Introduction bias: Cultivated alien plant species germinate faster and more abundantly than native species in Switzerland

Traits that differ between invasive alien and native plant species are frequently interpreted as traits conferring invasiveness. However, such differences could reflect an introduction bias of alien species, particularly cultivated ones, or human-mediated selection of cultivars of these species with certain traits. We tested whether this is the case for germination characteristics that are frequently reported to be associated with invasiveness. In a glasshouse experiment, we compared germination characteristics of 42 plant species native to Switzerland and 47 cultivated alien species including 26 cultivars. To test whether differences in germination between these groups of species depend on an important environmental factor, we used two light levels. Cultivated alien plant species germinated earlier and more successfully than related native plant species under both light conditions. Similarly, among cultivated alien species, cultivars germinated earlier and with higher proportions than non-cultivars. Our results indicate that previously reported differences in germination characteristics between invasive alien and native species might reflect introduction bias and human-mediated selection for these characteristics. Nevertheless, because fast and abundant germination is also frequently associated with naturalization and invasiveness among alien introduced species, our results suggest that biased introduction and human-mediated selection of ornamental plants with these characteristics might increase the risk that these species ultimately become invasive. Therefore, it is important that studies testing for determinants of invasiveness include non-invasive alien species in addition to invasive alien species.