复杂群落中的生态入侵行为 Collective dynamical regimes predict invasion success and impacts in microbial communities

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NatureEcology&Evolution

natureecology&evolution

Article

/10.1038/s41559-024-02618-y

Collectivedynamicalregimespredict

invasionsuccessandimpactsinmicrobialcommunities

Received:22February2024

Accepted:25November2024

Checkforupdates

JiliangHu1,2,MatthieuBarbier3,4,GuyBunin

5&JeffGore

1

Theoutcomesofecologicalinvasionsmaydependoneithercharacteristicsoftheinvadingspeciesorattributesoftheresidentcommunity.Herewe

useacombinationofexperimentsandtheorytoshowthattheinterplay

betweendynamics,interactionstrengthanddiversitydeterminethe

invasionoutcomeinmicrobialcommunities.Wefndthatthecommunitieswithfuctuatingspeciesabundancesaremoreinvasibleanddiversethan

stablecommunities,leadingtoapositivediversity–invasibilityrelationshipamongcommunitiesassembledinthesameenvironment.Aspredictedby

theory,increasinginterspeciesinteractionstrengthandspeciespoolsize

leadstoadecreaseofinvasionprobabilityinourexperiment.Ourresults

showapositivecorrespondencebetweeninvasibilityandsurvivalfractionofresidentspeciesacrossallconditions.Communitiescomposedofstronglyinteractingspeciescanexhibitanemergentpriorityefectinwhichinvaderspeciesarelesslikelytocolonizethanspeciesintheoriginalpool.However,ifaninvasionissuccessful,itsecologicalefectsontheresidentcommunityaregreaterwheninterspeciesinteractionsarestrong.Ourfndingsprovideaunifedperspectiveonthediversity–invasibilitydebatebyshowingthat

invasibilityandinvasionefectareemergentpropertiesofinteractingspecies,whichcanbepredictedbysimplecommunity-levelfeatures.

Ecologicalinvasions,characterizedbythespreadofnon-nativespeciesintonewenvironments,haveimportantconsequencesforbiodiversity,ecosystemfunctionandhabitatresilience

1

.Overdecades,ecologistshavesoughttounravelthemyriadfactorsinfluencingwhysomespe-ciesinvadesuccessfullyandwhysomeofthosehavelargeimpactsonresidentspeciescommunities,whileothersdonot.Ecologistshavepositedarangeofdeterminants,fromthefitnessandadaptabilityoftheinvaderstotheresilienceandcompositionofnativecommunities

2

–4

.Amongstudiesfocusingontheinvaderspecies,manyhavesoughttoidentifytraits,suchasgrowthanddispersalstrategies,thatmayshapeinvasionoutcomes

5

.Othershaveemphasizedtheroleoftheinvaders’initialpopulationsizeinthelikelihoodofestablishmentand

spread

6

,7

.Yetothershaveemphasizedinteractionswithresidentspe-cies;forexample,theenemyreleasehypothesisthatinvasivespeciesoftensucceedinnewenvironmentsbecausetheylackconsumersorpathogens

8

.Thishasledtoresearchonhowpropertiesofresidentcommunitiesasawholecandeterminetheinvasionoutcome.Forinstance,thebioticresistancehypothesissuggeststhatcommunitieswithhighnativebiodiversityaremoreresistanttoinvasionthanlessdiversecommunities,duetomoreefficientresourceuseorpresenceofnaturalenemies,butitisnotconsistentlysupportedbyempiricalresults

9

–

12

.Beyondthecharacteristicsofinvaderspeciesandresidentcommunities,environmentalconditionshavebeenshowntoplayacrucialroleinshapingtheinvasionoutcome

1

.Forexample,theories

1PhysicsofLivingSystems,DepartmentofPhysics,MassachusettsInstituteofTechnology,Cambridge,MA,USA.2DepartmentofMechanicalEngineering,

MassachusettsInstituteofTechnology,Cambridge,MA,USA.3CIRAD,UMRPHIM,Montpellier,France.4PHIMPlantHealthInstitute,MontpellierUniversity,

CIRAD,INRAE,InstitutAgro,IRD,Montpellier,France.5DepartmentofPhysics,Technion—IsraelInstituteofTechnology,Haifa,Israel.e-mail:

gore@

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suchasthestorageeffectandthefluctuatingresourceavailabilityhypothesispositthatenvironmentaldisturbancesandfluctuationsmightfavourinvaderspeciesinspecificperiods

13

–

15

.

Morerecently,theissueofecologicalinvasionhasbecomesalientinthestudyofmicrobialcommunities,rangingfromsoilandaquaticecosystemstothehumanbody

16

–22

.Theseinvasionscanhaveprofoundimpactsonecosystemservicesandhumanhealth

16

,

17,

19,

20

.Pathogenicmicroorganismscaninvadehost-associatedmicrobialcommunities,leadingtoinfectionsanddisease

19

,23

,24

.Forexample,theinvasionofthepathogenicmicroorganismClostridiumdifficileintothegutmicrobiotacanleadtoseverediseases,includingdiarrhoeaandcolitis

23

,25

.Under-standingthemechanismsunderlyinginvasionsuccessandecologicalconsequencescanhelptoinformstrategiesfordiseaseprevention,aswellasthedevelopmentoftargetedtherapiestocontrolinvasivepathogens

25

,

26

.Similartolarger-scaleecologicalsystems,ithasbeensuggestedthatmicrobialcommunitieswithhigherdiversity(numberofspecies)arelesslikelytobeinvadedbecausediverseresidentspeciesmayoccupyallavailablenichesbyconsumingallresources,leavinglessroomforinvaders

18

,27

–29

.Furthermore,itwasshownthatfacilitativeandcompetitiveinteractionsbetweenmicroorganismscanfavourandpreventsuccessfulinvasions,respectively

27,

30

–

32

.Paralleltoobserva-tionsinmacroorganisms,externaldisruptions,suchasantibioticinterventionsornutrientlevelshifts,canheightenthevulnerabilityofmicrobialcommunitiestoinvasions

16

,33

–

35

.

Whileresearchinmicrobialinvasionshasmadeimportantstrides,itremainsunclearwhatcharacteristicsofresidentcommunitiesdeter-minethesuccessandimpactsofaninvasion

17

,18

,

36

,37

.Speciesdiversityisaneasilymeasuredindicator,butitsrelationshiptoinvasibilitymaynotbestraightforward,whereasspeciesinteractionsareprobablyimportantbutoftendifficulttoquantify.Ararelyemphasizedpropertyisthedynamicsoftheresidentcommunity:arethespeciesabundancesconstantovertime,consistentwithastablestateoraretheydetermin-isticallyfluctuating?Itisnotobviousthatwecancharacterizedynamicsatthelevelofthecommunity;yet,buildinguponthegroundbreak-ingworkofRobertMay,ecologistshaveexploredthepossibilityofcommunity-wideemergentdynamics,whichcanbeclassifiedintoonlyafewqualitativelydistinctregimesandpredictedfromafewmacroscopicparameters

10

,38

–

43

.Inarecentstudy

40

,weexperimentallyassembledcommunitiesfromvariouspoolsofmicrobialspeciesindifferentconditionsandconfirmedthatsimplecommunity-levelfea-tures,includingspeciespoolsizeandinterspeciesinteractionstrength,determineddistinctdynamicalregimescharacterizedbythefractionofsurvivingspeciesandtheemergenceofdeterministicabundancefluctuationsovertime.Asspeciespoolsizeandstrengthofinterac-tionsincrease,wefoundthatmicrobialecosystemstransitionbetweenthreedistinctdynamicalphases,fromastableequilibriuminwhichallspeciescoexisttopartialcoexistencetotheemergenceofpersistentfluctuationsinspeciesabundances

40

.

Hereweperforminvasionexperimentsindiverseassembledmicrobialcommunitiesandobservethattheforemostpredictorofinvasionoutcomesappearstobethedynamicalstateoftheresidentcommunity.Wethenuseacombinationofexperimentsandtheory,exploringseveraldynamicalregimesandspanningtheircontrolparam-eters(speciespoolsizeandinteractionstrength)toshowthat,takentogether,theyexplainmanyfeaturesofinvasibilityandinvasioneffects.Communitiesofweaklyinteractingspeciesreachastablecomposition,whereafractionoftheinitialspeciespoolsurvives,andfurtherinva-sionsdisplaythesamefractionofsuccesses,onlyweaklyperturbingresidentspecies.Largerspeciespoolsandstrongerinteractionscangiverisetofluctuatingstates,wherespeciesabundancesfluctuateovertime.Wefoundthatthesefluctuatingcommunitiesaremoreinvasibleanddiversethanstablecommunities,leadingtoapositivediversity–invasibilityrelationshipamongcommunitiesassembledinthesameenvironmentandthesamespeciespoolsize.Thesedetermin-isticfluctuationsincommunitiesarechaoticdynamicsorlimitcycle

oscillationsdrivenbyinterspeciesinteractions,ratherthanstochasticfluctuationsdrivenbydemographicnoise.Finally,communitieswithstronginteractionscanalsoreachalternativestablestateswhereinva-sionssucceedmorerarelythanpredictedbysurvivalfraction,butstronglyimpacttheresidentcommunitywhentheydo.Thelowerinva-sionprobabilitycomparedtothesurvivalfractionsuggestsapriorityeffect,wherebyearlierinvadersprecludelateronesfromgrowingfromsmallabundances,leadingtosituationswherethesequenceandtimingofspeciesintroductioncaninfluenceinvasionsuccess

10

,44

,45

.

Studyinginvasionsthroughtheprismofcommunity-widedynami-calregimesallowsustoconnectseveralstrandsofecologicalthinking,regardingwhatcountsasasuccessfulinvasion,whenfactorssuchaspopulationsizeandhistorymatter,andwhatconsequencesinva-sionshaveonresidentcommunitystructureandfunctioning

46

,47

.Fur-thermore,ithelpsclarifythehypothesisthatincreasedcommunitydiversityresultsinreducedinvasionprobabilityduetofeweravailableniches

18

,27

–29

.Withinfixedconditions(giventhesameinitialspeciespoolsizeandenvironment),morediversecommunitiestendtobefoundinfluctuatingstates,andareactuallymorelikelytobeinvaded.Depend-ingonhowwechangeconditions—forexample,increasingspeciespoolorreducinginteractionstrength—diversitymaypositivelyornegativelycorrelatewithinvasibility,providingoneexplanationforinconsistentobservations

48

–50

.Throughoutthesedifferentconditions,however,thefractionofsurvivingspeciesduringtheinitialcommunityassemblyremainsabetterpredictorofinvasibility,displayingauniversalposi-tivecorrespondencewithinvasibilityacrossallconditions,modulatedbythepresenceofpriorityeffects.Ourresultsdemonstratethatbothinvasibilityandinvasioneffectsareemergentproperties,shapedbytheinteractionsofresidentspecies,whichcanbepredictedbysimplecommunity-levelfeatures.

Resultsanddiscussion

Toexperimentallycharacterizeinvasionsinmicrobialcommunities,webuilt17differentsyntheticcommunitiesofsizeS=20usingalibraryof80bacterialisolatesfromriverandterrestrialenvironments(Fig.

1a

andSupplementaryFig.1).Weexposedeachcommunitytodailycyclesofgrowthanddilutionintofreshmedia,withdispersalfromthespeciespool(S=20)tomimicspeciesdispersalinnaturalhabitats(Fig.

1a

).After6daysofculturing,weexposedeachcommunitytoaninvaderspe-cies(Fig.

1a

)andwecontinuedtoculturethecommunitiesforanother6dayswithdispersalofallspeciesoneachdilutioncycle(Fig.

1a,b

).Foreachresidentcommunity,weperformedseventonineindepend-entinvasiontestswithdifferentrandomlychoseninvaderspeciesonday6,andmonitoredthegrowthoftheinvaderandresidentspecies(Fig.

1b

).Analysingspeciesabundancesthrough16Ssequencing,wefoundthat7%±2%ofinvasiontestsweresuccessful(relativeinvaderabundanceexceededextinctionthreshold8×10−4onthelastday12;therationalebehindthechoiceofextinctionthresholdisexplainedintheSupplementaryMaterialsandMethods)(Fig.

1c

andSupplementaryFigs.2and26).Althoughdiverseecosystemsaretypicallythoughttobemoreresistanttoinvaders

18

,27

–29

,ourexperimentalresultsdisplayasig-nificant(P=0.036)positivecorrelationbetweeninvasionprobabilityandcommunitydiversity,wherethediversityisdefinedasthenumberofspeciesthatsurvivedtheassemblyprocessover6days(correlationcoefficient=0.5;Fig.

1c

).Amongcommunitiesoflowdiversity(twotofivesurvivingspecies),only3%±2%ofinvasionsweresuccessful,whereasamongcommunitiesofhighdiversity(sixtoninesurvivingspecies)13%±5%ofinvasionsweresuccessful.Throughoutthemanu-script,weusedthestandarderrorofthemean(s.e.m.)asthemeasureofdispersion.Wethereforefindthatlessdiversecommunitiesmayresistinvasionsbetterthanhighlydiverseonesunderthesameinitialspeciespoolsizeandnutrientconditions.

Tobetterunderstandwhythemorediversecommunitiesweremoreinvasible,wenextquantifiedthedynamicsoftheresidentcom-munitiesbeforeinvasion.Wefoundthatjustunderhalf(8/17)the

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a

20bacterialisolates

...

Speciesinvadeonday6

Dilution

Dispersal

InoculateOnday0

Growth

Invade

12days

Failure

Measurements

16SrRNA

amplicon

sequencing

Biomass

b

~9invaderspecies

cdResidentcommunities

0.31.0

Fluctuation

Stable

17residentcommunities

...

?

?

...

?

...

?

...

?

...

...

?

...

Invasionprobability

Biomass(OD)

0.8

0.2

0.6

?

0.4

0.1

?

...

0.2

0

12

3456

2468

RichnessofresidentcommunityTime(days)

?

e

100

Relativespeciesabundance

10–1

10–2

10–3

Invasionprobability

StablecommunityfFluctuatingcommunityg

0.20

BeforeinvasionAfterinvasionBeforeinvasionAfterinvasion*

0.15

Invader

0.10

0.05

2

0

1012

2

468Time(days)

1012

Stablefluctuation

468Time(days)

Fig.1|Experimentsusingsyntheticmicrobialcommunities.Theinvasion

probabilityinfluctuatingcommunitiesishigherthanstableones,leadingtoapositivediversity–invasibilityrelationship.a,WeusedalibraryofbacteriatogeneratedifferentsyntheticcommunitieswithS=20speciesinthepool(under‘high’nutrientconditions;Methods).Communitiesunderwent

serialdailydilutionswithadditionaldispersalfromthepool.Weintroducedinvaderspeciestotheresidentcommunitiesonday6andcontinuedtoapplydailydispersalofinvaders.Communitycompositionandtotalbiomasswere

monitoredvia16Ssequencingandopticaldensity(OD).b,Weformed17residentcommunitieswithdifferentsetsofspecies(S=20).Weaddedinvaderspecies

outsidethepoolintotheresidentcommunitiesonday6,andthenmeasured

thecommunitycompositionsandbiomassonday12todeterminetheoutcome

andeffectoftheinvasions.c,Theinvasionprobabilityinresidentcommunitiespositivelycorrelatewiththeirrichness(correlationcoefficient=0.5,P=0.047)underthesamespeciespoolsizeandnutrientconditions.d,Outofthe17

residentcommunities,8reachfluctuationinbiomass(orange)andtheother

9communitiesreachstablestates(purple).e,Representativetimecourseof

relativespeciesabundancevia16Ssequencingshowthatthestablecommunitywasnotinvaded.f,Therepresentativetimecourseofrelativespeciesabundanceshowsthattheinvadersuccessfullyinvadesandgrowsinthefluctuating

community.g,Theinvasionprobabilityinfluctuatingresidentcommunitiesis

statisticallyhigherthanthatofstablecommunities(twoindependentsamples

two-sidedStudent’st-test,P=0.016,thenumberofinvasiontestsisn=61(60)forfluctuating(stable)communities).Errorbars,s.e.m.

residentcommunitiesdisplayedpersistentanddeterministicfluctua-tionsinbiomassandspeciescomposition,withtheremainderreachingstablecommunitystates(Fig.

1d–f

andSupplementaryFigs.3–12).Wefoundthatbiomassfluctuationswerehighlycorrelatedwithspeciesabundancefluctuations(SupplementaryFig.12)andtheclassifica-tionofstableandfluctuatingcommunitieswasrobusttodifferentmethods(SupplementaryFig.12).Thesedeterministicfluctuations

incommunitiesarechaoticdynamicsorlimitcycleoscillationsdrivenbyinterspeciesinteractions,ratherthanstochasticfluctuationsdrivenbydemographicnoise,becauseofthelargepopulationsizeregimeinthisstudy(SupplementaryMaterialsandMethods).Consistentwithourpreviousresults,wefoundthatthediversityoffluctuatingcom-munitiesisapproximatelytwicethediversityinstablecommunities(Fig.

1c

)

40

.Giventhishigherdiversityinfluctuatingcommunities,we

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nextanalysedtheinvasibilityofcommunitiesseparatelyforthestableandfluctuatingcommunitiestodetermineifthiscouldbedrivingthepositivediversity–invasibilityrelationshipthatweobserved.Indeed,wedetectedeightsuccessfulinvasionsoutof61invasionteststofluctu-atingcommunities,whiletherewasonlyonesinglesuccessfulinvasionoutof60invasionteststostablecommunities(SupplementaryFig.2).Ourresultsthereforeshowthattheprobabilitytosuccessfullyinvadefluctuatingcommunities(13%±4%)isstatisticallyabouteightfoldlargerthantheprobabilityofinvadingstablecommunities(1.7%±1.7%)(Fig.

1g

).Ourexperimentaltestsofinvasiondemonstratethat,forfixedenvironmentandspeciespoolsize,morediversecommunitiesaremoreinvasiblebecausefluctuatingcommunitiesarebothmorediverseandmoresusceptibletoinvasion.However,wewillshowlaterthat,whenspeciespoolsizeornutrientconcentrationisvaried,thisrelationshipdoesnotalwayshold.Thisincreasedinvasibilityoffluctu-atingcommunitiescanbeinterpretedthroughthelensofnichetheory,wherefluctuatingcommunitiescreatefluctuatingnicheavailabilityforinvaderspecies

13

.Temporalfluctuationsinresourceavailabilityandenvironmentalconditionsallowinvaderstoexploitnichesthatmaynotbeconsistentlyavailableinstablecommunities

13

–

15

.

Togaininsightintothesesurprisingrelationshipsbetweendiversity,stabilityandinvasibility,wenextstudiedinvasionsinthewell-knowngeneralizedLotka–Volterra(gLV)model,modifiedtoincludedispersalfromaspeciespool:

whereNi(Nj)istheabundanceofspeciesi(j)(normalizedtoitscarryingcapacity),tisthetime,αijistheinteractionstrengththatcaptureshowstronglyspeciesjinhibitsspeciesi(withself-regulationαii=1)andDisthedispersalrate,whichissettoD=10−5(SupplementaryFigs.24and25).WesimulatedthedynamicsofcommunitieswithdifferentspeciespoolsizesSandcompetitiveinteractionmatricesbecausecompetitionisthedominantinteractiontypeinourexperiments

40

.WesampledtheinteractionstrengthfromauniformdistributionU[0,2<αij>],where<αij>isthemeaninteractionstrengthbetweenspecies(predictionsofthismodelareinsensitivetotheparticulardistributionchosen

40

).Mod-ellingspeciesinteractionsasarandominteractionnetworkcapturesspeciesheterogeneitywithoutassuminganyparticularcommunitystructure

10

,

38

–

40

.Weintroducedinvadersintoresidentcommunitiesatt=103andcontinuedtosimulatethedynamicsuntilt=2×103todeterminetoinvasionoutcome.

Oursimulationsrevealedawiderangeofdynamicsandinvasionoutcomesunderstronginteractionstrengthbetweenspecies(Fig.

2a

andSupplementaryFig.31).Somesuccessfulinvasionscausedramaticeffectsonthestructuresofresidentcommunities,whereasotherinva-sionsonlyyieldweakchangeincommunities(Fig.

2a

).Consistentwithourexperimentalresults(Fig.

1c,g

),wefoundapositivecorrelationbetweeninvasionprobabilityandrichness(numberofresidentspe-ciescoexistingbeforeinvasion)(Fig.

2b

),whichisbecausefluctuatingcommunitiesexhibitlargerinvasionprobabilitythanstablecommuni-tiesunderthesameconditions(Fig.

2c

).OursimulationresultswiththeLotka–Volterramodelalsopredictthattheinvasionprobabilitydecreaseswhenmeaninteractionstrength<αij>andthespeciespoolsizeSincrease(Fig.

2d–f

).Itisimportanttonotethatalthoughfluc-tuatingcommunitiesexhibitlargerinvasionprobabilitythanstablecommunitiesunderthesameconditions,stablecommunitiescanstillyieldlargerinvasionprobabilityunderweakerinteractionstrength<αij>orsmallerspeciespoolsizeS(Fig.

2d–f

).Ifweinterpretthesephenomenologicalinteractionsintermsofnichetheoryandresourcecompetition

51

,strongerinteractionstrengthcorrespondstolargernicheoverlapandgreaterresourceconsumption,makingitharderforinvaderstoestablish.Similarly,alargerspeciespoolincreasesthetotalinteraction(morenicheoverlap)betweencommunityspeciesand

invaderspecies,therebyinhibitinginvasionmorestrongly

51

.WealsodevelopedamodelthatintegratesexplicitpH-mediatedgrowthwiththeLotka–Volterraframework,allowinginteractionstobeexpressedasafunctionofpHmodification.ThisnewmodelsuggeststhatthepresenceofpHeffectsincreasestheeffectiveinterspeciesinteractionstrengths,butotherwiseyieldspredictionssimilartothoseofthecanonicalLotka–Volterramodel(SupplementaryFig.23).Inaddition,wefoundthatneitherserialdilutionsnortheexistenceofpositive(facilitative)interspeciesinteractionsqualitativelyaffectsthisresult(SupplementaryFigs.28–30).TheLotka–Volterramodelthereforeexplainswhyourdiverseandfluctuatingcommunitiesaresusceptibletospeciesinvasionandmakesnewpredictionsregardinghowinvasibil-itywouldchangewiththesizeofthespeciespoolandthestrengthofinterspeciesinteractions(Fig.

2d–f

).

Toexperimentallytestthepredicteddependenceofinvasionprobabilityoninteractionstrengthandspeciespoolsize,wetunedtheinterspeciesinteractionstrengthbytuningtheconcentrationofsupplementedglucoseandureaintheculturemedium

40

,

52

,

53

.Asdis-cussedinourpreviouswork

40

,52

,53

,increasingtheconcentrationofsup-plementedglucoseandurealeadstostrongerstrengthofcompetitiveinteractionsbetweenbacterialspeciesduetoextensivemodificationofthemedia(forexample,pH).Wemeasuredtheinvasionofaboutnineinvaderspeciesto15syntheticresidentcommunitiesunderlownutrientconditions(weakinteraction)and25communitiesunderhighnutrient(stronginteraction)conditions.Consistentwithourtheoreti-calpredictions,wefoundthatincreasinginteractionstrengthleadstoadecreaseofinvasionprobabilityinresidentcommunities(Fig.

3a

).Specifically,theinvasionprobabilitywas56%±8%inlownutrientconditions(weakinteraction),eightfoldhigherthantheinvasionprob-abilityof7%±2%observedinhighnutrientconditions(stronginterac-tion)(Fig.

3a

).WealsodecreasedthespeciespoolsizefromS=20toS=12andfoundthatinvasionprobabilityincreasedto85%±6%from56%±8%inlownutrientconditions(weakinteraction)(Fig.

3b

),con-sistentwithourtheoreticalpredictions.Weonlyobservedstablecom-munitiesunderlownutrients(weakinteraction)becausefluctuationsonlyemergewhenspeciespoolsizeandinteractionstrengtharelargeenoughtocrossthestabilityboundary

40

.Ourtheoryandexperimentbothindicatethatincreasingeitherinteractionstrengthorspeciespoolsizeleadstoadecreaseincommunityinvasibility

10

,18

,

27

–29

.

Tounifydifferentinvasibility-richnessrelationshipsintheexperi-mentsdependinguponhowtherichnessischanged(byvaryinginterac-tionstrength,speciespoolsizeordynamicalregime)(SupplementaryFig.13),wenextanalysedthedependenceofinvasionprobabilityonthesurvivalfractionofspeciesinresidentcommunities,definedasthefractionofspeciesintheinitialpoolthatsurvivetheassemblyprocess(onday6beforeinvasion).Theresultsshowastronglypositivecorrela-tionofinvasibilitywithsurvivalfraction,wherethecorrelationcoef-ficientis0.77(P=3.4×10−7)(Fig.

3c

).Microbialcommunitiesculturedinlownutrient(weakinteraction)mediadisplaybothalargerinvasionprobabilityandlargersurvivalfractionthancommunitiesunderhighnutrient(stronginteraction)(Fig.

3c

).Furthermore,fluctuatingcom-munities,whichareeasiertobesuccessfullyinvaded,alsoexhibitlargersurvivalfractionthanstablecommunitiesunderthesameconditions(Figs.

1c

and

3c

).Theseresultsdemonstratethatthesurvivalfractioncanserveasaunifyingpredictoroftheinvasibilityofaresidentcom-munity.Althoughithasbeensuggestedthatmicrobialcommunitieswithhigherdiversityarelesslikelytobeinvadedbecausetheyleavefeweravailablenichesforinvaders

18

,

27

–29

,ourresultsindicatethatthisisonlytruewhenthediversityisincreasedbyincreasingthesizeofthespeciespool(Figs.

1c

and

3c

).However,ifdiversityismodulatedbyachangeininteractionstrengthorstability,thenmorediversecom-munitiesareinsteadmoreinvasible.

Despitetheobservedcorrespondencebetweeninvasionprob-abilityandsurvivalfraction,wefindthatinvasionprobabilityunderhighnutrient(stronginteraction)conditionsisgenerallylowerthan

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