Native jewelweed, but not other native species, displays postinvasion trait divergence

Invasive exotic plants reduce the diversity of native communities by displacing native species. According to the coexistence theory, native plants are able to coexist with invaders only when their fi tness is not signifi cantly smaller than that of the exotics or when they occupy a diff erent niche. It has therefore been hypothesized that the survival of some native species at invaded sites is due to post-invasion evolutionary changes in fi tness and/or niche traits. In common garden experiments, we tested whether plants from invaded sites of two native species, Impatiens noli-tangere and Galeopsis speciosa , outperform conspecifi cs from non-invaded sites when grown in competition with the invader ( Impatiens parvifl ora ). We further examined whether the expected superior performance of the plants from the invaded sites is due to changes in the plant size (fi tness proxy) and/or changes in the germination phenology and phenotypic plasticity (niche proxies). Invasion history did not infl uence the performance of any native species when grown with the exotic competitor. In I. noli-tangere , however, we found signifi cant trait divergence with regard to plant size, germination phenology and phenotypic plasticity. In the absence of a competitor, plants of I. noli-tangere from invaded sites were larger than plants from non-invaded sites. Th e former plants germinated earlier than inexperienced conspecifi cs or an exotic congener. Invasion experience was also associated with increased phenotypic plasticity and an improved shade-avoidance syndrome. Although these changes indicate fi tness and niche diff erentiation of I. noli-tangere at invaded sites, future research should examine more closely the adaptive value of these changes and their genetic basis.