Why “Invasive Alien Species” is not a thing
Many invasion biologists are fond of the term “Invasive Alien Species” (often abbreviated to IAS), but for me it’s logically inconsistent and encompasses redundancy. Perhaps, the original reason for placing the three words together was in recognition that not all alien species are invasive, therefore we’d need to add the term invasive to underline the point that we are only referring to the subset of alien species, those that are invasive. However, the implication in this phrase is that it’s possible to be invasive and not alien; i.e. that “invasive native species” is another category. But it’s not. Most would agree that to be invasive you would first need to be alien. To this end, perhaps I should have titled this blog: Why "invasive native species" is not a thing... but there is a school of thought that suggests that invaders can be native (see Valéry et al 2008).
The Blackburn et al (2011) scheme (pictured below) formalised this in a way that makes this easy to understand.
To make it even easier, I’ve adapted the scheme into sets so that you can appreciate that each group of species is a subset of the other (this scheme is not to scale, as we’d expect to see much smaller sets inside each set – maybe even following the tens rule?). Note that if “invasive native species” were a thing, we could draw another set inside “All species” but separate from all the other sets. Does this seem logical?
If I said that this was an “invasive species”, would you then have to ask: “is it alien or native”?
Invasive species are a subset of alien species (i.e. the ones that spread), but we shouldn’t be adding words to each growing subset; otherwise we’d have the term “invasive established alien species” to distinguish between those that are merely “established alien species” or just “alien species”.
My appeal is to think about these terms instead of blindly following those who have gone before.
Yes, this is another rant on the blog, and I’d like to point the finger at John Wilson for infecting me with this particular pernicious titbit. Now I can only hope that it’ll spread and you’ll be able to point to this to help in your own war on IAS. We don't want to join those silly mechanistic definition folk with their non-biogeographic ideas of invasions. Otherwise we might end up going down an alley labelled 'invasion syndromes'.
