Species introduced by humans to areas where they don’t naturally occur are a worldwide problem. These “alien species” can cost a lot of money to deal with, and they’re the number one driver associated with recent extinctions worldwide. The rate at which new populations of alien species are establishing is increasing rapidly, and ideally we’d like to stop the process in its tracks.
But first we need to answer two key questions: firstly, what features of species make them good at becoming alien? For instance why has the Indian myna bird managed to establish itself across the world while its relative the crested myna has not? And, secondly, what is it that makes some locations more susceptible to aliens establishing there? While we have made progress in answering the first question, we haven’t done nearly so well with the second.
The reason for this is that the environment is very complicated and so many elements of it might be important for an arriving alien species. For instance the climate at a new location is likely to matter, not just in absolute terms, but in how well it matches what the alien is used to back home. Climatic extremes may be as or more important than averages in this regard.
Which species are already present at the location is also likely to be important. Too many and the alien may get easily outcompeted or eaten, too few might mean there wasn’t enough food in the area. Again, the identity of the alien matters in these regards. Habitat will matter, as will the extent to which humans have modified it. Finally, all of these features of the location vary over space and time, and which species get introduced as aliens also is not random, all of which introduces a range of complications.
In collaboration with colleagues in Australia and the US, my research group set out to attempt a rigorous global analysis of why some locations are more susceptible to alien establishment. We first assembled a catalogue of more than 4,000 alien bird introductions, featuring everything from Australian magpies in New Zealand to Southeast Asian zebra doves in Hawaii.
We then overlaid it with information on features of the local environment, species traits, and how many individuals got introduced (important because small founding populations can easily fail by accident), and used complex statistical methods to assess the impacts of all these different factors.
Our results are now published in Nature. Overall, we found that environmental features were the most important, particularly human impacts on the environment – how many other alien species groups were already present, for example – and how well the new environment matched what the species was used to from its native range.
The matching effect is not surprising – introduce a tropical parrot to the Arctic and it’s probably not going to do very well – but it is reassuring, as it gives us confidence in our analysis. This makes the anthropogenic effects more worrying though, in particular that alien birds are more likely to establish populations in areas that already have more aliens of other sorts. This is consistent with the “invasional meltdown” hypothesis that previous invasions help facilitate future alien arrivals.
The meltdown result does not simply reflect general environmental disturbance, as we also found that alien birds are more likely to establish themselves in less disturbed habitats – for example, areas where lots of habitat had been converted to cropland in the run up to the introduction.
Another sort of disturbance that matters is bad weather – a big storm in the period immediately following introduction can cause the alien population to disappear. There was already anecdotal evidence for this – for example, the initial extinction of the alien house crow population from Mauritius following a storm – but our analysis shows that this is a general effect.
We did find some evidence that lots of native bird species meant alien birds were less likely to establish themselves, but alien birds do slightly better in areas with at least some similar native species rather than none. This makes sense – for example, an alien bird that eats insects might be expected to do better in areas where native insectivores live, as long as there aren’t too many for the aliens to make a living. Overall, however, the types of native plants and animals had a relatively weak effect on alien bird establishment success.
While the environment explained most variation in alien bird establishment overall, we also found large effects of species traits and number of individuals introduced. Founding population size needs to be large enough (more than about 50 birds) to avoid the alien population failing simply by chance (the same reason we worry about very small populations of threatened native species). Assuming there are enough birds, then it helps if they breed fast but don’t die too young. It also helps if they are not too fussy about what they eat or where they live.
Taken together, our results show how features of the environment, the species, and the founding population size have all influenced the global history of alien bird establishment. They also suggest reasons to worry. The world is showing signs of a global invasion meltdown, while ever more alien species are being introduced to new locations and getting the chance to test their environmental matches. Increasingly, the future is looking alien.