Revisiting the “Tens Rule”: Vulnerability to Plant Invasions Worldwide Is Higher Than Previously Estimated

The widely cited “tens rule” in invasion ecology suggests that approximately 10% of established, non-native species will become invasive–or display negative impacts—when introduced to new environments. However, the accuracy of the tens rule, originally formulated  in 1996, has been widely questioned since it is based on small sample sizes of species from only a handful of locations in Great Britain and Australia.  Interrogating the validity of this rule, NE CASC and NE RISCC Management Network team members Will Pfadenhauer and Bethany Bradley have completed a new study that determines global plant invasion rates–or the percentage of established, non-native species that display negative impacts–using distribution data from more than 12,000 species across hundreds of locations worldwide. Their work was recently published in Ecological Applications.

This study demonstrates that the tens rule is a poor estimate of invasion rates on a global scale. Instead, a ‘twenties rule’ is a better rule of thumb, meaning that roughly 20% of all established, non-native species eventually become invasive. Crucially, however, the publication also reveals that northeastern states and Canadian provinces demonstrate average invasion rates of only 11.1%. Consequently, while a ‘twenties rule’ more reliably estimates a global invasion rate, the tens rule still applies for most areas within the NE CASC region. 

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.