Chris Thomas argues that natural hybridization of invasive and native species could have unforeseen ecological benefits (Nature 502, 7; 2013). However, it is dangerous to underestimate non-native species' potential for ecological and economic damage, which can manifest after decades of remaining innocuous, or in subtle but enormously harmful ways (D. Simberloff et al. Trends Ecol. Evol. 28, 58–66; 2013).

Thomas cites Spartina anglica as a potential contributor to future biodiversity. But this plant is so widespread and destructive that it is on the International Union for the Conservation of Nature's list of '100 of the World's Worst Invasive Alien Species'.

Any increase in local biodiversity that might arise from introduced species will be countered by an overall global decrease. Hawaii, for example, once had 114 native bird species: 55 of these are now extinct and 53 have been introduced, so the local change in species number is minimal. Globally, however, the change is a disaster. The introduced species are common elsewhere, but 48% of the islands' native species are gone forever.

The draft European Union legislation for controlling introduced species does not assume that their impact is always negative, as Thomas implies. Species will be risk-assessed, and states will then be obliged to control at most 50 of those confirmed to be harmful.

We maintain that combating invading species after gauging the risk they pose is well justified on scientific grounds and is not “irrational”.