Using native ranges, invaded ranges, and introduction pathways to understand invasive plant risk

Background/Question/Methods

Since a substantial proportion of introduced species become invasive and non-native plants are continually introduced to new areas, we must keep improving risk assessments in order to successfully reduce the impacts of invasive plants. Previous work has revealed spatial biases in the global movement of non-native plants, but whether the same patterns exist for invasive plants remains unclear. Invasive species may demonstrate different spatial biases if numerous invaders evolved in similar environments, or if some global regions are particularly vulnerable to invasion. Understanding these differences could improve risk assessments and more efficiently target invasive species without reducing the benefits associated with harmless non-native plants. Therefore, we asked: Which source countries and introduction pathways pose the greatest risk? And: Which countries are most vulnerable to invasion? To answer these questions, we used a list of invasive vascular plant species from the Global Plant Invaders database. Our total list includes 3,226 species, of which we recorded the countries associated with the native and invaded ranges for 2,184 species, and the main introduction pathway for 1,183 species. We then determined which countries act as disproportionately large sources or destinations for invasive plants, and which introduction pathways are most common.

Results/Conclusions

Sampled species each had an average of 23.01 (±23.90 SD) countries in their native ranges and 6.32 (±9.46 SD) countries in their invaded range. Two species were invasive in more than 100 countries, and 664 species were invasive in just one country. Thirty species were native to more than 100 countries and 194 were native to only one country. China was a source for more invasive plant species (641) than any other country, but it was followed closely by India (610), and Russia (577). Less evenly distributed were the number of invasions that took place in each country. The United States, New Zealand, and Australia were invaded far more often than elsewhere (invasions of 1159, 412, and 404 species, respectively), although these invasion statistics are biased towards English-speaking countries. Nevertheless, these results suggest that invasion risk varies widely by location. Most sampled species (75%) were purposefully introduced and subsequently escaped cultivation. Additionally, 71% of these escaped species were originally introduced for ornamental purposes. Therefore, future management strategies could improve by finding new ways to evaluate the risks posed by ornamental plant species, especially those that are not already classified as invasive in another country.