Propagule Pressure from Historic U.S. Plant Sales Explains Establishment But Not Invasion

The marketing and sale of non-native plants in the United States has been ongoing since the opening of the first commercial nursery in the early 18th century. Since then, the industry has rapidly expanded, and thousands of new species have been introduced for sale across the country. The ecological impact of this industry has been immense, as the ornamental plant trade has become the leading pathway for non-native plant invasions in the U.S. Non-native species have been sold at varying numbers of locations and for varying lengths of time, leading some to escape gardens and become established in the landscape, others to spread and have negative impacts as invasive species, with the rest remaining only introduced. Using a historical dataset of nursery catalog offerings, former NECASC fellow Matthew Fertakos and NECASC Principal Investigator Bethany Bradley sought to disentangle the relationship between introduction history and invasion success of non-native plants. Their results are now published in Ecology Letters.

Their study shows that introduction history is a highly significant and a strong predictor of plant establishment into the landscape, but only a marginally significant and a poor predictor of plant invasive success. Propagule pressure, or the number of locations a plant species was sold, was a stronger predictor of establishment than residence time, the length of time a species was sold. Species were most likely to establish when they were offered at only eight nursery locations. These results suggest that modern ongoing plant introductions through nursery sales across the country will lead to widespread establishment of non-native species; they may not, however, directly increase invasive success. It is possible, though, that these established species, could become invasive through other means governed by their biotic traits and interactions with other species.. More proactive approaches to the regulation and risk-assessment of non-native plants are needed to prevent the introduction and sale of potentially invasive species.