Under pressure: historic U.S. plant sales explain establishment but not invasion
Summary
Introduction history, including the number of locations a species was sold (propagule pressure) and the length of time it has been available in the market (residence time) have long been proposed drivers of plant invasions. However, it is unclear whether introduction history only increases the likelihood that a species will establish in the landscape (i.e. an introduced species with one or more self-sustaining populations) or if it also increases the likelihood that a species will become invasive (i.e. spreading and having negative impacts). Using a dataset of 11,408 species sold by nurseries in the U.S. from 1719 to 1978, Fertakos et al. found that introduction history is a highly significant and strong predictor of plant establishment into the landscape, but only a marginally significant and poor predictor of plant invasive success. Thus higher propagule pressure clearly increases establishment but does not directly affect the transition from established to invasive. Comparing species that successfully established vs. failed to establish showed that a species was more likely to establish if it was offered in only eight (or more) nursery locations. These results suggest that modern ongoing plant introductions through their sale at thousands of nurseries across the country will lead to widespread establishment of introduced species. These established species, though, could then go on to become invasive through other means governed by their biotic traits and interactions with other species and larger numbers of established species will ultimately lead to more invasions.
Take home points
Past ornamental plant sales better explain plant establishment than invasion
Species sold at only 8 nursery locations were more likely to be established today
While plant sales may not directly influence invasive success, more established species means more species that could become invasive.
Management implications
Practitioners working within horticulture should continue emphasizing the sale of native plant alternatives to reduce the propagule pressure of non-native plants across the landscape.
As markets continue to expand and the climate continues to change, new species will establish in the landscape and require attentive management to prevent invasion.
Developing more proactive regulations to prohibit the sale of high-risk introduced plants is paramount to preventing invasion.
Keywords: Management efficacy, Impact studies, Ornamental plants, Propagule pressure, Invasion success