Are plants really larger in their introduced ranges?

The rule that individuals of nonindigenous plant species are larger where they are introduced than where they are native is not borne out in detailed comparisons of European species introduced to California or the Carolinas and species from California and the Carolinas introduced to Europe. On average, individuals of California species are taller in California than in Europe, while individuals of species native to Europe do not differ between Europe and California. Similarly, individuals of species from the Carolinas are, on average, taller in the Carolinas than in Europe, while individuals of European species are the same height in Europe and the Carolinas or, depending on the nature of the statistical analysis, taller in Europe. Results for herbaceous species only are substantially the same. Although there is no general tendency for species to be taller in their introduced ranges, many species are, in fact, taller in some regions where they are introduced than in their native ranges. Absence of natural enemies in the introduced range is one hypothesis for such observations, but other hypotheses are possible, and the specific reasons for height differences must be sought case by case. The absence of a general tendency casts doubt on the biological control strategy of introducing sequences of phytophages, none of which delivers a knockout blow to a weed, with the expectation that each successive phytophage will force the plant to devote more resources to defense and fewer to traits such as increased size that make it more competitive.