One hypothesis is that the plant's resources devoted in the native range to a defense strategy can, in the absence of enemies, be devoted to increased growth and reproduction (the EICA Hypothesis, Blossey & Nötzold, 1995). However, this is not the case with introduced populations of E. californica in Chile: the Chilean populations were actually more resistant to Californian caterpillars than native populations (Leger and Forister, 2005).
