The establishment of species in ecosystems to which they are not native. Invasive species cause significant ecological harm: they can alter ecosystem processes, act as vectors of disease, and reduce biodiversity. Worldwide, out of 256 vertebrate extinctions with an identifiable cause, 109 are known to be due to biological invaders (Olsen and Santanou (2002) Am. J. Agric. Econ. 84, 5). Simberloff and Alexander (in P. Calow, ed. 1998) estimate that around a quarter of the value of US agricultural output is lost to non-indigenous plant pests or the costs of controlling them.

Case studies abound, among them the UK Environment Agency’s material on American signal crayfish and Japanese knotweed, which is also of interest for its use of anthropomorphic phrasing: ‘because [Japanese knotweed] does not originate in the UK, it does not compete fairly with our native species’ (this author’s italics). Olson and Roy (Univ. Maryland, Ag. & Res Econ., W. Paper 03–06) develop an economic model of a biological invasion, and Peterson (2003) Q. Rev. of Biol. 78 claims that the geographic course that invasions are able to take can be predicted.