Invasive species hotspots

Human-mediated transport beyond biogeographic barriers has led to the introduction and establishment of alien species in new regions worldwide. However, we lack a global picture of established alien species richness for multiple taxonomic groups. 

The number of established alien species varies across the world and it is where the most established alien species can be found and which factors influence their distribution. An international team created a database for eight animal and plant groups (mammals, birds, amphibians, reptiles, fishes, spiders, ants and vascular plants) that were found to occur in regions outside their original habitat. The study of the distribution of these species led the research team to identify 186 islands and 423 mainland regions in total thereby illustrating the global distribution of established alien species. 

The highest number of alien species can be found on islands and in the coastal regions of continents. The island of Hawaii was found to have the most alien species, followed by the north island of New Zealand and the small Sunda Islands of Indonesia. What these places have in common is that they are remote islands that used to be very isolated, lacking some taxa altogether, e.g. mammals. Today, these island regions are economically highly developed and maintain intense trade relationships with the mainlands. 

We found the number of alien species to be particularly high in densely populated areas as well as in economically highly developed ones. These factors increase the likelihood of humans introducing many new species to an area. This almost invariably results in the destruction of natural habitats, which in turn allows non-indigenous species to spread. Islands and coastal regions seem to be particularly vulnerable because they occupy leading roles in global overseas trade. There is yet another considerable risk besides the introduction of new alien species. Many of the alien plants and animals that, until now, have been kept in people's homes and gardens and are not yet to be found in the wild might well spread in the future. Given the word-wide effects of climate change, this is in fact a distinct possibility.