Increased susceptibility to enemies following introduction in the invasive plant Silene latifolia

EcologyLetters,(2004)7:813–820doi:10.1111/j.1461-0248.2004.00649.x

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IncreasedsusceptibilitytoenemiesfollowingintroductionintheinvasiveplantSilenelatifolia

Abstract

Oneofthein uentialhypothesesinvokedtoexplainwhyspeciesbecomeinvasivefollowingintroductionisthatreleasefromnaturalenemiesfavoursashiftininvestmentfromdefencetotraitsenhancinggrowthandreproduction.SilenelatifoliawasintroducedfromEurope(EU)toNorthAmerica(NA)c.200yearsagowhereitexperienceslowerdamagebynaturalenemies.AcommongardenexperimentinEUusingseedsfrom20EUand20NApopulationsrevealed(1)genetically-baseddifferencesinlifehistorybetweenplantsfromEUandNA;plantsfromNAhaveevolvedaweedyphenotypethatflowersearlier,andhasatwo-tothreefoldhigherreproductivepotential;(2)highersusceptibilityofNAplantstofungalinfection,fruitpredation,andaphidinfestation.resultssuggestthattheinvasiveNAphenotypehasevolvedattheexpenseofdefensiveabilities.Despitethisincreasedsusceptibilitytoenemies,NApopulationsstilloutperformedEUpopulationsinthiscommongarden.

Keywords

Aphid,biologicalinvasion,commongarden,Brachycauduspopuli,enemyreleasehypothesis,fruitpredation,Hadenabicruris,Microbotryumviolaceum,pathogeninfection,Silenelatifolia.

EcologyLetters(2004)7:813–820

LorneM.Wolfe,1*JelmerA.Elzinga2andArjenBiere2

1

DepartmentofBiology,

GeorgiaSouthernUniversity,Statesboro,GA30460,USA

2

DepartmentofPlant

PopulationBiology,

NetherlandsInstituteofEcology(NIOO-KNAW),POBox40,6666ZGHeteren,TheNetherlands*Correspondence:E-mail:wolfe@georgiasouthern.edu

INTRODUCTION

Despitethefactthatbiologicalinvasionsrepresentoneofthegreatestthreatstobiodiversity,westillknowrelativelylittleabouttheirunderlyingcauses(Cox1999;Pimenteletal.2000;Simons2003).Whydoesaspeciesthatisabenignentityinitsnativeecosystembecomeinvasivefollowingitsmovementtoanewlocation?Thetraditionalexplanationintheliteraturearguesthatenhancedperformanceisaplasticresponsetoareleasefromdamagebyenemiesnormallypresentinthenativerange(Crawley1987).Indeed,therehavebeenanumberofrecentlypublishedstudiesthathavefoundsupportfortheescape-from-enemieshypothesisbothwithrespecttoabove-ground(Wolfe2002;Mitchell&Power2003;Torchinetal.2003)andbelow-ground(Reinhartetal.2003)naturalenemies.Yet,reducedinterac-tionswithpathogens,herbivoresandpredatorsisaphenomenonthatisecologicalinnatureandmayalsohaveevolutionaryconsequences.Whilemechanicaland/orchem-icaldefensivecharactersmaybeadaptiveinthepresenceofenemies,theybecomeunnecessaryandcostlywhentheseenemiesareabsent.Asaresult,selectionshouldfavourreducedinvestmentintodefence,andsavedresourcesmay

bereallocatedtocontributetoenhancedgrowthandreproduction(Blossey&No¨tzold1995).Clearly,thechallengetounderstandinginvasionsrestsindeterminingtherelativecontributionsofecologicalandgenetically-basedevolutionaryforces(Sakaietal.2001;Lee2002).

Themostdirectwaytodetermineifinvasivenessistheresultofphenotypicplasticityorgeneticsistogrownativeandintroducedspecimensinacommongarden.Undersuchcontrolledconditions,anydifferencesthatariseinpheno-typictraitsbetweenmaterialfromthetworangesareconsideredtobegenetically-based.Incontrast,ifnativeandintroducedindividualsdonotdifferwhengrowntogether,itmaybeconcludedthattherehasnotbeenevolutionarychangesinceintroduction.

Asmallnumberofstudieshaveusedthecommongardenapproachtoexplorethebasisofbiologicalinvasionsandthesehavefoundequivocalresults.Willisetal.(2000)foundnodifferencesinfourplantspeciessampledfromtheirintroducedandnativeranges.Ontheotherhand,introducedSapiumsebiferumattainedgreatersizeafter14yearsofgrowthcomparedwithnativematerial(Siemann&Rogers2001).Similarly,Leger&Rice(2003)reportedthatintroducedCaliforniapoppies

Ó2004BlackwellPublishingLtd/CNRS

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