You might think they should be easy to identify – invasive species are seemingly everywhere we look, even carried inadvertently by people to the polar regions. And as one of the biggest threats to biodiversity, identifying which species are likely to become invasive is important.
Australia has some of the strictest quarantine measures in the world, based partly on trying to identify which species will be permitted to enter the country because the risk of invasiveness is low. Yet the number of naturalized species in Australia continues to rise, and the species are becoming more phylogenetically diverse, with increasing diversity of trading partners.
Attempts to identify invasive species have usually compared their traits to those of non-invasive species. Yet invasive species are classified using all sorts of criteria. Firstly, they might be exotic species that reach high abundance when present in an area. Or they might be species that have wide geographic ranges (because the environment in which they occur is common), or have wide environmental tolerances. Or they might be species that spread rapidly. Or they might be species with various combinations of these criteria.
If we just think of these four criteria, then a species might have any one of 16 possible combinations. And only one of these sets of criteria would represent a species that would categorically be “non-invasive”. So we might have (at least) 15 forms of invasiveness (24 −1 = 15). And if you add a fifth category (e.g., species having adverse impact), then we would end up with 31 possible forms of invasiveness (25 −1 = 31).
Our paper examines the literature (112 papers) to determine how many of these forms of invasiveness have been used to classify invasive species. What do you know? All 15 forms of invasiveness appear in the literature.
This wide range of ways in which invasive species are classified will obscure efforts to identify a consistent set of traits of “invasive species”. Because traits that imbue species with broad environmental tolerances, for example, will be different from those that permit high rates of spread.
Examining traits linked with the four demographic dimensions of invasiveness (rather than “invasiveness” per se) should help highlight species at risk of becoming dominant, spreading quickly or occupying large ranges. That seems like a more fruitful way to identify invasive species because the meaning of “invasiveness” is defined more clearly.
The new paper:
Catford, J. A., Baumgartner, J. B., Vesk, P. A., White, M., Buckley, Y. M. and McCarthy, M. A. (2016). Disentangling the four demographic dimensions of species invasiveness. Journal of Ecology. doi:10.1111/1365-2745.12627 [Online] [Submitted version of manuscript]