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).