Is the increased vigour of invasive weeds explained by a trade-off betweengrowth and herbivore resistance?

Blossey and Notzold (1995) recently hypothesised that the increased vigour of certain invasive plant species has been at the expense of defences again st natural enemies. A prediction of their evolution of increased competitiv e ability (EICA) hypothesis is that invasive genotypes are relatively poorl y defended. We tested this prediction with herbivore bioassays and with dir ect quantification of plant secondary metabolites comparing non-indigenous genotypes of Lythrum salicaria L. (purple loosestrife) with indigenous form s. The herbivore bioassays revealed no significant intra-specific variation in herbivore resistance between indigenous and nonindigenous hosts. The ph enolic content of L. salicaria leaves was significantly higher in indigenou s genotypes, as predicted by the EICA hypothesis. The average phenolic cont ent of leaves (regardless of their origin) was, however, low, implying that the role of plant phenolics in purple loosestrife anti-herbivore defence i s probably limited. It is suggested that the ESCA hypothesis, as tested in the current study, does not explain the increased vigour of L. salicaria in non-indigenous habitats.