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1、Ecology,85(10),2004,pp.2682±2686q2004bytheEcologicalSocietyofAmericaMEASURINGPLANTINTERACTIONS:ANEWCOMPARATIVEINDEXCRISTINAARMAS,1RAMOÂNORDIALES,ANDFRANCISCOI.PUGNAIREEstacioÂnExperimentaldeZonasAÂridas,ConsejoSuperiordeInvestigacionesCientõ®cas,GeneralSegura1,04001Alm
2、erõÂa,SpainAbstract.Weproposeanindextomeasuretherelativeinteractionintensityinplants(RII)withstrongmathematicalandstatisticalpropertieswhichovercomeproblemsshownbyotherfrequentlyusedindices.RIIhasde®nedlimits[21,11];issymmetricalaroundzero,withidenticalabsolutevaluesfor
3、competitionandfacilitation;islinear;anddoesnothavediscontinuitiesinitsrange.Itisthereforesafetouseinstatisticalandmathematicaloperations.RIIdistributionisapproximatelynormal,withmeansequaltothetruepopulationindexandasamplingvariancethatcanbederived.Itsstrongstatisticalp
4、ropertiesmakeRIIproperforuseinparametricmeta-analyses.Itcanbeappliedtoanykindofinteraction(fromcompetitiveexclusiontosymbiosis)andincommonlypublishedrangesofinteractionintensityitoffersthemostconsistentresults.BecauseRIIusesbasicarithmeticaloperators,itcanbescaledupandu
5、sedtomeasuremultispeci®cinteractionsatthecommunitylevel.Keywords:competition;facilitation;interactionindices;interactionintensity.INTRODUCTIONotherindicesmaybemisleadingbecauseoftheirlackofsymmetry,theirpoorstatisticalproperties,orbe-Interactionamongspeciesiscentraltoco
6、mmunitycausetheyproduceartifactsundersomecircumstancesecology(e.g.,Connell1983,Schoener1983,Brunoet(Hedgesetal.1999).al.2003),butaccuratequanti®cationofspeciesinter-Indicesusedtomeasurethenetbalanceofaninter-actionsishotlydebated(Reynolds1999,WeigeltandJolliffe2003).Tes
7、tinghypothesesincommunityecol-actionshouldmeetseveralbasicproperties.Theymustogyoftenrequiresquanti®cationofthemagnitudeofbesymmetrical,i.e.,foragiveninteractionintensityinteractionsbetweenspeciessothatthesemagnitudestheindexshouldprovidethesameresultforcompe-canbecompa
8、redamonggroupsoftaxaorofenviron-titionandforfacilitationbutwithoppositesigns.TheReportsments(Goldbergetal.1999
