The meaning of "invasive"

A bundle of destructive cultural prejudices seeks respectability...

This is a draft chapter from the book-in-progress, “Stop Blaming the Messengers,” that I am co-authoring with Nikki Hill.

Alien species seem practically designed to excite public concern. Almost by definition they are most abundant, and most visible, in the most highly human-modified habitats, such as towns and cities. Personal encounters with aliens are routine, so everyone has an opinion, and it’s often ‘obvious’ that aliens are actively supplanting natives, even if that isn’t what’s happening at all. It’s equally ‘obvious’ that something must be done, even if it’s not clear what that should be, and even if ill-judged intervention might only make things worse.ⁱ ~Ken Thompson

The dodginess of definitions

For “invade,” Merriam-Webster gives us, “to enter for conquest or plunder, to encroach upon, infringe” and for “invasive,” “tending to spread especially in a quick or aggressive manner” and “relating to, or characterized by military aggression.” The Cambridge Dictionary provides “to enter a country by force with large numbers of soldiers in order to take possession of it; to enter a place in large numbers, usually when unwanted and in order to take possession or do damage; to enter an area of activity in a forceful and noticeable way" for "invade" and "moving into all areas of something and difficult to stop" for "invasive."

That’s clear enough. But what is the definition of an an “invasive species”?

The short answer is that there is none.