Chapter 15MutualismWord下载.docx

Chapter 15MutualismWord下载.docx

《Chapter 15MutualismWord下载.docx》由会员分享，可在线阅读，更多相关《Chapter 15MutualismWord下载.docx（43页珍藏版）》请在冰豆网上搜索。

Meanwhile,backabovegroundadeerenterstheforestgladeandwandersovertotheplantrecentlyvisitedbythehummingbird.Thedeersystematicallygrazesittotheground,lightlychewstheplantmaterial,andthenswallowsit.Astheplantmaterialentersthedeer'

sstomach,itisattackedbyavarietyofprotozoansandbacteria.Thesemicroorganismsbreakdownandreleaseenergyfromcompoundssuchascellulose,whichthedeer'

sownenzymaticmachinerycannothandle.Inreturn,theprotozoansandbacteriareceiveasteadyfoodsupplyfromthefeedingactivitiesofthedeeraswellasawarm,moistplaceinwhichtolive.

FIGURE15.1Hummingbirdsfeedingonnectartransferpollenfromflowertoflower.

Theseareexamplesofmutualism,thatis,interactionsbetweenindividualsofdifferentspeciesthatbenefitbothpartners.Somespeciescanlivewithouttheirmutualisticpartnersandsotherelationshipiscalledfacultativemutualism.Otherspeciesaresodependentuponthemutualisticrelationshipthattheycannotliveinitsabsence.Sucharelationshipisanobligatemutualism.ItisacuriousfactthatthoughobserversofnatureasearlyasAristotlerecognizedsuchmutualisms,mutualisticinteractionshavereceivedmuchlessattentionfromecologiststhanhaveeithercompetitionorexploitation.Doesthislackofattentionreflecttherarityofmutualisminnature?

Asyouwillseeinthefollowingpages,mutualismisvirtuallyeverywhere.

Mutualismmaybecommon,butisitimportant?

Doesitcontributesubstantiallytotheecologicalintegrityofthebiosphere?

Theanswertoboththesequestionsisyes.Withoutmutualismthebiospherewouldbeentirelydifferent.Let'

sremovesomeofthemoreprominentmutualismsfromthebiosphereandconsidertheconsequences.Anearthwithoutmutualismwouldlackreef-buildingcoralsasweknowthem.SowecanerasetheGreatBarrierReef,thelargestbiologicalstructureonearth,fromourhypotheticalworld.Wecanalsoeliminateallthecoralatollsthatdotthetropicaloceansaswellasallthefringingreefs.Thedeepseawouldhavenobioluminescentfishesorinvertebrates.Inaddition,thedeep-seaoasesoflifeassociatedwithoceanfloorhot-watervents,discoveredjusttwodecadesago（seechapter6）,wouldbereducedtononmutualisticmicrobialspecies.

Onland,therewouldbenoanimal-pollinatedplants:

noorchids,nosunflowers,andnoapples.Thepollinatorsthemselveswouldalsobegone:

nobumblebees,nohummingbirds,andnomonarchbutterflies.Gonetoowouldbealltheherbivoresthatdependonanimal-pollinatedplants.Withoutplant-animalmutualismstropicalrainforests,themostdiverseterrestrialbiomeontheplanet,wouldbeallbutgone.Manywind-pollinatedplantswouldremain.However,manyofthesespecieswouldalsobesignificantlyaffectedsinceapproximately90%ofallplantsformmycorrhizae.Thoseplantscapableofsurvivingwithoutmycorrhizalfungiwouldlikelyberestrictedtothemostfertilesoils.

Evenifwind-pollinated,nonmycorrhizalplantsremainedonourhypotheticalworldtherewouldbenovastherdsofAfricanhoofedmammals,nohorses,andnoelephants,camels,orevenrabbitsorcaterpillars.Therewouldbefewherbivorestofeedontheremainingplantssinceherbivoresanddetritivoresdependuponmicroorganismstogainaccesstotheenergyandnutrientscontainedinplanttissues.Thecarnivoreswoulddisappearalongwiththeherbivores.Andsoitwouldgo.Abiospherewithoutmutualismwouldbebiologicallyimpoverished.

Theimpoverishmentthatwouldfollowtheeliminationofmumalism,however,wouldgodeeperthanwemightexpect.LynnMargulisandothers（MargulisandFester1991）haveamassedconvincingevidencethatalleukaryotes,bothheterotrophicandautotrophic,originatedasmutualisticassocia,tionsbetweendifferentorganisms.Eukaryotesareapparentlytheproductofmutualisticrelationshipssoancientthatthemutualisticpartnershavebecomecellularorganelles（e.g.,mitochondriaandchloroplasts）whosemumalisticoriginslongwentunrecognized.Consequently,withoutmutualismalltheeukaryotes,fromHomosapienstotheprotozoans,wouldbegoneandthehistoryoflifeonearthandbiologicalrichnesswouldbesetbackabout1.4billionyears.

Butbackhereinthepresent,let'

sacceptthatmutualismisanintegralpartofnatureandreviewwhatisknownoftheecologyofmutualism.Thefirstpartofthisbriefreviewemphasizesexperimentalstudies.Then,inthelastpartofthechapter,weexaminesometheoreticalapproachestothestudyofmutualism.

CONCEPTS

●Plantsbenefitfrommutualisticpartnershipswithawidevarietyofbacteria,fungi,andanimals.

●Beef-buildingcoralsdependuponmutualisticrelationshipswithalgaeandanimals.

●Theorypredictsthatmutualismwillevolvewherethebenefitsofmutualismexceedthecosts.

CASEHISTORIES:

plantmutualisms

Plantsbenefitfrommutualisticpartnershipswithawidevarietyofbacteria,fungi,andanimals.

Plantsarethecenterofmumalisticrelationshipsthatprovidebenefitsrangingfromnitrogenfixationandnutrientabsorptiontopollinationandseeddispersal.Itisnoexaggerationtosaythattheintegrityoftheterrestrialportionofthebiospheredependsuponplant-centeredmutualism.However,tounderstandtheextenttowhichecologicalintegritymaydependupontheserelationshipsweneedcarefulobservationalstudiesandexperiments.Herearesomedrawnfromstudiesofmycorrhizae.

PlantPerformanceandMycorrhizalFungi

Thefossilrecordshowsthatmycorrhizaearoseearlyintheevolutionoflandplants,perhapsaslongas400millionyearsago.Overevolutionarytime,amutualisticrelationshipbetweenplantsandfungievolvedinwhichmycorrhizalfungiprovideplantswithgreateraccesstoinorganicnutrientswhilefeedingofftherootexudatesofplants.Thetwomostcommontypesofmycorrhizaeare

（1）arbuscularmycorrhizalfungi（AMF）,inwhichthemycorrhizalfungusproducesarbuscules,sitesofexchangebetweenplantandfungus,hyphae,fungalfilaments,andvesicles,fungalenergystorageorganswithinrootcortexcells,and

（2）ectomycorrhizae（ECM）,inwhichthefungusformsamantlearoundrootsandanetlikestructurearoundrootcells（fig.15.2）.Mycorrhizaeareespeciallyimportantinincreasingplantaccesstophosphorusandotherimmobilenutrients（nutrientsthatdonotmovefreelythroughsoil）suchascopperandzinc,aswellastonitrogenandwater.

FIGURE15.2Mutuatisticassociationsbetweenfungiandplantroots:

（a）arbuscularmycorrhizalfungusstainedsothatfungalstructuresappearblue;

and（b）ectomycorrhizae,whichgiveawhitefuzzyappearancetotheseroots.

MycorrhizaeandtheWaterBalanceofPlants

Mycorrhizalfungiappeartoimprovetheabilityofmanyplantstoextractsoilwater.EdieAllenandMichaelAllen（1986）studiedhowmycorrhizaeaffectthewaterrelationsofthegrassAgropyronsmithiibycomparingtheleafwaterpotentialsofplantswithandwithoutmycorrhizae.Figure15.3showsthatAgropyronwithmycorrhizaemaintainedhigherleafwaterpotentialsthanthosewithoutmycorrhizae.Thismeansthatwhengrowingundersimilarconditionsofsoilmoisture,thepresenceofmycorrhizaehelpedthegrassmaintainahigherwaterpotential.Doesthiscomparisonshowthatmycorrhizaearedirectlyresponsibleforthehigherleafwaterpotentialobservedinthemycorrhizalgrass?

No,theydonot.Thesehigherwaterpotentialsmaybeanindirecteffectofgreaterrootgrowthresultingfromthegreateraccesstophosphorusprovidedbymycorrhizae.

FIGURE15.3InfluenceofmycorrhizaeonleafwaterpotentialofthegrassAgropyronsmithii（datafromAllenandAllen1986）.

Plantswithgreateraccesstophosphorusmaydeveloprootsthataremoreefficientatextractingandconductingwater;

mycorrhizalfungimaynotbedirectlyinvolvedintheextractionofwaterfromsoils.KayHardie（1985）testedthishypothesisdirectlywithaningeniousexperimentalmanipulationofplantgrowthformandmycorrhizae.First,shegrewmycorrhizalandnonmycorrhizalredclover,Trifoliurnpratense,inconditionsinwhichtheirgrowthwasnotlimitedbynutrientavailability.Theseconditionsproducedplantswithsimilarleafareasandroot:

shootratios.Underthesecarefullycontrolledconditions,mycorrhizalredclovershowedhigherratesoftranspirationthannonmycorrhizalplants.

Hardietookherstudyonestepfurtherbyremovingthehyphaeofmycorrhizalfungifromhalfoftheredcloverwithmycorrhizae.Shecontrolledforpossiblesideeffectsofthismanipulationbyusingatracerdyetocheckforrootdamageandbyhandlingandtransplantingallstudyplants,includingthoseinhercontrolgroup.Removinghyphaesignificantlyreducedratesoftranspiration（fig.15.4）,indicatingadirectroleofmycorrhizalfungiinthewaterrelationsofplants.Hardysugges