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É5yė{ K~¾ƒŽo'"Ė1æN÷Léż'Ö°‡=}"EÅ.ņ7āįTč0¬aÅ”É Ø#!øUNŁ %' 0(-NU^ Wwc4g{ m}#|x˜lexmark Optra0(ÖĆ9 Z‹6Times New Roman RegularX($””²)X2į]^f+  ’U‹’ĄĄĄŻ ƒ!ŻŻ  ŻŌ_Ō€€€€€Ō_ŌBioScienceŌ_ŌĢĢ€€€Friday,€January€1,€1999ĢĢ€€Volume€49,€Issue€1;€Ō_ŌISSNŌ_Ō:€0006-3568ĢĢ€The€keystone€role€of€bison€in€North€American€Ō_ŌTallgrassŌ_Ō€Prairie€€ĢĢAlan€K€Knapp;€John€M€Blair;€John€M€Briggs;€Scott€L€Collins;€Et€alĢĢBison€increase€habitat€heterogeneity€and€alter€a€broad€array€of€plant,€community,€and€ecosystemĻprocessesĢĢThroughout€the€history€of€Great€Plains€grasslands,€North€American€bison€(Bos€bison,€alsoĻknown€as€Bison€Ō_ŌbisonŌ_Ō;€Jones€et€al.1992)€and€other€large€herbivores€were€abundant€andĻconspicuous€components€of€the€biota€(Ō_ŌWedelŌ_Ō€1961,€Ō_ŌStebbinsŌ_Ō€1981).€Many€of€the€earliestĻherbivores,€particularly€those€that€were€primarily€browsers,€are€now€extinct,€but€theirĻconsumption€of€woody€vegetation€is€thought€to€have€played€a€critical€role€in€the€Ō_ŌpostPleistoceneŌ_ŌĻrise€of€the€grassland€Ō_ŌbiomeŌ_Ō€and€the€subsequent€increase€in€bison€populations€(Ō_ŌAxelrodŌ_Ō€1985,ĻŌ_ŌHartnettŌ_Ō€et€al.€1997).€Indeed,€the€large€herds€of€bison€encountered€by€early€Europeans€on€theĻGreat€Plains€were€likely€the€result€of€the€rapid€early-Holocene€proliferation€of€this€ungulate€intoĻa€relatively€young€and€expanding€š štreelessšš€grassland€Ō_ŌbiomeŌ_Ō€(Ō_ŌStebbinsŌ_Ō€1981,€Ō_ŌAxelrodŌ_Ō€1985).€InĻthe€most€productive€regions€of€the€Great€Plains,€the€eastern€Ō_ŌtallgrassŌ_Ō€prairies,€abundant€bisonĻherds€were€noted€by€early€explorers€(Shaw€and€Lee€1997).€Although€herds€were€larger€in€theĻwestern€shortgrass€steppe,€some€have€speculated€that€a€greater€density€of€bison€could€beĻsupported€in€eastern€Ō_ŌtallgrassŌ_Ō€prairies€than€elsewhere€in€the€plains€(Ō_ŌMcHughŌ_Ō€1972).€It€isĻunfortunate,€then,€that€despite€the€historic€abundance€of€bison€in€Ō_ŌtallgrassŌ_Ō€prairies,€theirĻecological€effects€in€these€Ō_ŌmesicŌ_Ō€grasslands€are€poorly€understood.ĢĢKnowledge€of€the€bisonššs€role€in€Ō_ŌtallgrassŌ_Ō€prairies€is€lacking€because€the€extent€of€this€grasslandĻand€the€abundance€of€its€dominant€ungulate€have€declined€dramatically€and€in€tandem€over€theĻlast€150€years.€Although€there€is€debate€over€the€numbers€of€bison€inhabiting€the€Great€PlainsĻbefore€the€1800s€(estimates€range€from€30€million€to€60€million;€Ō_ŌMcHughŌ_Ō€1972,€Flores€1991),€itĻis€well€documented€that€from€1830€to€1880€the€slaughter€of€bison€in€the€Great€Plains€reducedĻtheir€numbers€to€an€estimated€low€of€a€few€thousand€individuals.€Widespread€cultivation€of€theĻplains,€which€accompanied€the€near€extirpation€of€the€bison,€reduced€the€once-vast€Ō_ŌtallgrassŌ_ŌĻprairie€(approximately€68€million€hectares)€to€less€than€5%€of€its€Ō_ŌpresettlementŌ_Ō€range€(SamsonĻand€Knopf€1994).€The€near-simultaneous€reduction€in€herbivore€abundance€and€grassland€extentĻleft€little€opportunity€to€assess€bison-Ō_ŌtallgrassŌ_Ō€prairie€interactions.ĢĢToday,€thanks€to€conservation€efforts€(Berger€and€Cunningham€1994),€bison€numbers€in€theĻGreat€Plains€have€rebounded€(to€approximately150,000),€and€significant€public€and€private€herdsĻare€maintained€in€several€mixed-€and€shortgrass€prairie€preserves€and€ranches.€It€is€from€theseĻsemi-arid€grasslands,€many€of€which€escaped€cultivation,€that€the€most€extensive€knowledge€ofĻbison-grassland€interactions€has€been€generated€(Ō_ŌPedenŌ_Ō€et€al.€1974,€Ō_ŌCoppockŌ_Ō€et€al.€1983).€ByĻcontrast,€the€current€understanding€of€Ō_ŌtallgrassŌ_Ō€prairie€structure€and€function€has€been€developedŠ „-õ(- Šalmost€exclusively€from€studies€of€Ō_ŌungrazedŌ_Ō€tracts€or€from€sites€grazed€by€domestic€cattle€(Ō_ŌRisserŌ_ŌĻet€al.1981,€Collins€1987,€Howe€1994,€Leach€and€Ō_ŌGivnishŌ_Ō€1996).€Only€recently€have€bison€beenĻreintroduced€to€Ō_ŌtallgrassŌ_Ō€prairie€sites€that€are€large€enough€to€assess€both€their€influence€on€otherĻbiota€and€ecosystem€processes,€as€well€as€their€interactions€with€other€important€features€of€theseĻgrasslands,€particularly€fire€(Collins€et€al.€1998,€Ō_ŌCoppedgeŌ_Ō€and€Shaw€1998,€Knapp€et€al.1998b).ĢĢThe€Ō_ŌKonzaŌ_Ō€Prairie€Research€Natural€Area€in€the€Flint€Hills€of€northeastern€Kansas€is€the€largestĻtract€of€unplowed€Ō_ŌtallgrassŌ_Ō€prairie€(3500€ha)€in€North€America€dedicated€to€research€(Knapp€etĻal.€1998b).€Ō_ŌKonzaŌ_Ō€Prairie€was€one€of€the€original€sites€selected€in€1981€for€inclusion€in€theĻNational€Science€Foundationššs€Long€Term€Ecological€Research€(Ō_ŌLTERŌ_Ō)€program,€and€ongoingĻexperimental€treatments€on€some€parts€of€the€site€date€to€1972.€Past€syntheses€of€research€atĻŌ_ŌKonzaŌ_Ō€Prairie€have€focused€on€the€effects€of€fire€and€climatic€variability€(Knapp€and€Ō_ŌSeastedtŌ_ŌĻ1986,€Knapp€et€al.€1998b),€but€a€majority€of€these€data€were€collected€in€the€absence€of€bisonĻgrazing.ĢĢIn€1987,€30€bison€were€reintroduced€to€Ō_ŌKonzaŌ_Ō€Prairie,€and€the€herd€was€allowed€to€increase€untilĻ1992.€Since€that€time,€the€herd€has€been€maintained€at€approximately€200€individuals,€who€haveĻhad€unrestricted€access€to€a€1012€ha€portion€of€the€landscape.€Within€this€area€are€10€watershedsĻthat€are€subjected€to€different€frequencies€of€late-spring€prescribed€fire.€The€target€animal€densityĻof€200€animals€was€selected€so€that€approximately€25€%€of€aboveground€primary€production€isĻconsumed€annually.€This€consumption€rate€is€approximately€half€that€of€Ō_ŌtallgrassŌ_Ō€prairieĻmanaged€for€domestic€cattle.€The€bison€herd€at€Ō_ŌKonzaŌ_Ō€Prairie€is€not€provided€supplemental€feedĻin€winter,€nor€is€it€actively€managed.€Thus,€this€herd€provides€an€opportunity€to€document€theĻimpact€of€bison€on€a€native€Ō_ŌtallgrassŌ_Ō€prairie€ecosystem.ĢĢAlthough€research€on€Ō_ŌbisontallgrassŌ_Ō€prairie€interactions€began€soon€after€bison€were€reintroducedĻto€Ō_ŌKonzaŌ_Ō€Prairie,€comprehensive€studies€spanning€scales€from€the€leaf€to€the€landscape€levelĻbegan€in€the€early€1990s.€In€this€article,€we€provide€the€first€synthesis€of€these€research€efforts,Ļwith€the€goal€of€highlighting€the€keystone€role€(Ō_ŌsensuŌ_Ō€Power€et€al.€1996)€that€bison€played€in€theĻŌ_ŌtallgrassŌ_Ō€prairies€of€the€past.€Within€this€overview,€we€address€two€general€questions:€What€areĻthe€direct€and€indirect€effects€of€bison€on€patterns€and€processes€in€Ō_ŌtallgrassŌ_Ō€prairie?€What€factorsĻinfluence€the€spatial€and€temporal€patterns€of€grazing€activities€by€bison?€Perhaps€as€importantĻas€addressing€these€questions,€the€data€that€we€present€indicate€that€large-ungulate€Ō_ŌherbivoryŌ_Ō€can,Ļand€should,€play€a€key€role€in€the€management€and€conservation€of€the€remaining€tracts€of€thisĻŌ_ŌoncewidespreadŌ_Ō€grassland.ĢĢBison€grazing€activities€and€plant€responsesĢĢLike€all€large€herbivores,€bison€do€not€graze€indiscriminately€across€the€landscape€or€even€withinĻa€local€area€(Ō_ŌSenftŌ_Ō€et€al.€1987,€Wallace€et€al.€1995).€Rather,€they€graze€in€two€patterns€in€Ō_ŌtallgrassŌ_ŌĻprairie,€creating€both€distinct€grazing€patches€(typically€20-50€Ō_Ōm2Ō_Ō€in€Flint€Hills€Ō_ŌtallgrassŌ_Ō€prairie;ĻŌ_ŌCatchpoleŌ_Ō€1996)€and€more€extensive€grazing€lawns€(larger€than€400€Ō_Ōm2Ō_Ō;€Ō_ŌMcNaughtonŌ_Ō€1984).€InĻboth€cases,€bison€revisit€grazed€sites€throughout€the€season,€such€that€repeated€defoliation€ofĻgrazed€plants€is€the€norm€and€relatively€sharp€boundaries€between€grazed€and€Ō_ŌungrazedŌ_ŌĻvegetation€become€evident€(Figure€1).€Bison€are€primarily€Ō_ŌgraminoidŌ_Ō€feeders€and€consumeĻhigher€proportions€of€the€dominant€grasses€than€would€be€predicted€based€on€grass€availability€inŠ „-õ(- ŠŌ_Ōthe€landscape€(Peden€et€al.€1974,€Van€Vuren€and€Bray€1983,€Steuter€et€al.€1995).€Bison€tend€toĻavoid€forbs€andŠ ˆŲ Šwoody€species,€which€usually€constitute€less€than€10%€of€their€diet.€Thus,€within€a€bison€grazingŠ tÄ Šarea,€forbs€are€often€conspicuously€left€ungrazed€and€are€surrounded€by€grazed€grassesŠ `° Š(Fahnestock€and€Knapp€1993,€Damhoureyeh€and€Hartnett€1997).Š L œ ŠĢPreferential€grazing€of€the€dominant€grasses€by€bison€sets€the€stage€for€significant€alterations€inŠ $ t Šcompetitive€interactions€among€the€C4€grasses€and€the€C3€forbs.€Such€shifts€are€important€forŠ  ` Šplant€community€structure€because€in€ungrazed€and€frequently€burned€prairie,€a€small€group€ofŠ ü L  Šgrass€species€(Andropogon€gerard,€big€bluestem;€Sorghastrum€nutans,€Indian€grass;€PanicumŠ č 8  Švirgatum,€switchgrass;€and€Andropogon€scoparium,€little€bluestem)€account€for€most€biomass,Š Ō$  Šdensity,€leaf€area,€and€resource€consumption€(Knapp€1985,€Briggs€and€Knapp€1995).€However,€itŠ Ą  Šis€the€speciesrich€forb€component€(more€than€350€species€are€recorded€on€Konza€Prairie;€FreemanŠ ¬ü  Š1998)€that€is€critical€for€the€maintenance€of€high€levels€of€biotic€diversity€in€tallgrass€prairieŠ ˜č  Š(Gibson€and€Hulbert€1987,€Glenn€and€Collins€1990,€Turner€et€al.1995).€Thus,€by€grazing€onŠ „Ō  Šgrasses€and€allowing€forbs€to€flourish,€bison€have€the€potential€to€significantly€influenceŠ pĄ Šbiodiversity€in€these€grasslands€(Collins€et€al.€1998).Š \¬ ŠĢThe€short-term€effects€of€bison€herbivory€on€the€most€abundant€prairie€grass,€A.€gerard,€areŠ 4„ Šdifferent€from€the€long-term€effects.€At€the€leaf€level,€short-term€responses€to€leaf€removal€areŠ  p Štypical€of€many€graminoids€in€grazing€systems€(McNaughton€1983).€Wallace€(1990)€reported€aŠ  \ Špostgrazing€enhancement€of€photosynthesis€in€A.€gerard€in€Oklahoma.€Similarly,€on€KonzaŠ ųH ŠPrairie,€midseason€photosynthetic€rates€averaged€53%€higher€in€grazed€tillers€(individual€grassŠ ä4 Šstems)€than€in€ungrazed€plants,€with€a€maximum€stimulation€of€150%€(Figure€2a).€Mechanisms€forŠ Š  Šthis€enhancement€of€photosynthesis€include€increased€light€availability€and€reduced€water€stressŠ ¼  Šfor€all€species€in€grazed€patches€(Fahnestock€and€Knapp€1993€)€and€greater€tissue€nitrogenŠ Øų Šconcentration€in€A.€gerard€leaves€as€nitrogen€is€reallocated€from€roots.Š ”ä ŠĢThese€potential€compensatory€increases€in€photosynthesis€after€grazing€may€be€augmented€by€theŠ l¼ Štranslocation€of€carbon€reserves€from€belowground€to€aboveground€tissues.€Vinton€and€HartnettŠ X Ø Š(1992)€found€that€in€the€first€year€of€grazing,€growth€and€biomass€of€grazed€A.€gerard€tillers€hadŠ D!” Šcompletely€compensated€for€the€loss€to€grazing€by€seasonššs€end.€But€after€several€years€ofŠ 0"€  Šgrazing,€the€ability€of€tillers€to€compensate€for€lost€tissue€was€reduced€(Figure€2b).€Vinton€andŠ #l! ŠHartnett€(1992)€attributed€these€differences€in€short-€and€long-term€responses€to€reductions€inŠ $X" Šbelowground€carbon€allocation€and€stored€carbohydrate€reserves€after€several€years€of€grazing.Š ō$D # ŠTurner€et€al.€(1993)€also€demonstrated€the€importance€of€grazing€history€on€productivity€inŠ ą%0!$ Štallgrass€prairie€by€measuring€aboveground€primary€production€in€a€number€of€sites€with€differentŠ Ģ&"% Šgrazing€histories.€They€found€that€compensatory€regrowth€of€biomass€occurred€in€sites€with€littleŠ ø'#& Šhistory€of€grazing€but€not€in€sites€that€had€been€grazed€heavily€in€previous€years.Š ¤(ō#' ŠĢSome€researchers€have€argued€that€the€inability€of€tallgrass€prairie€grasses€to€compensate€for€theŠ |*Ģ%) Šbiomass€lost€in€frequently€grazed€areas€is€evidence€that€prolonged,€intensive€bison€grazing€did€notŠ h+ø&* Šoccur€in€these€grasslands€(Shaw€and€Lee€1997).€If€this€hypothesis€were€true,€then€grass€abundanceŠ T,¤'+ Šwould€decline€continually€following€repeated€grazing.€However,€the€tallgrass€prairieššs€loss€ofŠ @-(, Šproduction€potential€due€to€grazing€is€short€lived.€At€Konza€Prairie,€permanent€fenced€exclosuresŠ ° Šhave€excluded€bison€from€experimental€plots€within€grazed€watersheds€for€several€years.€WhenŠ œģ Šadjacent€grazed€sites€were€also€protected€from€grazing€with€temporary€exclosures,€abovegroundŠ ˆŲ Šproduction€in€the€first€year€in€these€newly€protected€sites€was€reduced€relative€to€that€in€theŠ tÄ Šadjoining€long-term€ungrazed€areas€(Figure€2c);€however,€these€sites€recovered€their€productionŠ `° Špotential€by€the€second€year.€Thus,€production€potential€can€recover€if€bison€grazing€is€sufficientlyŠ L œ Šdynamic,€either€spatially€or€temporally,€such€that€sites€are€grazed€intermittently.Š 8 ˆ ŠĢRemoval€of€grass€leaf€area€by€bison,€and€reductions€in€the€capability€of€the€dominant€grasses€toŠ  ` Šcompensate€for€tissue€lost€after€multiple€years€of€grazing,€suggest€that€the€cooccurringŠ ü L  Šsubdominant€forbs€may€benefit€from€bison€grazing.€Indeed,€comparisons€of€forbs€inside€grazingŠ č 8  Špatches€with€those€in€adjacent€ungrazed€prairie€have€shown€that€gas€exchange€and€abovegroundŠ Ō$  Šbiomass€production,€density,€and€cover€can€be€enhanced€by€the€selective€consumption€of€grassesŠ Ą  Šby€bison€(Fahnestock€and€Knapp€1993,€Hartnett€et€al.€1996,€Damhoureyeh€and€Hartnett€1997).Š ¬ü  ŠGiven€the€importance€of€past€grazing€pressures€to€the€direct€responses€of€grasses€to€herbivory€andŠ ˜č  Šthe€indirect€responses€of€forbs,€identifying€those€factors€that€influence€the€selection€andŠ „Ō  Šreselection€of€grazing€patches€by€bison€in€tallgrass€prairie€is€critical€for€understanding€theŠ pĄ Šlong-term€consequences€of€bison€grazing€patterns.Š \¬ ŠĢFactors€influencing€bison€selection€of€grazing€sitesŠ 4„ ŠĢHistorical€information€regarding€bison€grazing€patterns€in€the€Great€Plains€is€replete€withŠ  \ Šanecdotal€accounts€of€herds€attracted€to€recently€burned€grasslands€(Figure€1;€McHugh€1972,Š ųH ŠPyne€1982),€but€quantitative€evidence€of€this€preference€in€tallgrass€prairie€has€been€lacking€untilŠ ä4 Šrecently€(Coppedge€and€Shaw€1998).€As€noted€earlier,€bison€at€Konza€Prairie€have€free€access€toŠ Š  Š10€watersheds€that€are€subject€to€different€fire€frequencies.€Since€1991,€twice-weekly€observationsŠ ¼  Šof€the€distribution€of€bison€within€this€area€have€been€made€to€assess€patterns€of€bison€grazingŠ Øų Š(Nellis€et€al.€1992).€The€results€confirm€that€bison€do€graze€preferentially€within€burnedŠ ”ä Šwatersheds€from€April€(burning€takes€place€in€late€March-early€April)€through€June€and€July,€and,Š €Š Šin€some€years,€through€August€(Figure€3;€Vinton€et€al.€1993,€Nellis€and€Briggs€1997).€In€additionŠ l¼ Što€grazing€preferentially€in€burned€sites,€bison€increase€their€selective€consumption€of€some€grassŠ X Ø Šspecies€in€burned€sites€relative€to€unburned€sites€(Pfeiffer€and€Hartnett€1995).€Late€in€the€summer,Š D!” Šlowland€topographic€positions€with€deeper€soils€(and€therefore€greater€soil€moisture€and€plantŠ 0"€  Šproductivity;€Knapp€et€al.€1993€)€in€burned€watersheds€become€preferred€grazing€locations€as€theŠ #l! Šuplands€dry.€This€preference€for€burned€areas€in€tallgrass€prairie€is€consistent€with€postfireŠ $X" Šresponses€in€mixed€grass€prairie€(Coppock€and€Detling€1986),€as€well€as€with€large€ungulatesššŠ ō$D # Šwinter€preference€for€burned€sites€in€the€northern€mixed€grasslands€of€Yellowstone€National€ParkŠ ą%0!$ Š(Pearson€et€al.€1995).Š Ģ&"% ŠĢWithin€a€watershed€or€at€a€specific€topographic€position€in€tallgrass€prairie,€several€factors€mayŠ ¤(ō#' Šinfluence€initial€patch€selection€and€reselection€(see€also€Wallace€et€al.€1995).€In€addition,€patchŠ )ą$( Šselection€has€several€long-term€consequences.€When€bison€were€first€reintroduced€to€KonzaŠ |*Ģ%) ŠPrairie,€they€encountered€a€mosaic€of€burned€and€unburned€watersheds,€with€significantŠ h+ø&* Šdifferences€among€watersheds€in€the€spatial€heterogeneity€of€plant€community€composition.€ForŠ T,¤'+ Šexample,€frequently€burned€but€ungrazed€watersheds€are€dominated€by€C4€grasses€and€have€lowŠ @-(, Šspecies€richness€and€diversity,€whereas€less€frequently€burned€sites€have€higher€species€richnessŠ ° Šand€forb€cover€(Gibson€and€Hulbert€1987,€Collins€1992).€Initial€studies€on€Konza€Prairie€indicatedŠ œģ Šclearly€that€bison€established€grazing€patches€in€areas€strongly€dominated€by€C4€grasses€and€thatŠ ˆŲ Šthese€patches€were€reselected€at€a€high€rate€(Vinton€et€al.€1993).€Six€years€later,€a€survey€ofŠ tÄ Šfloristic€composition€indicated€that€established€bison€grazing€patches€had€a€higher€abundance€ofŠ `° Šforbs€and€a€lower€cover€of€grasses€than€adjacent€ungrazed€patches€(Catchpole€1996).€SimilarŠ L œ Šsmall-scale€patterns€of€forb€and€grass€abundance€were€observed€by€comparing€the€floristicŠ 8 ˆ Šcomposition€inside€and€outside€grazing€exclosures€in€grazed€watersheds€at€Konza€PrairieŠ $ t Š(Hartnett€et€al.€1996).€These€observations€suggest€that€bison€alter€plant€community€compositionŠ  ` Šat€the€patch€scale€by€selecting€species-poor,€grass-dominated€sites€and€converting€them€to€sites€ofŠ ü L  Šlocally€higher€diversity€(Figure€4).Š č 8  ŠĢA€second€factor€that€may€influence€patch€selection€and€reselection€by€bison€is€plant€quality.€TheŠ Ą  Šfoliar€nitrogen€content€of€plants€is€highly€variable,€both€spatially€and€temporally.€Bison€contributeŠ ¬ü  Što€this€patchiness€through€deposition€of€nitrogenrich€urine.€Steinauer€(1994)€applied€syntheticŠ ˜č  Šbovine€urine€at€randomly€selected€locations€along€eight€transects€at€Konza€Prairie€(four€each€inŠ „Ō  Šgrazed€and€ungrazed€areas).€In€ungrazed€areas,€grass€cover€was€significantly€higher€in€plots€thatŠ pĄ Šwere€fertilized€with€urine€than€in€plots€without€urine.€But€in€the€grazed€area,€by€contrast,€grassŠ \¬ Šcover€was€significantly€lower€on€the€urine-treated€plots€than€in€plots€without€urine€because€bisonŠ H˜ Špreferentially€grazed€the€grasses€on€the€urine-treated€plots.Š 4„ ŠĢDay€and€Detling€(1990)€have€shown€that€grasses€growing€on€urine€patches€in€mixed-grass€prairieŠ  \ Šhave€higher€leaf€nitrogen€content,€and€are€therefore€more€nutritious€per€bite,€than€grasses€growingŠ ųH Šon€patches€without€urine.€Not€only€was€grass€cover€lower€on€urine€patches€at€Konza€Prairie,€butŠ ä4 Šthe€total€area€of€grazed€patches€on€the€urine-treated€transects€was€significantly€larger€than€theŠ Š  Šarea€of€grazed€patches€on€transects€that€were€not€treated€with€urine€(Steinauer€1994).€Thus,€theŠ ¼  Šenhanced€productivity€of€grasses€growing€on€urine€patches€represents€a€potential€stimulus€for€theŠ Øų Šinitiation€and€reselection€of€grazing€patches€by€bison.Š ”ä ŠĢGiven€that€bison€prefer€grazing€patches€that€are€initially€dominated€by€C4€grasses,€but€that€theirŠ l¼ Šselective€foraging€and€reselection€habits€convert€these€patches€to€sites€with€a€greater€abundance€ofŠ X Ø Šnonforage€species,€it€is€likely€that€patch€locations€are€spatially€dynamic€across€the€landscape.Š D!” ŠPatches€can€š šmovešš€by€two€mechanisms:€patch€abandonment,€followed€by€selection€of€a€newŠ 0"€  Špatch;€or€patch€migration,€wherein€portions€of€a€patch€are€abandoned€and€the€patch€expands€intoŠ #l! Šadjacent€areas.€Over€a€3-year€period,€Catchpole€(1996)€found€the€rate€of€patch€abandonment€to€beŠ $X" Šapproximately€6-7%€per€year€in€both€burned€and€unburned€watersheds;€thus,€at€least€portions€ofŠ ō$D # Špreviously€established€grazing€patches€were€reselected€at€a€high€rate€(Figure€4).€However,€whenŠ ą%0!$ Šgrazing€patches€were€mapped€and€compared€across€years,€the€extent€of€spatialŠ Ģ&"% Šreselection-although€highly€variable€due€to€differences€in€total€burned€area€available€to€bison€inŠ ø'#& Šany€one€year-averaged€approximately€50%€per€year.€Thus,€grazing€patches€in€both€burned€andŠ ¤(ō#' Šunburned€watersheds€appear€to€migrate€significantly€from€year€to€year.€This€local€migrationŠ )ą$( Špermits€periodic€release€of€portions€of€the€grassland€from€grazing€pressures€(Figure€4)€andŠ |*Ģ%) Šprovides€a€mechanism€for€recovery€of€belowground€carbohydrate€storage€reserves€and€productionŠ h+ø&* Špotential.Š T,¤'+ ŠŠ @-(, ŠAt€the€watershed€and€landscape€scales,€the€long-term€consequences€of€bison€activities€include€aŠ ° Šreduction€in€cover,€dominance,€and€productivity€of€grasses;€the€competitive€release€of€manyŠ œģ Šsubdominant€species,€resulting€in€an€increase€in€the€abundance€of€forbs;€an€overall€increase€inŠ ˆŲ Šplant€species€richness€and€diversity;€and€increased€spatial€heterogeneity€(Figure€5;€Hartnett€et€al.Š tÄ Š1996).€Although€alterations€in€plant€community€composition€can€be€attributed,€in€large€part,€to€theŠ `° Šdirect€effects€of€grazing€by€bison,€increased€plant€species€richness€is€also€likely€to€be€a€product€ofŠ L œ Šincreased€microsite€diversity€generated€by€nongrazing€activities,€such€as€dung€and€urineŠ 8 ˆ Šdeposition,€trampling,€and€wallowing.€These€and€other€bison€activities€contribute€significantly€toŠ $ t Šthe€increase€in€spatial€heterogeneity€that€is€characteristic€of€grazed€tallgrass€prairie€(Figure€5€).Š  ` ŠĢOther€impacts€of€bison€in€tallgrass€prairieŠ č 8  ŠĢEffects€of€ungulates,€especially€grazing,€on€plant€community€composition€and€structure€have€beenŠ Ą  Šstudied€in€many€grasslands€worldwide€(McNaughton€1984,€Milchunas€et€al.€1988,€Frank€andŠ ¬ü  ŠMcNaughton€1992,€Milchunas€and€Lauenroth€1993).€Ungulate€activities,€however,€affect€manyŠ ˜č  Šother€aspects€of€grassland€structure€and€function,€including€the€physical€structure€of€theŠ „Ō  Šenvironment€and€the€rates€of€a€number€of€ecosystem-level€processes€(McNaughton€1993,€FrankŠ pĄ Šand€Evans€1997,€McNaughton€et€al.1997).€There€are€several€other€important€mechanisms€byŠ \¬ Šwhich€bison€alter€ecosystem-level€processes€and€physical€habitat€structure€in€tallgrass€prairie.Š H˜ ŠĢNutrient€redistribution€and€cycling.Š  p ŠĢBison€can€substantially€alter€nutrient€cycling€processes€and€patterns€of€nutrient€availability€inŠ ųH Štallgrass€prairie.€Their€effects€on€nitrogen€cycling€are€critical€because€nitrogen€availability€oftenŠ ä4 Šlimits€plant€productivity€in€these€grasslands€(Seastedt€et€al.€1991,€Blair€1997,€Turner€et€al.1997)Š Š  Šand€influences€plant€species€composition€(Gibson€et€al.€1993,€Wedin€and€Tilman€1993).Š ¼  ŠSimulation€models€of€tallgrass€prairie€responses€to€grazing€(Risser€and€Parton€1982)€and€studiesŠ Øų Šof€grazers€in€other€grasslands€(Frank€and€Evans€1997,€McNaughton€et€al.1997)€haveŠ ”ä Šdemonstrated€a€disproportionate€influence€of€ungulates,€including€bison,€on€the€regulation€ofŠ €Š Šnitrogen€cycling€processes.€Preliminary€data€from€Konza€Prairie€suggest€that€bison€are€similarlyŠ l¼ Šimportant€in€controlling€nitrogen€cycling€in€tallgrass€prairie.Š X Ø ŠĢBison€influence€nitrogen€cycling,€conservation,€and€availability€in€tallgrass€prairie€ecosystems€byŠ 0"€  Šaltering€several€soil€and€plant€processes.€Ungulates€in€grasslands€consume€relatively€recalcitrantŠ #l! Šplant€biomass€and€return€labile€forms€of€nitrogen€(i.e.,€urine)€to€soils€(Ruess€and€McNaughtonŠ $X" Š1988),€thus€bypassing€the€otherwise€slow€mineralization€of€nitrogen€in€plant€litter.€Nitrogen€inŠ ō$D # Šbison€urine€is€largely€urea,€which€can€be€hydrolyzed€to€ammonium€in€a€matter€of€days€(Ruess€andŠ ą%0!$ ŠMcNaughton€1988).€Indeed,€application€of€synthetic€bison€urine€increased€concentrations€ofŠ Ģ&"% Šammonium€and€nitrate€in€Konza€Prairie€soils€over€130fold€and€30-fold,€respectively,€8€days€afterŠ ø'#& Šapplication€(J.€R.€Matchett€and€Loretta€C.€Johnson,€unpublished€data).Š ¤(ō#' ŠĢBison€grazing€can€decrease€the€export€of€nitrogen€from€tallgrass€prairie€by€altering€the€magnitudesŠ |*Ģ%) Šof€two€major€pathways€of€nitrogen€loss-combustion€and€ammonia€volatilization.€Fire€is€the€majorŠ h+ø&* Špathway€of€nitrogen€loss€from€ungrazed€tallgrass€prairie€(Dodds€et€al.€1996,€Blair€1997);€nitrogenŠ T,¤'+ Šloss€from€burning€averages€1-4€gm-2€yr-1€(Blair€et€al.€1998).€Grazing€lowers€combustion€losses€ofŠ @-(, Šnitrogen€in€tallgrass€prairie€by€reducing€the€aboveground€plant€detritus€and€increasing€theŠ ° Špatchiness€of€a€prairie€fire€(Figure€2;€Hobbs€et€al.€1991).€Although€volatilization€of€ammonia€canŠ œģ Šbe€increased€by€grazing€in€other€types€of€grassland€(Detling€1988),€Hobbs€et€al.€(€1991€)Š ˆŲ Šsuggested€that€any€increase€in€ammonia€volatilization€in€tallgrass€prairie€will€be€more€thanŠ tÄ Šcompensated€for€by€a€reduction€in€combustion€losses€of€nitrogen.Š `° ŠĢFinally,€bison€grazing€affects€the€amount€and€quality€of€plant€litter€returned€to€soils.€GrazingŠ 8 ˆ Šincreases€plant€uptake€of€nutrients€(Ruess€1984)€and€shoot€nitrogen€content€in€many€grasslandsŠ $ t Š(Holland€and€Detling€1990,€Milchunas€et€al.€1995),€including€tallgrass€prairie€(Turner€et€al.€1993Š  ` Š).€However,€the€effects€of€grazers€on€root€growth€and€chemistry€vary€among€grasslandsŠ ü L  Š(Milchunas€and€Lauenroth€1993).€On€Konza€Prairie,€root€productivity€and€root€biomass€wereŠ č 8  Š30%€and€20%€lower,€respectively,€in€bison€grazing€lawns€than€in€ungrazed€exclosures.€InŠ Ō$  Šaddition,€the€nitrogen€concentration€of€new€root€growth€in€bison€grazing€lawns€at€Konza€PrairieŠ Ą  Šincreased€significantly,€from€0.6%€to€0.9%,€and€the€C:N€ratio€of€roots€decreased.€A€lower€C:NŠ ¬ü  Šratio€reduces€microbial€immobilization€and€enhances€nitrogen€availability€within€grazed€areas.Š ˜č  ŠIndeed,€recent€studies€on€Konza€Prairie€indicated€that€net€nitrogen€mineralization€in€bison€grazingŠ „Ō  Šlawns€was€153%€greater,€and€net€nitrification€126%€greater,€than€in€ungrazed€prairie€(Figure€6).Š pĄ ŠFurthermore,€net€nitrogen€mineralization€rates€were€proportional€to€the€intensity€of€bison€use€of€aŠ \¬ Šgiven€area.€Thus,€the€net€effect€of€bison€grazing€appears€to€be€increased€rates€of€nitrogen€cycling,Š H˜ Šcoupled€with€a€significant€increase€in€spatial€heterogeneity€in€nitrogen€availability;€together,€theseŠ 4„ Šeffects€can€alter€patterns€of€plant€productivity€and€species€composition€in€tallgrass€prairie€(FigureŠ  p Š5;€Steinauer€and€Collins€1995).Š  \ ŠĢWallowing.€One€aspect€of€bison€behavior€that€differs€from€that€of€cattle,€and€is€primarily€aŠ ä4 Šphysical€activity,€is€wallowing.€Wallows€in€Flint€Hills€tallgrass€prairie,€which€are€establishedŠ Š  Šprimarily€in€level€upland€or€lowland€sites,€dramatically€alter€the€patch€structure€of€this€prairie.Š ¼  ŠBison€wallows€develop€as€the€animals€paw€the€ground€and€roll€in€the€exposed€soil.€Continued€useŠ Øų Šof€wallows€by€bulls,€cows,€and€calves€creates€a€soil€depression€of€3-5€m€in€diameter€(and€10-30Š ”ä Šcm€in€depth)€that€is€devoid€of€vegetation.€These€denuded€patches€either€gradually€revegetate€orŠ €Š Šremain€as€bare€soil,€depending€on€the€frequency€of€revisitation€by€bison.€With€the€vast€numbers€ofŠ l¼ Šbison€that€once€occupied€the€Great€Plains,€these€soil€depressions€were€probably€abundant€andŠ X Ø Šwidespread€features€of€the€landscape€(England€and€DeVos€1969).€For€example,€a€number€of€relicŠ D!” Šwallows€had€to€be€filled€to€level€the€playing€field€for€the€first€University€of€Oklahoma€homeŠ 0"€  Šfootball€game€in€1895€(University€of€Oklahoma€Athletic€Department€1986).€Relic€wallows€stillŠ #l! Šexist€in€many€areas€where€bison€have€not€occurred€in€the€past€125€years.Š $X" ŠĢEnvironmental€conditions€in€relic€and€newly€established€wallows€strongly€influence€prairie€patchŠ ą%0!$ Šdynamics€(Polley€and€Collins€1984,€Polley€and€Wallace€1986).€Because€of€soil€compaction,Š Ģ&"% Šwallows€often€retain€rainwater€in€the€spring,€creating€localized€habitats€that€are€suitable€forŠ ø'#& Šephemeral€wetland€species,€similar€to€vernal€pools€in€California€(Holland€and€Jain€1981,€UnoŠ ¤(ō#' Š1989).€In€the€summer,€however,€the€same€wallows€support€only€plants€that€can€tolerate€severeŠ )ą$( Šdrought.€Vegetation€composition€and€structure€in€wallows€is€different€from€that€in€theŠ |*Ģ%) Šsurrounding€prairie€(Polley€and€Collins€1984),€and€these€differences€are€enhanced€by€fire€(CollinsŠ h+ø&* Šand€Uno€1983),€which€may€not€spread€through€wallows€because€of€low€fuel€loads.€Consequently,Š T,¤'+ Šat€larger€spatial€scales,€grazed€prairie€that€contains€bison€wallows€has€higher€plant€speciesŠ @-(, Šdiversity€than€grazed€prairie€without€wallows€(Collins€and€Barber€1985).€Thus,€bison€canŠ ° Šphysically€alter€grasslands€in€ways€that€increase€environmental€heterogeneity€and€enhance€bothŠ œģ Šlocal€and€regional€biodiversity€(Hartnett€et€al.€1997).Š ˆŲ ŠĢBison€carcasses.€Bison€not€only€affect€vegetation€patterns€and€soil€processes€through€their€grazingŠ `° Šactivities€but€also€have€profound€and€lasting€localized€effects€after€they€die.€Although€legalŠ L œ Šrequirements€and€management€practices€dictate€the€removal€of€carcasses€of€domestic€herbivoresŠ 8 ˆ Šfrom€public€and€private€grasslands,€native€herbivores€routinely€die€of€natural€causes€and€theirŠ $ t Šbodies€remain€in€situ.€As€part€of€the€minimal€management€strategy€at€Konza€Prairie,€bison€that€dieŠ  ` Šon€site€are€not€removed.€As€a€result,€these€carcasses€create€unique€local€disturbances€(Figure€7)Š ü L  Šthat€are€the€focus€of€studies€to€assess€their€effects€on€soil€nutrients€and€vegetation€responses€inŠ č 8  Štallgrass€prairie.Š Ō$  ŠĢWhen€an€individual€bison€dies,€copious€quantities€of€fluids€(with€high€nitrogen€concentration)€areŠ ¬ü  Šreleased€during€decomposition.€Adult€bison€can€weigh€more€than€800€kg,€and€these€carcassesŠ ˜č  Štypically€kill€underlying€and€adjacent€plants,€creating€a€denuded€zone€of€4-6€m2€(Figure€7).Š „Ō  ŠAlthough€the€fluids€that€are€initially€released€are€toxic€to€vegetation,€these€sites€eventually€becomeŠ pĄ Šzones€of€high€fertility.€For€example,€soil€cores€extracted€from€the€center€of€carcass€sites€on€KonzaŠ \¬ ŠPrairie€3€years€after€death€had€inorganic€nitrogen€concentrations€that€were€two€to€three€timesŠ H˜ Šhigher€than€the€surrounding€prairie.€This€nutrient€enrichment€may€extend€up€to€2.5€m€away€fromŠ 4„ Šthe€original€carcass€site€and€results€in€patches€dominated€initially€by€early€successional€species.Š  p ŠThe€aboveground€primary€production€in€these€patches€is€two€to€three€times€higher€than€inŠ  \ Šundisturbed€prairie.Š ųH ŠĢAlthough€disturbances€created€by€bison€carcasses€are€sporadic€and€localized€on€Konza€Prairie,Š Š  Šthey€provide€nutrient€pulses€that€exceed€all€other€natural€processes,€even€urine€and€fecal€deposits.Š ¼  ŠOverall,€we€can€only€speculate€about€historical€rates€of€bison€mortality.€Given€the€enormous€sizeŠ Øų Šof€the€bison€population€before€their€widespread€slaughter€in€the€1800s,€annual€mortality€wasŠ ”ä Šprobably€high.€High€death€rates€would€have€been€especially€common€during€droughts,€when€thereŠ €Š Šwould€be€the€potential€for€large€numbers€of€carcasses€to€occur€across€the€landscape.€Even€thoughŠ l¼ Špredators€and€scavengers€may€have€consumed€and€relocated€many€of€these€carcasses,Š X Ø Šdecomposition€of€the€remaining€and€partial€carcasses€would€still€have€resulted€in€patches€of€locallyŠ D!” Šhigh€nutrient€concentration.€Thus,€although€it€was€variable,€bison€mortality€would€have€led€to€aŠ 0"€  Šcontinual€cycle€of€disturbance€and€recovery€of€these€patches€in€presettlement€grasslands.Š #l! ŠĢAre€bison€keystone€species?Š ō$D # ŠĢThe€net€effects€of€selective€bison€grazing€activities€at€the€landscape,€patch,€and€individual€plantŠ Ģ&"% Šlevel€include€shifts€in€plant€species€composition,€alterations€of€the€physical€and€chemicalŠ ø'#& Šenvironment,€and€increased€spatial€and€temporal€heterogeneity€in€vegetation€structure,€soilŠ ¤(ō#' Šresource€availability,€and€a€variety€of€ecosystem€processes€(Figures€4,€5,€and€6).€Before€bisonŠ )ą$( Šreintroduction€at€Konza€Prairie,€the€long-term€burning€experiments€produced€clear€patterns€ofŠ |*Ģ%) Šresponse€in€the€vegetation.€As€fire€frequency€increased,€the€dominance€of€C4€grasses€increased,Š h+ø&* Šand€the€cover€of€C,grasses,€forbs,€and€woody€species€decreased€(Figure€4;€Gibson€and€HulbertŠ T,¤'+ Š1987).€Overall,€plant€species€diversity€declined€as€fire€frequency€increased€in€ungrazed€tallgrassŠ @-(, Šprairie€(Collins€et€al.€1995).Š ° ŠĢThese€patterns€in€community€structure,€which€had€developed€over€20€years€of€burning€treatmentsŠ ˆŲ Šat€Konza€Prairie,€are€being€rapidly€and€dramatically€altered€by€the€grazing€activity€of€theŠ tÄ Šreintroduced€bison.€In€particular,€grazing€by€bison€has€lowered€the€abundance€of€the€dominant€C4Š `° Šgrasses,€increased€the€abundance€of€the€subdominant€C3€grasses€and€forbs,€and€markedlyŠ L œ Šincreased€plant€species€diversity€(by€23€%€),€richness€(by€38€%),€and€community€heterogeneity€(byŠ 8 ˆ Š13%)€relative€to€ungrazed€sites,€even€under€annual€burning€conditions€(Hartnett€et€al.€1996,Š $ t ŠCollins€and€Steinauer€1998,€Collins€et€al.1998).Š  ` ŠĢBecause€of€the€multiple€and€dramatic€effects€of€bison€on€this€landscape,€we€believe€that€bison€areŠ č 8  Škeystone€species€in€the€tallgrass€prairie.€Other€authors€have€noted€the€potential€of€large€grazingŠ Ō$  Šmammals€to€act€as€š škeystone€herbivoresšš€capable€of€maintaining€open€grassland€vegetation€thatŠ Ą  Šwould€otherwise€undergo€succession€to€shrubland€or€woodland€(Owen-Smith€1987).€Indeed,€theŠ ¬ü  Šdisappearance€of€a€grazing€megafauna€at€the€end€of€the€Pleistocene€may€have€played€a€major€roleŠ ˜č  Šin€the€widespread€transition€from€steppe€to€tundra€at€that€time€(Zimov€et€al.€1995).€However,€theŠ „Ō  Šconcept€of€keystone€species€has€been€controversial€since€its€inception€(Power€et€al.€1996).€One€ofŠ pĄ Šthe€problems€with€this€concept€has€been€the€variable€interpretation€of€criteria€by€which€species€areŠ \¬ Šdetermined€to€be€keystone.€Power€et€al.€(1996)€consider€a€keystone€species€to€be€š šone€whoseŠ H˜ Šimpact€on€its€community€or€ecosystem€is€large,€and€disproportionately€large€relative€to€itsŠ 4„ Šabundance.šš€To€make€this€definition€operational,€these€authors€proposed€a€measure€of€communityŠ  p Šimportance€(CI)€to€be€used€as€an€index€of€the€strength€of€the€impact€of€a€given€species:Š  \ ŠĢAre€bison€and€cattle€functional€equivalents€in€tallgrass€prairie?Š ä4 ŠĢThe€historical€presence€of€immense€herds€of€large€ungulates€in€Great€Plains€grasslands€isŠ ¼  Šundisputed€(McHugh€1972),€and€we€have€emphasized€the€keystone€role€that€bison€played€inŠ Øų Šdetermining€the€structure€and€function€of€tallgrass€prairies€at€multiple€spatial€and€temporal€scales.Š ”ä ŠWith€the€replacement€of€native€bison€by€domesticated€cattle€in€the€remaining€grasslands,€anŠ €Š Šobvious€issue€is€the€degree€of€similarity€between€these€two€ungulates€with€respect€to€their€effectsŠ l¼ Šon€tallgrass€prairie.€In€other€words,€can€bison€and€cattle€be€considered€ecological€equivalents?Š X Ø ŠĢThere€have€been€several€previous€attempts€to€answer€this€question,€but€the€results€have€beenŠ 0"€  Šequivocal€at€best€(Plumb€and€Dodd€1993,€Hartnett€et€al.€1997).€The€primary€barrier€to€resolvingŠ #l! Šthis€issue€rests€with€a€lack€of€comparative€studies€in€which€management€is€held€constant€and€theŠ $X" Štype€of€grazer€is€varied.€Such€studies€have€recently€been€initiated€at€Konza€Prairie.€Results€after€3Š ō$D # Šyears€indicate€that€the€abundance€and€richness€of€annual€forbs,€and€the€spatial€heterogeneity€ofŠ ą%0!$ Šbiomass€and€cover,€are€higher€in€sites€with€bison€than€in€sites€with€cattle.€No€dramatic€differencesŠ Ģ&"% Šhave€been€detected,€however,€between€cattle-€and€bison-grazed€sites€in€cover€of€the€dominant€C4Š ø'#& Šgrass,€A.€gerard,€or€the€dominant€forb,€Ambrosia€psilostachya;€total€plant€species€richness€is€alsoŠ ¤(ō#' Šnot€dramatically€different€(E.€Gene€Towne€and€David€C.€Hartnett,€unpublished€data).Š )ą$( ŠĢResults€at€Konza€Prairie€are€consistent€with€previous€assessments€(e.g.,€Schwartz€and€EllisŠ h+ø&* Š[1981],€Van€Vuren€and€Bray€[1983],€and€Plumb€and€Dodd€[1993]€in€mixed€and€shortgrassŠ T,¤'+ Šprairie),€which€noted€that€both€bison€and€cattle€are€generalist€herbivores€that€graze€preferentiallyŠ @-(, Šon€graminoids.€Nevertheless,€some€differences€in€the€foraging€patterns€of€bison€and€cattle€haveŠ ° Šbeen€documented€that€may€have€long-term€implications€for€grasslands.€For€example,€bison€have€aŠ œģ Šhigher€proportion€of€graminoids€in€their€diet€than€do€cattle;€consequently,€forb€and€browse€speciesŠ ˆŲ Šare€more€common€in€cattle€diets€(Van€Vuren€and€Bray€1983,€Hartnett€et€al.€1997).€Also,€bisonŠ tÄ Šspend€less€time€grazing€than€cattle€and€more€time€in€nonfeeding€activities€(Plumb€and€DoddŠ `° Š1993),€and€bison€strongly€prefer€open€grassland€areas€for€grazing,€whereas€cattle€use€wooded€andŠ L œ Šgrassland€habitats€opportunistically€(Hartnett€et€al.1997).Š 8 ˆ ŠĢStudies€that€have€focused€exclusively€on€cattle€generally€concur€that€their€grazing€activitiesŠ  ` Šincrease€spatial€heterogeneity€and€enhance€plant€species€diversity,€so€long€as€stocking€density€isŠ ü L  Šnot€too€high€(Collins€1987,€Hartnett€et€al.€1996).€Because€bison€grazing€in€tallgrass€prairie€has€aŠ č 8  Šsimilar€effect,€one€could€conclude€that€either€herbivore€can€alter€resource€availability€andŠ Ō$  Šheterogeneity€and€reduce€the€cover€of€the€dominant€grasses€sufficiently€to€enhance€the€success€ofŠ Ą  Šthe€subdominant€species.€Perhaps€of€greater€importance€than€differences€in€foraging€patternsŠ ¬ü  Šbetween€bison€and€cattle,€however,€are€the€number€of€nongrazing€activities,€such€as€wallowingŠ ˜č  Šand€horning€(i.e.,€rubbing€on€trees)€that€are€associated€exclusively€with€bison€(Coppedge€andŠ „Ō  ŠShaw€1997,€Hartnett€et€al.1997).€These€activities,€when€combined€with€the€spatial€redistributionŠ pĄ Šof€nutrients€and€selective€consumption€of€the€dominant€grasses,€may€further€increase€plant€speciesŠ \¬ Šrichness€and€resource€heterogeneity,€particularly€at€the€landscape€scale.Š H˜ ŠĢNevertheless,€it€is€likely€that€because€bison€and€cattle€are€functionally€similar€as€largeŠ  p Šgrass-feeding€herbivores,€management€strategies€(stocking€intensity€and€duration)€will€have€aŠ  \ Šgreater€influence€on€the€degree€of€ecological€equivalency€achieved€than€inherent€differences€inŠ ųH Šthese€ungulates€(Hartnett€et€al.€1997).€Clearly,€the€degree€of€overlap€in€diet€and€foraging€patternsŠ ä4 Šis€greater€between€bison€and€cattle€than€between€cattle€and€other€historically€important€nativeŠ Š  Šherbivores€(Hartnett€et€al.€1997),€such€as€antelope€(Antilocapra€americana),€deer€(OdocoileusŠ ¼  Švirginianus),€and€elk€(Cervus€canadensis).€Indeed,€the€loss€of€antelope€and€elk€from€the€tallgrassŠ Øų Šprairie,€coupled€with€dramatic€increases€in€deer€populations,€presents€additional€challenges€forŠ ”ä Šmanaging€these€ecosystems.Š €Š ŠĢConservation€implicationsŠ X Ø ŠĢConserving€small€and€moderate-sized€tracts€of€once-vast€biomes,€such€as€the€tallgrass€prairie,Š 0"€  Špresents€a€unique€set€of€problems€that€are€distinct€from€those€associated€with€spatially€restrictedŠ #l! Šecosystems€because€many€of€the€defining€forces€that€historically€were€important€in€structuringŠ $X" Šthese€systems€occurred€at€spatial€scales€that€no€longer€exist.€For€example,€in€pre-1900sŠ ō$D # Šgrasslands,€fires€were€not€plot-level€or€even€watershed-level€events€but€operated€at€spatial€scalesŠ ą%0!$ Šencompassing€thousands€of€hectares.€This€large€spatial€scale€resulted€in€potentially€high€fireŠ Ģ&"% Šfrequencies€throughout€the€tallgrass€prairie€because€any€point€of€ignition€in€this€š šinland€sea€ofŠ ø'#& Šgrassšš€could€affect€grasslands€hundreds€of€kilometers€distant.Š ¤(ō#' ŠĢToday,€the€fragmentation€of€Great€Plains€grasslands€is€recognized€as€a€key€factor€in€reducing€theŠ |*Ģ%) Šfrequency€of€fire,€which€in€turn€contributes€to€species€loss€(Leach€and€Givnish€1996).€Indeed,€theŠ h+ø&* Šprimary€management€strategy€for€small€prairie€preserves,€which€are€most€prone€to€invasion€byŠ T,¤'+ Šwoody€vegetation€and€exotic€species,€is€to€burn€them€as€frequently€as€possible€to€suppressŠ @-(, Šinvasion€by€undesirable€plants€(Leach€and€Givnish€1996).€Unfortunately,€frequent€(annual€orŠ ° Šbiannual)€spring€fire€maintains€dominance€by€C4€grasses€but€reduces€plant€species€diversityŠ œģ Šrelative€to€grasslands€that€are€burned€infrequently.€One€alternative€is€to€conduct€burns€at€differentŠ ˆŲ Štimes€of€the€year€(Howe€1994),€but€summer€fires,€for€example,€may€not€prevent€invasion€orŠ tÄ Šreduce€the€abundance€of€woody€vegetation€(Adams€et€al.€1982).€In€addition,€burning€in€lateŠ `° Šsummer€may€be€difficult€because€of€other€considerations,€including€reduced€ability€to€control€theŠ L œ Šfire€under€dry,€windy€conditions.€Not€only€are€prairies€threatened€by€fragmentation€and€invasionŠ 8 ˆ Šby€undesirable€species,€but€grasslands€throughout€the€Great€Plains€are€now€affected€by€increasedŠ $ t Šatmospheric€nitrogen€deposition€(Wedin€and€Tilman€1996).€Thus,€remnant€grasslands€areŠ  ` Šsubjected€to€a€variety€of€anthropogenic€factors€that€can€reduce€the€diversity€of€native€prairieŠ ü L  Šspecies.Š č 8  ŠĢThe€spatial€and€temporal€impacts€of€bison€grazing€activities€caused€by€the€historically€large€andŠ Ą  Šnomadic€herds€are€also€best€characterized€as€landscape-level€forces.€These€too€are€difficult€toŠ ¬ü  Šreplicate€in€todayššs€fragmented€grassland€remnants.€Yet€just€as€some€of€the€ecologicalŠ ˜č  Šcharacteristics€of€natural€fires€can€be€reintroduced€to€grasslands€through€prescribed€fire,€the€keyŠ „Ō  Šelements€of€bison€grazing€activities€can€and€should€be€incorporated€into€conservation€andŠ pĄ Šrestoration€strategies€for€remnant€prairies€(Steuter€1997).€One€approach€to€accomplish€this€goal€isŠ \¬ Šthe€substitution€of€cattle€for€bison.€Plumb€and€Dodd€(1993)€argued€that€the€choice€of€whether€toŠ H˜ Šuse€cattle€or€bison€as€a€management€tool€in€grasslands€is€scale€and€context€dependent.€Clearly,Š 4„ Šreintroducing€bison€may€not€be€appropriate€for€small€prairie€remnants€with€public€access€and€lowŠ  p Šeconomic€resources.€But€cattle,€managed€for€their€ecological€rather€than€their€economic€value,Š  \ Šmay€be€suitable€in€such€cases.Š ųH ŠĢAlternatively,€mowing€can€be€used€to€reduce€the€dominance€of€the€tall€grasses€and€to€enhanceŠ Š  Šspecies€richness€(Gibson€et€al.€1993,€Collins€and€Steinauer€1998).€Results€from€a€longtermŠ ¼  Šexperiment€at€Konza€Prairie€incorporating€annual€fire,€nitrogen€addition,€and€mowing€(Collins€etŠ Øų Šal.€1998)€indicated€that€on€annually€burned€and€fertilized€treatment€plots,€productivity€of€theŠ ”ä Šgrasses€was€higher,€and€plant€species€diversity€lower,€than€in€control€plots.€However,€on€burned,Š €Š Šfertilized€plots€that€were€mowed€(with€removal€of€the€foliage;€a€rough€substitute€for€grazing),Š l¼ Šplant€species€diversity€was€restored€to€levels€similar€to€control€plots€(Collins€and€Steinauer€1998).Š X Ø ŠĢA€combination€of€frequent€spring€fire€to€maintain€populations€of€the€desirable€C4€prairie€grasses,Š 0"€  Šdecrease€nitrogen€availability€(Blair€1997),€and€suppress€growth€of€weedy€annuals€and€woodyŠ #l! Švegetation,€coupled€with€mowing€portions€of€the€site€to€reduce€the€competitive€dominance€of€C4Š $X" Šgrasses,€can€enhance€the€abundance€of€forbs€and€maintain€high€plant€species€diversity€in€smallŠ ō$D # Šremnant€prairies.€Ultimately,€management€designed€to€increase€the€spatial€heterogeneity€ofŠ ą%0!$ Šresources€in€a€manner€analogous€to€that€imposed€by€ungulate€activities€is€essential€if€significantŠ Ģ&"% Šand€sustainable€biotic€diversity€in€tallgrass€prairie€is€a€goal.€ConclusionsŠ ø'#& ŠĢDespite€less€than€a€decade€of€research€at€Konza€Prairie€on€bisontallgrass€prairie€interactions,€theŠ )ą$( Škeystone€role€that€bison€must€have€historically€played€in€this€grassland€is€clear.€Moreover,€muchŠ |*Ģ%) Šas€fire€is€now€recognized€as€an€essential€component€of€tallgrass€prairie€management€(becauseŠ h+ø&* Šwithout€fire€this€grassland€disappears),€the€need€for€reintroducing€the€forces€of€large€ungulateŠ T,¤'+ Šherbivory€to€this€grassland€is€evident.€Ō_ŌIndeed,€it€is€the€interaction€of€ungulate€grazing€activities€andŠ @-(, Šfire,€operating€in€a€shifting€mosaic€across€the€landscape,€that€is€key€to€conserving€and€restoringŠ ° Šthe€biotic€integrity€of€the€remaining€tracts€of€Ō_ŌtallgrassŌ_Ō€prairie.Š œģ ŠĢBefore€bison€were€reintroduced€to€Ō_ŌKonzaŌ_Ō€Prairie,€Knapp€and€Ō_ŌSeastedtŌ_Ō€(1986)€speculated€thatŠ tÄ Šbison€grazing€and€fire€could€act€in€similar€ways€by€reducing€the€accumulation€of€detritus€in€thisŠ `° Šsystem.€It€is€primarily€the€blanketing€effect€of€the€accumulation€of€dead€plant€material€aboveŠ L œ Šground€that€limits€productivity€in€undisturbed€Ō_ŌtallgrassŌ_Ō€prairie.€Like€fire,€bison€grazing€reducesŠ 8 ˆ Šaboveground€standing€dead€biomass.€Ō_ŌButŌ_Ō€it€is€now€clear€that€the€unique€spatial€and€temporalŠ $ t Šcomplexities€of€bison€grazing€activities€(Figure€5)€are€critical€to€the€successful€maintenance€ofŠ  ` Šbiotic€diversity€in€this€grassland.€This€grazing-induced€heterogeneity€contrasts€sharply€with€theŠ ü L  Šspatial€homogeneity€induced€by€fire€in€an€Ō_ŌungrazedŌ_Ō€landscape€(Figure€6).Š č 8  ŠĢŌ_ŌTallgrassŌ_Ō€prairie,€by€virtue€of€its€inherently€variable€climatic,€grazing,€and€fire€regimes,€is€anŠ Ą  Šecosystem€that€requires€long-term€study€to€document€patterns€and€quantify€processes€(Knapp€etŠ ¬ü  Šal.€1998b).€Through€the€partnership€of€The€Nature€Conservancy,€the€National€ScienceŠ ˜č  ŠFoundationššs€Ō_ŌLTERŌ_Ō€program,€and€Kansas€State€University,€ongoing€studies€at€this€site€willŠ „Ō  Šcontinue€to€explore€the€ecological€interactions€of€fire€and€grazing€in€the€Ō_ŌtallgrassŌ_Ō€prairieŠ pĄ Šlandscape.€Such€research€is€timely€because€conservation€and€management€issues€have€intensifiedŠ \¬ Šin€the€remaining€tracts€of€this€once-vast€Ō_ŌbiomeŌ_Ō,€particularly€in€response€to€predicted€alterations€inŠ H˜ Šglobal€climate€and€Ō_ŌlanduseŌ_Ō€changes.€Interdisciplinary€ecological€research,€such€as€that€ongoing€atŠ 4„ ŠŌ_ŌKonzaŌ_Ō€Prairie,€will€provide€the€basic€information€necessary€for€designing€optimal€conservation,Š  p Šrestoration,€and€management€strategies€in€this€and€other€grasslands.Š  \ ŠĢAcknowledgmentsŠ ä4 ŠĢResearch€summarized€here€was€supported€by€the€National€Science€Foundationššs€Long-TermŠ ¼  ŠStudies,€Ecology,€and€Ecosystems€Programs;€Ō_Ōthe€US€Department€of€Agricultureššs€NationalŠ Øų ŠResearch€Initiative€Program;Ō_Ō€Ō_Ōthe€National€Aeronautics€and€Space€Administration;Ō_Ō€Ō_ŌThe€NatureŠ ”ä ŠConservancy;Ō_Ō€Ō_ŌandŌ_Ō€the€Kansas€State€University€Agricultural€Experiment€Station€(99-156-J).Š €Š ŠĢAlan€K.€Knapp,€John€M.€Blair,€John€M.€Briggs,€Scott€L.€Collins,€David€C.€Ō_ŌHartnettŌ_Ō,€and€LorettaŠ X Ø ŠC.€Johnson€are€professors,€and€E.€Gene€Ō_ŌTowneŌ_Ō€is€a€research€associate,€in€the€Division€of€Biology,Š D!” ŠKansas€State€University,€Manhattan,€Kansas€66506.€Collins€is€also€an€adjunct€professor€with€theŠ 0"€  ŠDepartment€of€Zoology,€University€of€Maryland,€College€Park,€MD€20742,€and€a€programŠ #l! Šdirector€in€the€Division€of€Environmental€Biology,€National€Science€Foundation,€Arlington,€VAŠ $X" Š22230.€CŌ_Ō)Ō_Ō€1999€American€Institute€of€Biological€Sciences.Š ō$D # Š