Does remoteness promote biological invasions on islands worldwide?

There’s a new paper out that is getting a lot of attention from the invasion biology tweeps in my timeline (Moser et al. 2018. Remoteness promotes biological invasions on

islands worldwide. www.pnas.org/cgi/doi/10.1073/pnas.1804179115). It explores patterns in the distribution of alien species across tropical and sub-tropical islands, and finds that in general, the more isolated is an island from other major landmasses (continents or large islands), the more alien species are found to be naturalised on that island. This pattern runs counter to that shown by native species, where richness tends to decrease with isolation - the well-known species-isolation relationship (SIR).

The SIR most likely arises because it is harder for species to disperse to, and colonise, more remote islands. Why aliens should show the reverse pattern is less clear. Moser et al. argue that it is unlikely to be because more isolated islands are more easily reached by aliens – most aliens are moved deliberately or accidentally with international trade, but trade volumes are not higher to more isolated islands. Instead, they argue that because remote islands tend to be depauperate in native species, and those species that do live on them tend to be more ecologically naïve (competitively inferior or unused to predators), remote islands are easier to invade.

Moser et al. may be right, but there is a problem with their argument. Their analysis uses data on trade volumes as a proxy for the likelihood that aliens can reach those islands, because they don’t know how many alien species actually made the journey. They cannot completely rule out the possibility that islands with more alien species established are not more easily invasible, but rather are simply those that have had more alien species introduced.

Well, actually, for one taxon in their analysis, they can. For birds, we do have good data on how many species made it to different islands. As it happens, the number of bird species introduced to the different islands does not increase with isolation. Moser et al. argue that this supports their argument that more isolated islands do not get more species introduced to them. This is true, but birds is also the one group in their analysis where alien richness does not increase with island isolation. Moreover, we know from other analyses that alien bird richness is strongly positively related to how many species have been introduced. For birds at least, island richness is related to how many aliens have reached the island; there is no evidence that it is due to how easy islands are to invade. In fact, globally, we know that areas that are rich in native bird species are more likely to be rich in alien bird species, once we control for how many species have been introduced (Dyer et al. 2017. The global distribution and drivers of alien bird species richness. PLoS Biology, 15, e2000942).

In sum, I remain to be convinced that Moser et al. can robustly conclude that remote islands are easier for alien species to invade. To show this, they need to demonstrate that introduced alien species are more likely to establish naturalised populations on more isolated islands. Yet, for the only group they can, they do not. They might be right, but to my mind, the jury is still out.