Propagule pressure
The recent distributions of examined species were correlated with initial patterns of their introductions, in 1950s. Quite surprisingly, the two species revealed an opposite relationship to these historical patterns. In the case of S. gigantea , the pattern was rather simple and intuitive: the probability was highest in squares closer to the sites of initial distribution. However, S. canadensis quite surprisingly was the most likely to occur in squares 100 km from the initial sites of introduction. These results suggest different mechanisms of long-range dispersals. Recently, S. canadensis was considered to have a higher ornamental value (because of larger size, bigger inflorescences, and clump occurrence) than S. gigantea . As a result, it is offered by garden shops, but S. gigantea is not (Szymura M. personal observations, data from internet shops offering ornamental plants). A similar pattern of trade has been described in Estonia, Central Europe, where only S. canadensis is offered in markets (Ööpik, Bunce, & Tischler, 2013). Moreover, the honey fromS. canadensis has recently been promoted on social media, without supporting scientific data, as a ‘superfood’ with healing properties. This claim could encourage beekeepers to produce goldenrod honey, which would lead to further spread of S. canadensis and exacerbate its existing negative environmental impact. (Lenda et al., 2020).
The distribution of S. canadensis is positively correlated with human population density. This straightforward correlation breaks if the population density exceeds 5000/km2. This happened in a few of the most densely inhabited squares, representing strict city centres. It was generally found that the plant species richness in areas with moderate levels of urbanisation (e.g., suburban areas) exceeded the richness recorded in non-urbanised areas as well as in central, urban core areas (McKinney, 2008). The lack of a further increase in alien species richness in strict city centres, despite the high propagule pressure, was explained by the loss of suitable areas for plants (McKinney, 2008). Such generally limited neophytes’ richness caused by population density has previously been shown for this region (Szymura et al., 2018).
The results of the modelling support the assumption that recent S. gigantea dispersal has occurred mostly spontaneously without any human aid, while S. canadensis dispersal is still related to human presence and, additionally, intentional transport over longer distances via, for example, internet commerce (Lenda et al., 2014).