mcnaughton 1976

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The University of Chicago

Grazing Lawns: Animals in Herds, Plant Form, and CoevolutionAuthor(s): S. J. McNaughtonReviewed work(s):Source: The American Naturalist, Vol. 124, No. 6 (Dec., 1984), pp. 863-886Published by: The University of Chicago Press for The American Society of NaturalistsStable URL: http://www.jstor.org/stable/2461305 .

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Vol. 124,No. 6 The AmericanNaturalist December 1984

GRAZING LAWNS: ANIMALS IN HERDS, PLANT FORM,AND COEVOLUTIONS. J.McNAUGHTON

Biological Research Laboratories,Syracuse University, 30 College Place,Syracuse, New York 13210SubmittedNovember 3, 1983; Accepted May 23, 1984

Escape frompredation s commonlyrecognized as a major forcethatguidednatural election eading to the evolutionof gregariousness Williams 1966; Eibl-Eibesfeldt1970;Ewer 1968;Hamilton1971; Alexander 1974; Estes 1974; Jarman1974) since lone individuals nd those on colony peripheries re more predator-prone than those within aggregationsHorn 1968; Tenaza 1971; Kruuk 1972;Schaller 1972; Sinclair 1974; Estes 1976). However, a unitary xplanationofgregariousness s suspect Pulliam 1973;Hoogland 1981) since gregariousness anhave other benefits, uch as releasingtime from predator search to foraging(Caraco 1979a, 1979b; Caraco and Pulliam1984). Although his does notstrictlyincrease the efficiency f foraging,tincreases the proportion f the timebudgetthatcan be devoted to foraging.n thepresentpaper I examine data suggestingthatgregariousnessngrazing nimalsmay ncreaseforaging fficiencyy modify-ingvegetation tructure o increase foodyieldperbiteto the ndividual razer n aherd. Thatgregariousness lso leads to the evolutionof plants nresponse to thegrazing ntensity xperienced.Large grazingmammals are amongthe most spectacularly regarious nimalsknown, ften ormingarge,dense herds.Among he diversegrazingmammals orwhich herds are well known are manyspecies of Africanungulates Estes 1974;Jarman 974;Leuthold 1977; Delany and Happold 1979; Eltringham 980);bison(Bison bison) inNorth America Seton 1910; Roe 1951; Dary 1974); saiga (Saigatartarica) nEurasia (Bannikovet al. 1961); caribou Rangifer aradus) in north-ern latitudes Bergerud 1974); and members of the Macropodidae in Australia(Newsome 1971, 1975). The gregariousness fherding nimals,and consequentherd size and animal density,often varies throughouthe year. The ungulatestudiessuggest hattheadjustment f densities o vegetation roduction yfree-ranging erbivores ppears to be a generalfeature f animals thatfeedcompara-tivelyunselectively n plant foliage, .e., grazers.Herbivores have well-documented ffects n plantestablishmentWatt 1919;Shaw 1968; Gashwiler 1970; Louda 1983); growth NcNaughton 1976. 1979b,1983a; Morrow and LaMarche 1978); and reproductive uccess (Janzen 1969;Chew and Chew 1970;Louda 1982, 1983). They also have substantial ffects nAm. Nat. 1984.Vol. 124, pp. 863-886.? 1984byThe University fChicago. 0003-0147/84/2406-0009$02.00.ll rights eserved.

863

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864 THE AMERICANNATURALISTplantformMcNaughton 1976, 1979a, 1983b).Herbivores s distinct s browsingmammals Vesey-Fitzgerald 1973a) and stem gall insects (Hartnett nd Abra-hamson 1979)feeding n trees and herbs,respectively, lterplantformdramat-ically by activatingateralbuds thatproduce a dense,bushygeometry. rowsingby moas has been proposed as a selective factor eadingto the evolution of adivaricating hrubform n New Zealand (Greenwoodand Atkinson 977). Amongthe most conspicuous effects f large mammalian razersupon grasslandsare adrastic reductionof canopy height nd the activation of tillersthat lead to aprostrate,dense canopy (Vesey-Fitzgerald 1973b; McNaughton 1976, 1979a,1979b), hereafter eferredo as a grazing awn.A wide variety fgrazing nimals in additionto largemammalianherbivoresfeed as membersof dense aggregations f individuals.A pattern f vegetationexploitationhat an be characterized s herding asbeen describedforgrazers sdiverse as freshwaterHunter 1980) and marine Lein 1980;Bertnesset al. 1983)snails; tortoises Grubb1971;Gibsonand Hamilton1983);marine urtlesBjorn-dahl 1980); geese (Prinset al. 1980; Cargill 1981);deer Clutton-Brockt al. 1982);bison and smallmammalsfeedingon commongrazinggrounds Coppock et al.1983a, 1983b),many species of Africanungulatesranging n size over severalorders of magnitudeand with otherwise diverse behavioral systems (Vesey-Fitzgerald1960;Lamprey 1964;Bell 1969; Duncan 1975; Hoeck 1975; McNaugh-ton 1976, 1979a, 1979b; Sinclair 1977; Jarman1979; Tomlinson 1981; Murray1982). It seems unlikely that foraging ime would be released frompredatorwatching ime n snails or that predationupon tortoiseswould have led to theevolutionofaggregating ehavior. Althoughnotnecessarily xplicitlymentionedby theauthors,these studies reveal thatgrazing nimals commonly djusttheirdensities ndvegetation tilization atternsnrelation othevegetation'sproduc-tivity otential, ongregatingnd producing razing awns where thatpotential shigh nd dispersing rom reas where thatpotential s low. Africanungulates nareas of moderate ohigh ainfall oncentrate n small reasduringhewet seasonand disperse intoexpanded ranges during hedryseason. In low rainfall reas,theycommonlydisperse over large areas during he wet season and congregatearound ake margins nd ingroundwater-irrigatedrasslandswith highproduc-tivity otentialduring hedryseason (Lamprey 1963).Those behavioralpatternstend to producegrazing awns during eriodsofhighutilization nd allow tallermeadows to develop duringperiodsofgrasslanddisuse (Vesey-Fitzgerald1969,1973b, 1974;McNaughton 1976, 1979a).The creation ndmaintenance f grazing awns maybe caused by bothdevelop-mentalresponses by the grasses in ecological time,and the genetic ability togenerate hosedevelopmental esponses that s determinedn evolutionary ime.Grasses can evolve rapidly in response to the prevailing defoliationregime(Gregorand Sansome 1926; Stapledon 1928; Turesson 1929; Kemp 1937; Brad-shaw 1960; Lodge 1962; Warwick nd Briggs 1978b;McNaughton 1979b; Detlingand Painter 1983). Natural selection in infrequentlymown hay meadows thatreach a substantial eight eforeharvest s forcompetitive cotypes that re tall,erect,and largeleaved. Frequent, ntensivegrazing elects forprostrate, mall-leaved, dwarfed cotypes that re short n stature. ince grazing arely s spread



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ANIMALS IN HERDS, PLANT FORM,COEVOLUTION 865uniformlyn space or continuousn time, ven in commercial astures Burt1949;Hunter1964),both natural nd commercial rasslands re often mosaic ofshort,heavilygrazed areas and tall, ightly razed patches.Although eneticdifferentia-tion npasture mosaics has rarelybeen studied Kemp 1937; Detling nd Painter1983), plants in mosaics maintainedfor aesthetic and cultural reasons are acorrespondingmosaic of growth orm nd phenologicalgenotypes Warwick ndBriggs 1978a, 1978b,1979, 1980a, 1980b,1980c;Warwick 1980). There also, ofcourse, are oftenmajordifferencesn species composition nd associated growthforms n vegetation subject to different atternsof exploitationby herbivoressince there re limits o natural electionforecotypes tolerant f competition rgrazing Quinn andMiller 1967). Darwin 1872, p. 66) observed that If turfwhichhas long been mown andthe case would be the samewith urf losely browsedbyquadrapeds) be let to grow, the more vigorousplants graduallykill the lessvigorous, houghfully rownplants; thus out oftwenty pecies grownon a littleplot ofmown turfthreefeetby fourfeet),nine species perished,from he otherspecies beingallowed to grow up freely." Similar hanges nspecies compositionand growthformhave been well documented n marine Dayton 1975; Menge1976; Osman 1977; Lubchenko 1978) and terrestrialgrazing ecosystems(McNaughton 1979a, 1983c), suggesting hatgrowth orm election s a generalconsequence ofgrazing ndiverse ecosystems.Maintenance ofgrazing awns increasesthequalityof food available to herbi-vores, particularly hrough nhanced nitrogen ontent, n both aquatic (Hunter1980; Hunter and Russell-Hunter 1983) and terrestrial abitats McNaughton1979b; McNaughton et al. 1982; Coppock et al. 1983a; Detling nd Painter1983;Ruess 1984; Ruess et al. 1983). Grazing lso increasesthe digestibilityfforages(Olubajo et al. 1974; Misleavy et al. 1982) so that both nutrient ontentandrelativeyield to herbivores re greater n grazing awns.Quantitative haracterization f the effects f herbivory pon the structuralcharacteristics f vegetation McNaughton 1976, 1979a, 1979b) indicates thatgrazing awns have a high plant biomass concentrationforage mass per unitvolume) because of plantgrowth esponses thatpack productive,nutritious,ndherbivore-soughtissues nto a small volumenear the soil surface Stobbs 1973a,1973b). Similarly, rowsing ommonly roducesa dense, highly ranched anopysurfaceanalogous to a grazing awn that has a high foliage densitybut whichprotects he nterior oliagefrom rowsing y makingt physicallyess accessibleto browsers Vesey-Fitzgerald1973a). The higher iomass concentration epre-sents a potentiallyhigherfood yield to herbivoresper mouthful aten (Stobbs1973a, 1975; McNaughton1976), but t also createsa spatial refuge hatrendersportion fthefoliageunavailable McNaughton 1979b,1983a, 1983b).Suchphysi-cal refuges rom erbivory, enerated y plantdevelopmental esponsestodefoli-ation, maybe an importantontributorothedynamic tabilityfgrazing cosys-tems. The stability f analytical grazingmodels is highly ependentupon suchinvulnerable lanttissueclasses (Noy-Meir 1975, 1978; Caughley 1976;May 1977;Walkeret al. 1981).Those tissuesrepresentn induceddefense gainstherbivorythatproducesan invulnerable lass of productive oliage.The cranial and skeletal properties of ungulatesinfluence heir abilities to



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866 THE AMERICANNATURALISTexploitvegetation f differenteometries Bell 1969, 1970). Furthermore,oththedigestivetracts Hofmann 1968) and the behavioralrepertoire Jarman1974) ofungulates are related to their feeding patterns. Thus, the connectionsamongvegetationgeometry nd the morphologies, igestiveproperties, nd behavioralpatterns f grazers suggest n evolutionary artitioningfvegetation ased on itsgeometrynd theability f differentngulates o obtainfoodefficientlyrom hatgeometry. examine the sward geometry f East Africangrasslandsand theextent o which hatgeometrys inducedby grazers necologicalandevolutionarytime. These studies,when combined withdata on the foraging fficiencies fdomestic ungulates in awards of different eometries, uggestthat individualanimals may gain substantialfitness, hroughncreasedforaging fficiency, ybeing a member f a herd.

STUDY REGION AND METHODSThe Serengetiregionoccupies 25,000 km2 n a vast peneplainbetween 1135mand 1800 minelevationthat s tilted pwardfrom ake Victoria n thewest to theGregoryRiftbranchofthe Great Rift ystem nthe east (McNaughton 1983c). Itlies between 1?15' and 3?30' S at longitudes between 340 and 37? E. There is a meanannual rainfall radientbetween 350 mm n the rainshadowoftheCrater High-lands in the southeast and 1100 mm in the northwestwhere the Lake Victoriaconvergencezone is influentialNorton-Griffithst al. 1975). The nominal wetseason is Novemberto May, but t s shorter han hat nthe ow rainfall reas andlonger hanthat nhigher ainfall reas (McNaughton 1984). About 3 millionheadof over 25 species ofherbivorous ngulates ccupy thearea (Sinclair ndNorton-Griffiths979, 1982). Small mammals lso are abundant nmanyareas (Senzota1982). The woody vegetation ranges from comparatively are closed canopywoodland to the prevailing eciduous to semideciduous horn ree savanna (Her-locker 1974, 1976). Grasslands range from hortgrasslandsof Sporobolus in thesoutheast, throughmid-height rasslands of Themeda triandra n most of thesavanna regions, o tall grasslandsof Hyparrhenia nd Hyperthalia nthe north-

west (McNaughton 1983c).The data reportedhere were collected between May 1974 and October 1978.Complete vegetationdescriptions McNaughton 1983c) and functional ynamics(McNaughton 1984) are reportedelsewhere. Plant biomass was measured bycanopy spectroflectancend interception s described previously McNaughton1976, 1979a). Canopy heightwas measuredby allowing 4.5 g styrofoamheet 17x 26 cm to come to rest on the canopy McNaughton 1976). Biomass concentra-tionwas standing rop divided by canopy height McNaughton 1976).Plantpopulation samples (n = 3-5 per population,depending n survival)fortransplant arden studies were collected in the native sites and transplanted oSeronera,a location nthecenterof the ecosystemwith mean annual rainfall f837 mm.Plantswereirrigated ntil stablished nd grownunderweed-free ulti-vation n a fenced garden. Those studies concentrated n threewidespreadplantspecies, Cynodon dactylon (L.) Pers., Harpachne schimperiA. Rich, andThemeda triandraForsk. The first wo species are shortgrasses, the latter s a



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ANIMALS IN HERDS, PLANT FORM,COEVOLUTION 867mediumheight rassthat s themost bundant pecies in thegrasslands s a whole(McNaughton1983c). Cynodon s stoloniferous,he other wospecies are bunch-grasses. All were collected at locations ranging romthe heavilygrazed, lowrainfall erengeti lains to thehigh ainfall,owergrazingocalitiesof thenorthernpartof theregion.The datahere refrom he second year n thetransplant arden,bywhich timetherewas no evidence of any transplant hock or residual effectsrelated to collection. Principaltraits measured were leaf length,based on arandom ample of five ully xpanded eaves per plant,number ffloweringulmsper plant, nd height f the tallest ulmon each plant.Additional ata collectedonCynodon ncluded internode ength mean of five sampled randomly), rea col-onized by the second growing eason, densityof rootedshootswithin he col-onized area, and canopy height s in the field tudies.Data formean annual rainfallwere fromrecordsof the SerengetiResearchInstitute nd data on grazing ntensity McNaughton 1979b, 1983c, 1984)werefromfieldstudysites where the plants were collected. Grazing intensity G)rangesfrom ero forno grazing o one forcomplete harvestofstanding rop. Inthe data analyses presented below, species were analyzed togetherwhen datadistributions ere similar nd separatelywhentherewere statistically ignificantspecies differences. ainfall nd grazing ntensityre partially onfoundedvari-ables since theytend to be negatively ssociated across the rainfallgradient(McNaughton1983c)so regression esultswere based on stepwiseregressionwithpartialcorrelation s a guide to entry rder.

RESULTSCanopy Characteristics

The heightdynamics of vegetation n adjacent fenced and unfencedplots inheavily grazed shortgrasslandsof the SerengetiPlains were decidedly differentthroughhegrowing eason (fig. a). Grazedvegetationwas never aller han3 cmsincegrazersarrested hevegetationbelow that evel. The vegetation n fencedplots was consistently aller thanunfencedvegetation nd elongatedculms toreacha maximum anopy height f 39 cm in lateApril.Mean averageheight verthegrowing eason was 12 cm in fenced plots and only 2 cm in unfencedplots(pairedt15 = 3.936,P < .01). Biomass concentrationwas much higher n grazedplots, averaging2.57 mg/cc,while fenced vegetationhad an average biomassconcentration f 1.15mg/cc pairedt15 = 2.753,P < .02).Heightand biomass dynamics n the ess heavilygrazed mid-heightrasslandswere much more similarwithin encedand unfenced lots fig. b). Fenced plotsreached a maximumheight f 58 cm in late May and unfencedplots reached amaximumheightof 45 cm at the same time. Mean canopy heightover a yearaveraged21 cm within hefence and 9 cm in unfenced reas (pairedt14 = 3.67, P< .01). Biomass concentration as higherngrazedvegetation hanwithin encedplots pairedt14 = 2.729, P < .02), averaging .78 mg/cc omparedwith .51 mg/cc inside fences.The largedifferences etweenunfenced hort ndmedium rasslandswere due



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868 THE AMERICAN NATURALIST60 -a b

50-0

40 -~~~~~~~~~~~~~~~40-

20-0,~~~~~~~~~~

10 - o 00~~~~~~~~~~~~

Jan Mar May Jul Sep Nov Jan Mar May JulDateFIG. 1.-Seasonal course of canopy height n grazed (closed circles) and fenced opencircles) vegetation. a, Growingseason in a shortgrassland; b, annual cycle in a mediumheight rassland.Each point s the mean of 5 replicates.

to differentensities fanimals,particularlyuring hegrowingeason. The shortgrasslands upportdense herdsofnomadicwildebeests Connochaetestaurinus),gazelles (Gazella thomsoni ndG. grantii), nd zebras (Equus burchelli) hrough-out the wet season (Grzimekand Grzimek 1960; Watson and Kerfoot 1964;Pennycuick1975)while those animals exploitthemid-grasslandsuring hedryseason whenplant growth otential s limited.Some regions of the ecosystem supportresidentherdsof animals throughouttheyear.To examine the annualdynamics f vegetation tructurensimilar reasthat varied in the densityof residentherds, comparedgrasslands n the highrainfallregionsof the northwesternerengetiNational Park and Masai MaraGame Reserve. The sites were separatedby a lineardistanceof 45 kmand annualrainfall uring he study yearwas 1156mmin the SerengetiNational Park and1046mm ntheMasai Mara Game Reserve. The SerengetiNationalPark sitehadaresidentherd of buffalo Syncerus caffer) nd some elephants Loxodonta af-ricana), but lacked conspicuousnumbers f other nimals. The Masai Mara sitesupportedbuffaloes,wildebeests,gazelles, zebras, and a large topi Damaliscuskorrigum) erd.Averagedover the year,mean canopyheight n the low animaldensity ite was 51 cm inside fences and 34 cm outside;values at the high nimaldensity itewere15 cm and 9 cm,respectively,n fenced ndunfenced lots Fsites= 91.1, P < .001; Ffence= 12.6, P < .005; Finteraction 2.98, P = NS; df = 1,36 inall cases).Biomass concentration t the low-animal-densityite was comparativelyowand uniformhroughoutheyearand did notvarywithmonthly ainfallfig.2a),



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l~~~~~~0.8 a0~~~~~~a 0og 0.6

00~~~~~~~~~~~~~~~ 0~~~~~~0.4 - 0

0 ~~~~~~0(0C),Uo

0E 0.0 0.2 ? 0

0 0 50 100 150MonthlyRainfall (mm)

3 bE

o 20,E0 - -~~~~

0 0~~~~~o 50 100 150E -000 -o,

0 -00000

0 0 50 100 150MonthlyRainfall (mm)FIG. 2.-Relationship between monthly ainfall nd biomass concentration mg/cc) nfenced open circles)and grazed closed circles) vegetation ver an annualcycle in medium-

tallgrasslands. a, Site in the SerengetiNational Park with low density f resident nimals(r2 = 0.185 inside fences and 0.111 outside fences, bothP = NS). b, Site in Masai MaraGame Reserve with a high densityof resident nimals r-2 = 0.558 for outsidefencesand0.584 for nsidefences,bothP < .001). Solid line = best fit orgrazed plots; dashed line =fencedplots. Each point s the mean of5 replicates.



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870 THE AMERICAN NATURALISTalthough twas consistently igher pairedt1o= 2.464, P < .05) outside exclo-sures 0.44 mg/cc) han nsidethem 0.34 mg/cc).At the site with highdensity fresidentgrazers, biomass concentration aried conspicuouslythrough he year,responding o rainfall fig.2b). Although enced and unfencedplots had similarbiomass concentration ynamics nresponse to rain,the average also was consis-tently ower (paired t1o= 2.265, P < .05) inside fences 0.88 mg/cc) han t wasoutside 1.35 mg/cc).The patterns f biomass concentration t those two sites suggestedthatonlypartof the structural ariationdetermining iomass concentration esultedfromimpactof animals on thevegetation nd thatpartwas because of nherent egeta-tion differences. he tendency orthesitewith highresident-animalensity obe shorter,ndfor tsbiomass concentrationo be higher nd rainfall-responsiveboth nside and outside fences, suggested hattherewere intrinsic ifferencesnvegetation tructure. hat resultwas borne out bya comparison f28 study itesfrom throughout he rainfallgradient across the Serengeti region. Maximumcanopy heights utside fencesaveraged5 cm for hortgrasslands,29 cmformid-grasslands, nd 71 cm fortall grasslands F2,25 = 55.6,P < .001). Insidefences,maximumcanopy heightsreached by short grasslandsaveraged 32 cm, mid-grasslandswere 42 cm, and tall grasslandswere 80 cm (F2,25 = 23.4, P < .001).Those differencesre notparticularlynterestingincethey ust provide a quan-titativedocumentation f a qualitative assignationof grass to different eightgroups. It is ecologicallysignificant, owever,that both the actual and relativedifferencesetween fenced and unfencedplotswas much greater n shortgrass-lands than n tallgrasslands.Inside fences,mean maximum iomass concentra-tions reached during he year were 4.3 mg/cc or shortgrasses, 1.2 mg/cc ormediumgrasses,and0.7 mg/cc or allgrasses F2,25 = 23.3,P < .001).Maximumbiomass concentrations utsidefences averaged6.6 mg/cc or shortgrass sites,2.0 mg/cc ormid-grasslands,nd 1.9 mg/cc or tall grasses (F ,25 = 15.1,P


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12- a0

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and ~ ~ ~ ~ ~ ~ mgunside Fences .1,P


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872 THE AMERICAN NATURALIST10 c

Em 180)

E 1W0.2 0o 004

0

0 0 %~0U 4 s **.O

0~~~~~~~~~~0Co 2 . 0( "

0 20 40 60 80 100Maximum Height (cm)FIG. 3 (Continued).-c, Relationshipbetweenmaximumheight nd maximumbiomassconcentrationn fenced closed circles)andgrazed open circles) vegetation. olid line s thebestfit orplotsoutside fences ,-2 = 0.531, P < .001); dashed line s forplots nside fences

(,s2 = 0.239, P < .01).

and fencedvegetation fig. 3c). In fencedplots,the two variableswere linearlyrelated withmg/cc = 4.7 - 0.055 ht

but therelationshipwas decidedlycurvilinearngrazedvegetation,mg/cc = exp[3.99 - 0.95 (In ht)]

withht ncm. These equations ndicatethat he maximum iomass concentrationofgrazed vegetationwas achieved onlywhen thecanopyheightwas arrested tveryshort tatures, .e., ingrazing awns.Grass Traits n a UniformGarden

The general staturedifferences mong the threegrass species were evident nthe transplant arden.Mean foliage heightwas 23 cm forThemeda, 14 cm forCynodon,and 11 cm forHarpachne (F2 24 = 12.1, P < .001). Floweringculmheightwas 85 cm for Themeda, 39 cm for Cynodon, and 45 cm for Harpachne(F222 = 12.7,P < .001). Cynodon nd Harpachne were ndistinguishableorbothtraits y rangetest,and each was significantlyifferentromThemeda. Althoughthe species maintained heir verage staturedifferencesn the uniform arden,each species varied between populationsin patternsrelated to environmentalproperties f thenative sites wheretheywerecollected.Leaf length f Themedaplantswas positively orrelatedwithmean annual rainfall f collectionsites fig.



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ANIMALS IN HERDS, PLANT FORM,COEVOLUTION 8734a), but was unassociated withgrazing ntensity t those sites F1,5= 0.51; rp=0.3; r = 0.13). Flowering ulm height f Themeda was unrelated o either ainfall(r = 0.35) or grazing ntensityr = - 0.39) at collection ites. But within he shortgrasses, flowering ulm heightwas positively orrelatedwithrainfall fig. c) andnegatively orrelatedwith grazing ntensityfig.4d) at the locations where theplants were collected. Partial correlation oupled with multiple egression ndi-catedthat he rainfall orrelationwas spurious F1,15= 1.6; rp= 0.307) and onlygrazingF1,15= 18.49, P < .001) was significantlyssociated withfloweringulmheight.Therefore, lthough he statures f Cynodon and Harpachne were posi-tively correlated with the rainfallgradient, nd that association was visuallyobviousas theplantswerecollectedandgrownnthegarden, heassociationwithenvironmental roperties t local sites indicatedthatgrazingwas the importantfactor ontrolling eight f those two species.Cynodonvariedconspicuously n a suite of traitswhengrown nthetransplantgarden. ts stoloniferous rowthhabit made variationmorevisuallyevidentandquantifiable, o several traitsrelated to morphology nd canopy structureweremeasured for that species thatwere not measured for the two bunchgrasses.Plantsfromnorthern, igher ainfall, ess heavilygrazed grasslandswerevisuallymore robust, taller, and occupied a larger,but less densely packed space thanplants from outhern, ower rainfall,more heavilygrazed grasslands. That com-plexofstructural hanges nCynodonultimately as tracedto a relatively implemorphological hangethathadmajoreffects pon plantgrowth orm. herewas astrongnegativeassociation betweengrazing ntensityt the collectionsite andinternodeengthn thetransplant arden fig.5a) with

L = 16.6 - 17.8 GwhereL was internode engthncmandG was grazing ntensity. he differencesin nternodeength amifiedhroughouthegrowth orm oproduce conspicuouslydifferenteometriesnplantsfrom ifferentnvironments.he area colonizedbya clone after yr n thetransplant ardenwas relatedto stolon nternodeength(fig.5b) by

area (M2) = (0.06 + 0.079 L)112,theheight fvegetative hoots was describedby (fig.5c)

ht (cm) = 9.9 + .86 Landshootdensitywithin he colonizedareawas related o nternodeength y fig.Sd)

N/M2 = [9.72 - 2.26 (L)1/2]2where L was mean internode ength n cm. Floweringculm heightalso waspositively elated o mean nternodeengthrs = 0.633,P < .02withdf= 13),butthe distribution equired nonparametric tatisticsand no line was calculated.There was no evidence that nternode engthwas associatedwith nnual rainfall tthesites ofplantcollectionfrom itherpartial orrelationrp= 0.19) ormultipleregressionF1,14 = 0.5), although hesimple orrelationwas marginallyignificant(r15= 0.433, P < .1).



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40 a

30 -~~~~~~~~~~~

, 20-Ja)X

-J

10500 700 900 1100Annual Rainfall (mm)

25 b0

20 -E 0 0~~15 -

10 \

050~~~~~~~~~~~~~~0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Grazing IntensityFIG. 4.-Associations betweenpopulation amplemeans fortraits n a transplant ardenand environmental roperties t sites wherethe sampleswere collected. a, Leaf length fThemeda triandra n relation o meanannual rainfallF1,5 = 8.62,P < .05). b, Leaf length fCynodon dactylon closed circles) and Harpachne schimperi open circles) in relationtograzing ntensityF1,16 = 11.86,P < .005). (Continued)

874



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80 C

0E 60o 0 0

E 0 0~~~~~~~~~I *fi40 *f 040 0

o 0020-

0 500 700 900 1100Annual Rainfall (mm)

80 d0

60 -E 00Z~~~~~~~M 40 -

E20 - ~ ~ ~ ~0~ ~~~~~~~~~~~~0

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7Grazing IntensityFIG. 4 (Continued).-c, Flowering ulmheight fC. dactylon nd H. schimperinrelationto rainfall. , Flowering ulmheight fC. dactylon ndH. schimperi nrelation o grazing.Partially illed ircles are superimposeddata pointsforbothspecies.

875



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20 -a2015

' 10 \C.\

0~~~~~

0 0.1 0.2 0.3 0.4 0.5 06 0.7Grazing Intensity

1.2 -

CE~~~~~~~~C 0.0~~~~~~~~~~~~~~~0.4~~~~~~~~~~~~

0~~~~~~~~~~~~~

0 5 10 15 20Internode Length (cm)FIG. 5.GRelationship between nternodeength f C. dactylonnd: a, grazing ntensitytthe sites whereplantswere collected F1,14 = 22,P < .001); b, area colonized after yr n thetransplant arden F1,15 = 56, P < .001). (Continued)

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30 - cE~~~~~~~~~~U~~~~~~~~~~~

- 20 -0.~~~~~~~~~

CL0~~~~~~~~~~~~~

lo 0

0 5 10 15 20Internode Length (cm)

8 d

6z Z~~~~~

U,

4

2

0.~~~~~~~~~~~0 5 10 15 20Internode Length (cm)

FIG. 5 (Continued).-c, Canopy heightF1,15= 8.22, P < .025); and d, density f rootedshoots nthecolonized area (F1 15 = 29, P < .001). Significance est n a is from he multipleregressionwithrainfall s an additionalvariable, as explained n the text.

877



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878 THE AMERICANNATURALISTDISCUSSION

The grazing ungulates of the Serengeti region have effectsupon grasslandgeometrynbothecological and evolutionary ime.They maintain he shortgrassregions n a permanent razing awn throughout he year since theygo there ssoon as there s sufficientain o generate lantgrowth nd remain here s long asrain continues Grzimek and Grzimek 1960; Watson and Kerfoot1964; Penny-cuick 1975; Inglis 1976).Mean plant heightwithin enced plotsaveraged6 timesand maximumheightwas 16 times that of plants withinunfencedplots on theSerengeti Plains. Biomass concentration, owever, was over twice as highinunfencedplots since grazingreduced heightmore than t reducedplantbiomass.The tillering nd subsequent high canopy density associated withdefoliation(McNaughtonet al. 1983) led to higherbiomass concentrationsn grazed plots.Mediumheight rasslandswere ess distinctwhenfenced ndunfenced lotswerecompared, although hecomparisonof sitesdifferingn thedensitiesofresidentgrazers ndicated hatgrazers,whenpresent, lso controlled hestructure f tallergrasslands.The properties f fencedgrasslands covaried withthose ofadjacent unfencedplotson a regionalbasis,withbothheight nd biomass concentrationorrelatednthe comparison, ndicating hat only part of the structural ifference etweengrazed and ungrazedvegetationwas due to the proximal ffects f the animals.Transplantgarden studies documented genetic differences etween and withinspecies in relation o both therainfall nd the grazing ntensity fthe native sitefromwhich ndividualswere obtained. Therefore, he intrinsictructural iffer-ences ingrasses from ifferent abitats,determinedn partbyclimate nd inpartby grazing Coughenour 1984), are accentuated and amplifiedby the currentgrazing regimethat the plants experience. Those differences re related to thetendency of animals to formboth intraspecific nd interspecificggregationsduring ertain easons. The difference etween the two northernites thatvariedinresident nimaldensitywas accompanied by several more nimal pecies at themore dense site.

Whydo such a diversearrayof animals nbothaquatic and terrestrialcosys-temsconcentrate heir ctivities nspace andtime, hereby reating ndmaintain-ing grazing awns (Seton 1910;Roe 1951; Vesey-Fitzgerald 960;Bannikovet al.1961; Lamprey 1964;Bell 1970;Grubb1971; Dary 1974;Estes 1974;Jarman 974,1979;Duncan 1975;Hoeck 1975; McNaughton 1976, 1979a, 1979b;Leuthold1977;Sinclair1977; Bjorndahl1980;Hunter1980;Lein 1980;Prins et al. 1980; Cargill1981; Tomlinson 1981; Clutton-Brock t al. 1982; Murray 1982; Coppock et al.1983a, 1983b; Gibson and Hamilton 1983)? The combination of the geneticdwarfingfgrassesfrom razedareas (Gregor nd Sansome 1926;Stapledon 1928;Turesson 1929; Kemp 1937; Bradshaw 1960; Lodge 1962;Warwick and Briggs1978b;McNaughton 1979a;Detling nd Painter 983), animalmorphology egulat-ing bility oforage nvegetation f differenttaturesBell 1970),behavior eadingto close cropping fvegetationJarman 974),anddigestive raits elated o size-dependent egetation ualityHofmann1968) ndicate hat hegrassesandgrazingmammals have coevolved as interacting omponentsof a trophicweb. Various



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ANIMALS IN HERDS, PLANT FORM,COEVOLUTION 879factorsmay contribute o herdformation y grazingungulates nd other nimals(Williams 1966; Eibl-Eibesfeldt1970; Ewer 1968; Horn 1968; Hamilton 1971;Tenaza 1971; Pulliam 1973; Alexander 1974; Estes 1974; Jarman1974; Caraco1979a, 1979b, 1981; Hoogland 1981; Caraco and Pulliam 1984). No unitary xpla-nation is feasible, but the results of the present study suggestthat foragingfacilitation esulting rom he ability f arge, dense animalaggregationso createand maintain egetation f highbiomass concentration nd qualitymay have beenimportantn evolutionary imeand is importantn ecological time.Since dense herding educesforage tanding rop considerably,t would seemadvantageous to individualsto dissociate themselves fromthe herd and seekgreener pastures elsewhere if food was involved in herd formation.But thegrazing awns have higher roductivitiesMcNaughton 1976, 1979a, 1979b, 1984)and are food of higherquality (Sinclair 1977; McNaughton et al. 1982, 1983;Misleavy et al. 1982). Grazingalso enhances the productivityCooper 1973) andquality (Hunter 1980; Hunter and Russell-Hunter 1983) of the vegetation naquatic ecosystems. Concentrated nimal populationshave a drasticeffect ponvegetation eometry s well as its standing rop and productivity.Data from xperimentalmanipulations f forage tructure ithhormonal reat-ments Ludlow et al. 1982) ndicatethatforageyield per bite s positively elatedto biomass concentrationfig. ). Lower forage iomass concentrationeduces theforage hatcan be garneredbythe tongue-sweeping eeding f ungulates Stobbs1973a, 1973b, 1975). Low plant biomass concentrations, ven if total standingcrop s high, an result nforage onsumption ates nsufficientomeet herbivoreenergyand nutritional equirementsChacon and Stobbs 1976; Chacon et al.1978), eadingto decliningherbivore ondition Stobbs 1977; Chacon et al. 1978)amidhighplantbiomasses. Below a bitesize of about 0.3 g, a cow-sizedanimal sfood limited Stobbs 1973a). The best fitequation from the data of figure6indicatesthatcow bite size will fall below that evel at plant biomass concentra-tionsbelow 0.8 mg/cc.Mostvegetationwith canopy height reater han bout40cm in the Serengeti cosystemwould have a biomass concentrationowerthanthatmaintenance evel. By keeping hevegetation ven lower, particularlyelow10cm, grazers n theSerengeti an increasetheirharvestingatedramaticallyndthey also are foragingwithin a height range where biomass concentration sextremelyheightsensitive. Although some of this foragemay be below theanimal's harvesting eight Bell 1970), vegetationwithin hatforaging eightwillbe rapidlyrenewed afterdefoliationbecause of greatly ncreasedrates of leafelongation McNaughtonet al. 1983).Much of theconcentration f herbivore oragingn time ndspace, i.e., the herdformationypical fextremely iversegrazing nimals,maybe due tothegreaterforaging fficiencyf ndividualsn suchherds. The benefits fgreater oodyieldper bite froma dense canopy confers a major selective advantage upon theindividual. Although selection is at the individual evel, sward structures aconsequence of the behaviorof thepopulation s a whole. Lone animalscertainlycan concentrate heirforagingn small areas, thereby reating he local grazinglawn patches observed in vegetationgrazed by domesticanimals (Burt 1949;Hunter1964),but lone wild animals are likelyto be highly ubjectto predation



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880 THE AMERICAN NATURALIST0.34-

0.32 -~~~~~~~~~~~0.320' 0.30 -

0.28 -

0.26

0.4 0.6 0.8 1.0 1.2Biomass Concentration (mg/cc)FIG. 6.-Relationship between average size of bite taken by grazingcattle and biomassconcentration f the sward being grazed 12 = 0.828, P < .025); data fromLudlow et al.(1982).

(Kruuk 1972; Schaller 1972; Sinclair 1974; Estes 1976), particularlyince suchanimals wouldhave very predictablepatterns fspace utilization.Thus,whileasolitary orager ouldmaintain grazing awn,doing o wouldrequire sedentarylife stylethatwouldmakethe ndividualmore prone to predation.Alternatively,predation n lone animalswould tend to lead to a mobility otallowingfrequentenoughreturn o a givenpatch of vegetation omaintain t ntheactivelygrowing,highbiomass concentration, utritious tate characteristic f grazing awns. Infact, ome territorialrazing ishncoralecosystemsdo maintain razingawnsasloneindividuals ndexclude otherherbivores rom hose awns Sammarco1983).Their bility odo thismaybe related o thespatialrefuges rom redation reatedbythecoralline andscape and thetemporalpredictabilityfproductivityn thatenvironment.n open landscapes where productivitys less predictable patiallyand temporally,membersof a herd, less constrainedby predationthan loneindividuals,can both create and maintaingrazing awns, witheach individualreaping fitness ainresulting romncreasedpercapita forageyieldperbiteoversome rangeof herbivore ensity.Therefore, he nterplay etween thepredationsusceptibilityf one animals nlandscapes lacking patial refuges rom redationand thefeeding dvantagesthat ccrueto individuals y beingmembers f a herdprobably nteracted o contribute o the evolutionof gregarious ehavior.Plantshave therefore volved a variety f traits ssociated withheavy grazing.Genetic studiesofplantdwarfingngrainbreeding rograms ndicate hat singledominant enecontrollingnternodeength n wheat Triticum estivum) an lead



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ANIMALS IN HERDS, PLANT FORM,COEVOLUTION 881to the whole syndrome f properties Qualset et al. 1970) thatweredocumentedforCynodon from he Serengetigrasslands.Four recessive, additive, and inde-pendently ransmitted ene pairs in barley Hordeum vulgare) controldwarfing(Sears et al. 1981). Narrow sense heritabilitystimates orplantheight, rectness,and leafiness of indiangrass Sorthastrumnutans) were 0.75, 0.48, and 0.36,respectively Vogel et al. 1981). Coefficients f selection against erect plants nmowed awnswere between 0.53 and 0.68, and were 0.77 againstprostrate lantsin unmowed areas (Warwick and Briggs1978b). Thus, relatively imple geneticchanges,highheritabilities,nd strong electionpressures re sufficiento explainthe rapid evolution of grasses in response to the prevailingdefoliationregime(Kemp 1937;McNaughton 1979b;Detling ndPainter1983).The stature radientsrelated to rainfallmay create geneticgradients nd genetic variation mplifiedstrongly y grazing Coughenour 1984). In combinationwith themany traitsofanimals thatare related to theirforaging atterns, heproperties f thegrassesfromthe Serengeti region suggest coordinatedcoevolution of members of thesametrophicweb. Thoughselection s at the ndividualevel,theconsequencesofthat selection have important mplications ororganization f the trophicweb,species diversity, oexistence among members of the animal and plant com-munities, nd suchecosystemprocesses as energy low ndnutrientycling.Thehighrates ofenergy nd nutrient low ustainedper unitof plant standing rop inthe Serengetiregion (Ruess 1984; Ruess et al. 1983; McNaughton 1984) arestrongly egulatedby the adaptive propertiesof both the animals and plantsresulting rom heir nteractions nd consequentevolution ver ongtimeperiods.

SUMMARYMany grazing animals in both terrestrial nd aquatic ecosystems formdenseherds thatmaintain he vegetation n their oncentration reas at very ow stat-ures. Studiesoftheeffects f arge ungulates n the structure fgrasslands ntheSerengeti region of Tanzania and Kenya indicated that some vegetationwas

arrested n a shortform hroughout he wet season while othervegetationwasonly ightly razedduring hatseason and reached similarheights nfenced andunfenced reas. Biomass concentration, he mass offoliage per unit of canopyvolume, was consistently nd substantiallyhigher n unfenced plots but thecovariation fheight ndbiomass concentration crossplots alongtheSerengeti'shabitatgradientndicatedthat the tendency owardproduction f a dense, pros-trategrazing awn in ecological time was accentuatedby, but was not solely aconsequence of, proximal grazing intensity.Transplant garden studies docu-mented intrinsicdifferenceswithin and between species in traits related todwarfing. omparisonsof theSerengetidata with nformationn theforaging fdomesticungulates ndicatedthat ndividualgrazers obtaina foragingdvantageby membershipn a herdbecause of thegreater orageyieldperbitefrom razinglawns compared with lightly razed vegetation.Thus, natural selectionat theindividual evel,actingon both animals and plantsto producecoevolution mongmembersof the same trophic web, can regulatesuch ecosystem processes as



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882 THE AMERICANNATURALISTenergyflowand nutrient ycling, nd contribute o species coexistenceand theresultant pecies diversity f communities.

ACKNOWLEDGMENTSMargaret, ean, and ErinMcNaughtonwere essentialfield ssistants.Permis-sionto live and work n the SerengetiNationalParkwas granted ytheDirectorand TrusteesofTanzania NationalParksand permission owork n Masai MaraGame Reserve was grantedby the governments f Kenya and Narok District.Officersnd staff fthe SerengetiResearchInstitutend SerengetiNationalParkfacilitatedmyresearch nmanyways. Financialsupportwas fromNSF BMS 74-02043,DEB 77-20360, nd DEB 79-22991.

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