Professor University of Massachusetts Amherst, United States
Abstract: Understanding which environments are most susceptible to invasive species is a priority for researchers, managers, and regulators since this information can be used to minimize negative environmental and economic impacts. There are many hypotheses about why some environments are more susceptible to invasion. For example, the biotic resistance hypothesis suggests that areas with fewer native species will have more available ecological niches for invaders to exploit, and will therefore be more susceptible to invasion than species-rich environments. Additionally, the island susceptibility hypothesis is an extension of the biotic resistance hypothesis which posits that islands should be more susceptible to invasion than mainlands since islands have comparatively fewer native species per unit area. Despite the ubiquity of these hypotheses, empirical support for them is mixed, and often comes from localized studies with relatively small sample sizes. Therefore, a global analysis of invasion susceptibility is needed to resolve differences in existing studies, determine which global environments are most vulnerable to plant invasions, and design effective management strategies.
Here, we created a global database of plant invasions and used it to test the biotic resistance hypothesis and island susceptibility hypothesis. Our database included established and invaded distributions of 12,425 plant species spread across 230 countries. We assigned climates and island/mainland labels to each location, and we calculated the proportion of established plants that had become invasive (“invasion rates”) within each of these countries. The average invasion rate for all countries was 18%, with a 95% confidence interval between 15-20%.
Species-rich tropical regions had significantly higher invasion rates (24-27%) than species-poor polar regions (8-12%). Therefore, the preventative effects of biotic resistance appear to be outweighed by the promotive effects of environmental characteristics that are favorable for plant growth (e.g. consistent temperature and precipitation patterns). We found support for the island susceptibility hypothesis, as islands were invaded significantly more often than mainlands (18-20% vs 14-16%, respectively). However, tropical islands were far more susceptible to invasion than islands in continental or polar climates. Thus, in polar regions, it appears that harsh abiotic conditions may make all environments (i.e. both islands and mainlands) similarly inhospitable for plants. We conclude that tropical ecosystems, especially tropical islands, are most susceptible to invasions of non-native plants and should be prioritized for future management.
