A very cool new study about air travel as a vector for invasive species is out in the current issue of the Proceedings of the Royal Society of London: B (the "B" is for Biological Sciences). Authors Andrew Tatum and Simon Hay analyzed the networks that cross the globe (44,000+ flight routes!), and using the concept that a species is more likely to be a threat to a region with a climate that is similar to the region it originated from, they identified certain hotspots that were most at risk for non-native species introductions.
The most linkage between areas with similar climates occurs June through August, when air traffic reaches its peak. That means that at the same time that climates are best matched up, propagule pressure (numbers of non-native organisms that are introduced) is also at its highest. Hotspots where airports identified as being most at risk are clustered together include central and northern USA, the Caribbean, central-west Europe, Scandinavia and Eastern Asia. As the article points out in the concluding paragraph, "organisms have never had a better chance at expanding their ranges."