of the tenain west unconvincing.Schroedl did not comparethe of the RockyMountains was a mosaicofhabitats, numberof sitesin whichbison were found with many of them unsuitablefor bison.Distribution the number in rhich they were not found; a of bison probably was disjunct, with physio- higher numberof older sitescontaining bison graphic barriers such as mountains,, or could simplyreflect a highernumber of suchsites deepcanyons hindering movernent of individuals being excavated.The temporaldecrease in bison betweenmany populations, McDonald (1981, 22?) bonefragments resulted entirely from the dispro- found a high incidence of dental anomalies portionateinfluence of oneof the 22 sitesexca- amongbison skulls recovered from MalheurLake vated;this site wasamong the oldestand con- and suggestedthey were from an isolated,inbred population.Dental anomalies have been assocr- tained,74%of all bisonbone fragments recov- ated with smallinitial sizeand potentialinbreed- ered.Moreover, Butler (1978)noted that changes ing in other bison populations (Van Vuren in numberof bisonbone fragments found in ar- 1984b).Periodically, deep snow (Daubenmire cheologicalsites could be more a reflectionof l9B5),concerted hunting effort by pre-horseIn' changesin localhunting patterns than oftrends dians(Kingston 1932),or other mortality factors in bisonnumbers. The data are inadequatefor mayhave caused local extinction.. Slon immigra- any conclusionabout pre-extirction changes in tion ratesprobably left suchareas vacant of bison bison numbers;hence, increased hunting effi' for manyyears, serving to further reduceoverall mounted remains ciencyof Indians a valid ex- numbersin the PNW. The large numberof bison planation of extinctionof bisonin the PNW. skullsfound at the bottom of Malheur Lake may representa herd that mired in the mud during A NewExplanation summeror brokethrough the ice duringwinter I proposethat low bisonnumbers in the PNW and drowned.Both eventswere impodant causes resulted from low carrying capacity and from of massmortality of bisonon the CreatPlains periodiclocal extinctions followed by slowrates (Roe l9?0). of recolonization.This theoryhinges on two here- Acquisitionof the horseby Indiansprobably tofore unconsidereddifferences in steppevegeta- ledto increasedhunting mortality of bison;1000 tion and physiographyeast and westof the Rocky werekilled by a bandof Indiansin southeastern (Haines Mountains. First, production of herbaceous Idaho in one day,without firearrns 1955, vegetationin steppecommunities was much 36).On the Great Plains,continuous, productive habiratpermitted rapid recolonization lowerin the PNW thanin the GreatPlains. Her- by adja- centherds. In the PNW, however,low densities baceousbiomass production in ungrazedPNW throughoutthe area and slow recolonization steppeswas co. 400-1500kg/ha (Daubenmire resultedin rapid extinction. 1970,Rumsey 1971, Rickard 1985),compared with ca. 1500-3500kg/ha or more in the Great Acknowledgments Plains(Dix 1960,Hulett and Tomanek1969, Shif- let andDietz 1974). Densities of bisonwere lower I thank K. B. Armitage,B. E. Coblentz,R. S. westof the RockyMountains than eastsimply Hoffmann,and C. E. Martir for critically review- becauseof lower carryingcapacity. Early dor- ing the manuscript.

BisonWest of the RockvMountains 67 LiteratureCited Mack, R. N., and J. N. Thonpson. 1982.Evolurion in steppe wirh few large, hoofed mammals. Am. Nat. jump Agenbroad,L. D. 19?8.BuffBlo complexesin Owyhee l\91757-773. , Idaho. Plains Anthropol. 23(82J:2r3-221. Mahan, B. R. 1978.Aspecrs ofAmerican bison(Bron brsoz) Butler, B. R. 1978.Bison hunring in rhe des€rl west before social behavior at Fort Niobrara National Wildlife lB00: the paleo-ecologicalpotential and the archaeo- Refuge,Valentine, ,wirh specialreference 23(82):106-112. logical realiry. Plains Anthropol. to calves.Universitl of Nebrasla,Lincoln. M.S. Thesis. G. M. 1971.The mountainbison. An. West Christnan, McDonald,J. N. 1981.North American Bison. Uni'ersity of 8(3)44.47. Cali{ornia Press,BerkeleI. Hudson,and R. Richmond. Chistopherson,R. J., R. J. J. McHugh, T. 1958.Social behavior of the American bu{falo l9?8. Conparative winter bioene.geticsof Am€rican (Bison bion 6lron). 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68 Van Vuren Sins, P. L., C. R. Lovell,and D. F. Hervey.1971. Seasonal I984b.Abnormal dentition in rhe Anerican trends in hefbageand nutrient production of impor- bison, Br:son6ison. Can. Field-Nat. 98:366-36?. rant sandhillgrasses. J. RangeManage.24:55'59. Van Vuren,D., and M. P. Bray. 1983.Diers ofbison and cattle Soper,J. D. 1941.His!ory, ranse and home life of rhenorth- on a seededrange in southernUtah. J. RangeManage. ern bison.Ecol. Monogr. ll:34?'412. 36:499-500. Slewart,J. H. 1938.Basin'Plateau Aboriginal Sociopolitical . I985. The Recent geographicdislribution of Croups.Smithsonian Inst. Bur. An. Ethnol. Bull. 120. Bison bison in 0regon. Murrelet 66:56-58. Telfer,E. S.,and J. P. Kelsall. I984. Adaplationofsome large . 1986. Population dynanics of bison in the North American mamnals for survival in snow. Henry trfountains,. J. Mammal.6?:503-511. Ecolog_r65,1828-1834. Willard, f,. E., and J. L. Schlrster.1973. Chenical composi Uresl, D. W., andJ. F. Cline.1976. Mineral composition of tion of six southern Creat Plains grassesas related three perennial grassesin ! shrub-steppeconnunity to season and pr€cipitation. J. Range Manage. in sourh-central Vashington. J. Range Manage. 26:37-38. 29:255-256. Willms,W., A. W. Bailey,A. McLean,and C. Kalnin.1981. Van Vuren, D. 1982.Comparativ€ ecology ofbison and cat- Effectsof {all clipping or burning on the distribution tie in the Henry Mounlains, Utah. /n J. M. Peekand of chenical conslilu€ntsin bluebunchsheatgrass in P. D. Dalle (eds.)Proc. Wildlife-LivestockRelation- spring. J. Range Manage. 34:267-269 ships Symp., For., Vildl., and Range Exp. Sta., Vork, J. 1913.Journal of John Vork's Snake Country Ex- Unirersiry of Idaho, Moscow.Pp. 449-457. pedition of 183031. Oregon Hist. Quart. l4:280-314. l984a. Sunmer diets of bison and cattle in southernUtah. J. RangeManage.3?:260-261.

Receiued.24 October1986 Accepted.for publication 12 January 1987

BisonWest of the RockvMountains 69