(Not So) Great Expectations: Vulnerability to plant invasion worldwide is higher than previously estimated

Pfadenhauer, WG, Bradley BA (2024) Quantifying vulnerability to plant invasion across global ecosystems. Ecological Applications.

Summary

The “tens rule” in invasion ecology suggests that approximately 10% of established, non-native species will become invasive (i.e. have negative impacts) when introduced to new environments. However, the accuracy of the tens rule (originally published in 1996) has been questioned, since it is based on small samples of species from a few locations. This analysis revisited the tens rule by calculating “invasion rates” for plants (the percent of established plant species that had negative impacts) for hundreds of locations around the world using plant distribution data from more than 12,000 plant species. Results showed that globally, the tens rule is a poor estimate of invasion rates, with a ‘twenties rule’ being a better rule of thumb (i.e. globally, about 20% of established plants go on to become invasive). However, states in the Northeast U.S. and Canadian provinces had average invasion rates of 11.1%. Therefore, although a ‘twenties rule’ is a better global estimate of invasion rates, the tens rule still applies for most areas within the NE RISCC network.

Take home points

• For most locations worldwide, the tens rule underestimates invasion rates. This means that management strategies based on the tens rule are likely to underestimate the risks posed by introduced, established plants. 

• Invasion rates average 17-25% globally, but are strongly dependent on the scale, climate, and geography of the recipient environment. Tropical islands had invasion rates as high as 33%, while some continental mainlands had invasion rates of just 7.2%. 

• Future estimates of invasion rates should be based on the size, climate, and island/mainland status of each location, rather than only using a single value for all environments. 

Management implications

• In areas with high invasion rates (e.g. islands and areas with tropical climates),  risk assessment protocols should be adjusted to identify a higher proportion (20% or more) of incoming species as likely invasive. 

• Practitioners in areas with lower invasion rates (mainlands and areas with continental/polar climates) should continue to use risk assessments with an assumption that about 10% of introduced, established species are likely to become invasive.

Keywords: Management efficacy, impact studies, invasion susceptibility, tens rule, weed risk assessment