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 fitness is not significantly smaller than that of the exotics or when they occupy a different niche. It has therefore been hypothesized that the survival of some native species at invaded sites is due to post-invasion evolutionary changes in fitness 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 conspecifics from non-invaded sites when grown in competition with the invader (Impatiens parviflora). We further examined whether the expected superior performance of the plants from the invaded sites is due to changes in the plant size (fitness proxy) and/or changes in the germination phenology and phenotypic plasticity (niche proxies).
Invasion history did not influence the performance of any native species when grown with the exotic competitor. In I. noli-tangere, however, we found significant 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. The former plants germinated earlier than inexperienced conspecifics or an exotic congener. Invasion experience was also associated with increased phenotypic plasticity and an improved shade-avoidance syndrome. Although these changes indicate fitness and niche differentiation of I. noli-tangere at invaded sites, future research should examine more closely the adaptive value of these changes and their genetic basis.