Is there an urban effect in alien plant invasions?
Cities are known to be extraordinarily rich in alien plant species compared to rural environments. This is related to specific attributes of urban areas including the availability of natural resources and habitats (namely geological substrates and land cover), the dispersal pathways and associated propagule pressure due to trade and traffic, and the proximity many urban hubs have to rivers. Here we explored how richness and proportions of alien species introduced after the discovery of the Americas (so-called neophytes), can be explained by environmental covariates along the urbanization gradient from very rural to very urbanized grid cells. We tested whether there is a specific urban effect, either as an interaction effect of urbanized areas that changes these general relationships, or if there is an effect due to specific urban conditions. We found that the environmental covariates explaining richness as well as proportions of neophytes remain largely the same across the rural-urban gradient. There is, however, an effect of urbanized area on neophyte species richness and proportions, which also incorporates strictly urban conditions. Rivers, roads and railroads contribute disproportionately less to the increase of neophyte species diversity in more urbanized areas, which might be due to the already higher number of neophytes in cities. We argue that the conditions determining neophyte richness in cities are not fundamentally different from those in rural environments, but extend on the same environmental axis, i.e. having different positions along the gradient towards the upper end.