Abstract. We compared the demographic characteristics of native central-European and invasive USA-populations of Lythrum salicaria growing in similar habitats. Based on the ‘Evolution of increased competitive ability’ (EICA) hypothesis, we predicted that shoot density, height, and biomass, fertility, and fecundity would be less in the European populations due to greater loss of plant material caused by exposure to control agents. Shoot density was significantly greater in most USA-populations, but shoot height and biomass were similar in invasive and native populations growing in similar habitats, especially in growing seasons with greater water availability. The number of fertile shoots was greater for invasive populations, except for those growing in sandy, nutrient-poor substrates, while percent fertility did not differ between populations growing under similar field conditions. Fecundity was also similar for populations growing in nutrient-poor and intermediate habitats, but was significantly greater in USA-populations (89–103 seeds per fruit), compared to European populations (58–64 seeds per fruit), growing in nutrient-rich habitats; seed predators were found in these European populations only.
Log-linear analysis of transition frequency matrices showed that the growth of USA-populations is different from European populations, but that habitat effect was strong. Population dynamics were similar for populations growing in nutrient-poor habitats; location was of marginal importance only. USA-populations responded differently from European populations growing in intermediate and nutrient-rich habitats; differences were most pronounced between invasive (center of its North American distribution) and native populations growing in nutrient-rich habitats.
The use of insect herbivores as biological control agents in North America will work best against L. salicaria populations growing in nutrient-rich habitats in the center of its invasive distribution, but will be less effective against populations growing in other habitats or portions of its range. Other factors, including nutrient- or water availability, and climate, may be as important as herbivory in affecting invasive populations; these factors interact to control L. salicaria in a more complex manner than thought previously.