入侵植物和本地植物功能差異指數(Functional differences index, FDj)按照以下公式計算(Gross et al. 2013 Gross N, Brger L, Duncan RP, Hulme PE. 2013. Functional differences between alien and native species: do biotic interactions determine the functional structure of highly invaded grasslands? Functional Ecology 27: 12621272.)：
式中FDj為樣方j內入侵植物特徵加權值(CWTij)與本地植物特徵加權值(CWTnj)的功能差異指數。CWTi為所有樣方入侵植物特徵加權值，CWTn為所有樣方本地植物特徵加權值。若FDj大於0表明入侵植物和本地植物的生理生態特性指標差異較大(Functionally diverge)，存在生態位分化(Niche differentiation)；若FDj小於0表明入侵植物和本地植物的生理生態特性指標差異較小(Functionally converge)，存在生境過濾(Habitat filtering)。
其中植物特徵加權值(CommunityWeighted Trait Values, CWT)按照以下公式計算(Violle et al. 2007 Violle C, Navas ML, Vile D, Kazakou E, Fortunel C, Hummel I, Garnier E. 2007. Let the concept of trait be functional! Oikos 116: 882892.; Gross et al. 2013 Gross N, Brger L, Duncan RP, Hulme PE. 2013. Functional differences between alien and native species: do biotic interactions determine the functional structure of highly invaded grasslands? Functional Ecology 27: 12621272.)：
式中S為調查樣方內所有物種種類的數量。Pi為調查樣方內物種i的相對豐度，即Pi= ni/N，ni為調查樣方內物種i的植株數量，N為調查樣方內所有物種的植株數量。Xi為物種i生理生態特性指標的平均值。