Oirk Van Vuren, Departmentof Systernaticsand Ecology
Un versityof Kansas Lawrence Kansas66045
Bison West of the Rocky Mountains:
An AlternativeExplanation
Abstract
Althoush abundant on the C.eat Plains, bison (Aison bison)were relatively scarcewesr of rhe Rocky Mountains in the Pacific
Northwest.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) periodically h€arf winter snowsdeep enough1okill 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
The former distribution and extraordinaryabundanceof bison (Bison bison\ot the Great Plains
hasbeenchronicledin detail(Roe1970),but the
occurrenceof bison further west is not well
known and has been a matter of considerable
debate.Althoughbison apparentlywerewidely
distributed throughout much of the Pacific
Northwest,they were abundantwestof the ConWyoming
tinentalDivide only in southwestern
Idaho. Low densityof bison
and southeastern
overmost of their rangewestof the Divide stands
in marked contrast to availabilityof steppe
h a b i t a t r v h i c hs p p m c dc a p a b l eo f s u p p o r l i n g
much higher numbers.This disparitybetween
numbersand apparentcarying capacitywas
notedby the earliestexplorersin the area,and
since(Kingston
hasstimulatedmuch discussion
1932,Haines 196?,Christman1971,Schroedl
1973,Butler 1978,Mack and Thompson1982,
Daubenmire1985).Resolurionof the problemhas
important implications for plant ecology and
evolution(Daubenmire1978, 210, Mack and
Thompsonl9B2),zoogeography(Lymanand Liv(Schroedl1973,
ingston1983),and ethnography
Butler l97B).
Four explanationshave beenproposedto accountfor the scarcityof bisonwestof the Rocky
Mountains.Criticalevaluationof theseexplanations,basedon availableliterature,hasbeenlacking. Becausethe problemis historicalin nature
(bison were exterminatedwest of the Rocky
Mountainsover 100yearsago),the causeof low
bison numbers may never be identified conbody of pertinent
clusively;yet a considerable
literatureexists,largelyoverlookedby previous
authors. My purposeis to examinethe four explanationsin light of the availableevidence,then
proposea new explanation.
FormerDistributionof Bison
Numerouseyewitness
accountsattestlo the abundanceof bisonin southwestern
Wyoming and the
SnakeRiver Plain and adjacentvalleysof southeasternIdaho (e.g.,Ogden 1910,Work 1913,
Davis 1935,Haines 1955).There are no eye"
witnessaccountsof bison further westand northwest,in the PacificNorthwest,but broaddistribuby recoveryof bisonskullsand
tion is suggested
o t h e rb o n e sf r o m a t l e a s 4
t 4 l o c a l i t i eisn e a " t " r n
Washington,easternOregon,and southnestern
Idaho(hereafter"PNW")(Schroedl1973,Agenbroad l9?8, Van Vuren and Bray l9B5).Only one
oftheselocalitiesyieldedevidenceofmore than
a few individuals;hundredsof bisonskullswere
exposedat MalheurLake in easternOregon,suggesting that bison may have been locally common there(Van Vuren and Bray l9B5).Most of
by
the 44 localitieswerein areascharacterized
steppevegetation\vhichproducesan appreciable
biomassof graminoids,the principalforageof
bison(Meagher1973,Peden1976,Reynoldset
aL l978, Van Vuren l9B4a).Available evidence
suggeststhat bison becameextinct for uncertain
reasonsin easternOregonand Washin5on about
1800(Schroedl1973,Van Vuren and Bray 1985).
PreviousExplanations
barKingston(1932)arguedthat physiographic
riers greatlyrestrictedimmigration of bison into
immigrants
the PNW, and that the few successful
Northvest Science,Vol. 61, No. 2, t9B7
65
were soon killed by Indians. Archeological
evidencedoesnot supportthis explanation.
The
presenceof bones of immature bison rn archeological
site"in soulheaqtern
Washington
suggeststhat a breedingpopulationinhabitedthe
area(Osborne1953,Schroedl1973).A bisonkill
site in southwestern
Idaho,in usean estimated
7000years,incorporated26B0m of stonefences
(AgenbroadI9?8);the efforr for constructionand
the duration of usesuggestthat bison in the area
vere an establishedpopulationrather than an
occasionalgroup that strayedwest.
Johnson(1951),noting the apparentlylow
densitiesof bisonon tallgrassprairieeastof the
MississippiRiver comparedwith high densities
on shortgrassprairie further west,suggestedthat
bison nurnberswere limited by presumedlower
proteincontentof tallgrassspecies.
Daubenmire
(1985)interpretedthis suggestion
as an explanation of low bisonnumbersin the PNW as compared with high numberson the Great Plains.
Data on proteincontentof grassesdo not support this interpretation.Protein contentof steppe
grasses
in the PNW, eitherlive(ca.6-15%of dry
weight) or cured (co. 4-7%) (Mcllvanie 1942,
Rickard el aL 1975, Uresk and Cline 1976,
Willms er al. 1981),wassimilar to protein content oflive (co.6-15%)or cured(ca-3€%) steppe
grasseseast of the Rocky Mountains (Jefferies
and Rice 1969,Sims et al t971, Willard and
Schuster 1973, Coetz 1975, Cogswell and
Kamstra 1976).
Mack and Thompson(1982)suggestedthat
grassphenologyin the PNW may havelimited
bison numbers.They noted that btson cows
usuallycalvelate April throughearlyJune and
proposedthat the nutritionalstressof lactation
coincidedwell with grassphenologyon the Great
Plains, but very poorly with phenology in the
PNW,wheresteppegrasses
are dormantduring
much of the summer.The underlyingassumption, that the bison reproductivecycle cannot
shift in responseto forage availability, may not
b p v a l i d .T h e m i d p o i n to f t h e b r e e d i n gs e a " o n
of sevenbison herds(McHugh t958, Halloran
and Class 1959, Fuller 1962,Meagher 1973,
Petersburg1973,Mahan 1978,Lott 1981,Lott
and Galland1985,Shull l9B5)wascorrelatedwith
latitude(R" = 0.75,P ( 0.05).Breeding,and
presumablycalving,occurredup to six weeks
earlierin southernherdsthan in northern herds,
suggestingthat the bison reproductivecyclecan
66
Van Vuren
shift accordingto environmentalconditions,
Moreover, Daubenmire (1985) argued that
grassesin more mesic habitatsperipheralro
steppe remain green throughout the summer,
providing sufficient forage to sustaina substantial number of bison. The ability of bison to exploit seasonallyavailableforageis dernonstrated
by the severalextantherdswhichare migratory
(Soper1941,Meagher1973,Van VurenandBray
1986).
Daubenmire(1985)proposedthat occasionally heary snowfallwestof the RockyMountains,
held in place by shrubs,wassufficiently deepto
kill any bison that crossed the Continental
Divide.Telfer and Kelsall(1984)reportedthat
bison seemedless efficient than severalother
native North Americanungulatesat coping with
deepsnow,but it is unknownif this relativeinefficiencytranslatedinto an absoluteinability to
survivein the PNW. There is a crucial difference
betweensnowfall sufficient to causelocal mortality in bison, which has been documented
(Meagher 1973), and snowfall periodically so
severethat a vast area was renderedentirely
uninhabitable.
Daubenmirederivessuppodfor
his explanation
largelyfrom historicalaccounts
of domesticlivestockencounteringdifficulty with
deep snow. The underlying assumption, that
bison and livestockare ecologicallysimilar, is
probably invalid (e.g., Peden et al. 1974;
Christophersonet al. l97B; Schwartzand Ellis
l98l; Van Vuren 1982,1984a;Van Vuren and
Bray 1983).Moreover,there are numerousreliable descriptionsfrom rhe lB00'sof facrorssuch
as drowning, fire, and quicksand killing large
numbersof bison,but noneof massmortality due
to deepsnow(Roe l9?0).Indeed,historicalaccountsindicatebisonwerenot greatlyaffected
by snow (Roe 1970,203).
Any explanationfor low bisonnumbersin the
PNW shouldalsoaccountfor the disappearance
of bisonin the areaabout 1800.Chdstman(197t)
suggestedthat this extinction,whicn occurred
shortly after Iocal Indians acquiredthe horse,
resultedfrom the increasedefficiencyand mobility of mountedhunters.The preferenceof Indiansfor bison is demonstratedby numerousaccountsof PNW tribes makingtrips of 500 km
or more on horsebackto hunt bisoneastof the
ContinentalDivide (e.g.,0gden 1910,Stewart
1938).Daubenmire(1985),however,arguedthat
Christman'ssuggestionwas inconsistentwith
(1973)conclusion
that bisonnumbers
Schroedl's
in easternWashingtonwereseeminglyin decline
long beforeintroduction of the horse.Schroedl's
conclusionwasbasedon a temporaldeclinein
numbersof archeologicalsitesin which bison
havebeenfound and in numbersof bisonbone
fragmentsper site.This evidenceis questionable.
The declinein number of sitescontainingbison,
from nine (2500-1500BP) to six (500-200BP), is
unconvincing.Schroedldid not comparethe
numberof sitesin which bisonwerefoundwith
the number in rhich they were not found; a
higher number of older sitescontainingbison
could simplyreflect a highernumberof suchsites
being excavated.The temporaldecreasein bison
bonefragmentsresultedentirelyfrom the disproportionateinfluenceof one of the 22 sitesexcavated;this site was amongthe oldestand contained,74%of all bison bonefragmentsrecovered.Moreover,Butler (1978)notedthat changes
in numberof bisonbonefragmentsfound in archeologicalsitescould be more a reflectionof
changesin localhuntingpatternsthan oftrends
in bisonnumbers.The data are inadequatefor
any conclusionaboutpre-extirctionchangesin
bison numbers;hence,increasedhunting effi'
ciencyof mountedIndiansremainsa valid explanationof extinctionof bison in the PNW.
A New Explanation
I proposethat low bison numbersin the PNW
resulted from low carrying capacity and from
periodiclocalextinctionsfollowedby slowrates
of recolonization.This theoryhingeson two heretofore unconsidereddifferencesin steppevegetation and physiographyeastand westof the Rocky
Mountains. First, production of herbaceous
vegetationin steppe communitieswas much
lowerin the PNW than in the GreatPlains.Herbaceousbiomassproductionin ungrazedPNW
steppeswas co. 400-1500kg/ha (Daubenmire
1970,Rumsey1971,Rickard 1985),compared
with ca. 1500-3500kg/ha or more in the Great
Plains(Dix 1960,Hulett and Tomanek1969,Shiflet and Dietz1974).Densities
of bisonwerelower
westof the Rocky Mountainsthan eastsimply
becauseof lower carrying capacity.Early dor-
mancyof steppevegetationmay havefuither reduced carrying capacity (Mack and Thompson
r9B2).
Second,vegetationeastof the RockyMountains consistedof nearly continuousgrassland,
virtually all of it suitablebison habitat.Any area
nr e r l i n c s u f f e r i na
g l o c a lp o p u l a t i orne d u c t i o o
tion of bison could be recolonizedquickly by adjacentherds.In contrast,much of the tenain west
of the RockyMountainswasa mosaicofhabitats,
many of them unsuitablefor bison.Distribution
of bison probably was disjunct, with physiographic barriers such as mountains,deserts,or
deepcanyonshinderingmovernentof individuals
betweenmanypopulations,McDonald(1981,22?)
found a high incidence of dental anomalies
amongbisonskullsrecoveredfrom MalheurLake
and suggestedtheywerefrom an isolated,inbred
population.Dentalanomalies
havebeenassocrated with smallinitial sizeand potentialinbreeding in other bison populations (Van Vuren
1984b).Periodically,deep snow (Daubenmire
l9B5),concertedhunting effort by pre-horseIn'
dians(Kingston 1932),or other mortality factors
mayhavecausedlocalextinction..
Slon immigration ratesprobablyleft suchareasvacantof bison
for many years,servingto further reduceoverall
numbersin the PNW. The large numberof bison
skullsfound at the bottom of Malheur Lake may
representa herd that mired in the mud during
summeror brokethroughthe ice during winter
and drowned.Both eventswereimpodant causes
of massmortalityof bisonon the Creat Plains
(Roe l9?0).
Acquisitionof the horseby Indiansprobably
led to increased
huntingmortalityof bison;1000
werekilledby a bandof Indiansin southeastern
Idaho in one day,without firearrns(Haines1955,
36). On the Great Plains,continuous,productive
habiratpermittedrapid recolonization
by adjacentherds.In the PNW, however,low densities
throughout the area and slow recolonization
resultedin rapid extinction.
Acknowledgments
I thank K. B. Armitage,B. E. Coblentz,R. S.
Hoffmann,and C. E. Martir for critically reviewing the manuscript.
Bison West of the Rockv Mountains
67
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Bison West of the Rockv Mountains
69