Alien species are changing biogeography

Theba pisana (Credit: César Capinha)

When the first explorers sailed around the world they observed that the further away they traveled, the more different species and ecosystems they encountered. Today we know that there are geographical barriers to dispersal. Lineages that were separated for quite some time followed different evolutionary paths and have diverged. However, humans and their goods travelling over the last few centuries have fostered the dispersal of species to new places. This human-mediated dispersal has been hypothesized to homogenize biodiversity and perhaps breaking down biogeographic barries, but this hypothesis had never been tested globally.

An international team of researchers from Portugal, Austria and Germany tested this homogenization hypothesis by analyzing dissimilarities of species composition of 175 species of alien snails across 56 countries and subregions. For each location they mapped the distribution of snails after human-mediated dispersal. Then they went on to look at where these alien species lived before the human-mediated dispersal. 

The study comes in the wake of recent studies that could not find significant trends in biodiversity loss at the local scale over the last decades. 

We therefore took a different angle. We didn't test whether there has been species richness changes in communities over time. Instead we asked how is the similarity between species communities changing. As expected, before human-mediated dispersal, similar communities were found within each major biogeographic region. But after human-mediated dispersal, the communities of aliens follow a completely new pattern and are organized into only two large biogeographic regions: tropics and temperate areas,

Communities of species in temperate areas are more similar to other communities in temperate areas, independently of the continent they live on, and the same is true for tropical communities. Before, human-mediated dispersal, no species were shared by communities separated by more than 11,000 km, and very few species were shared between communities separated by more than 6.500 km. Now, even locations as far away as 20,000 km can share a large number of species.

In the past, geographical distance was the main factor determining community similarity. Now, climate is the major factor, influenced by distance and the extent of trade between countries, particularly those goods that serve as vectors for the transport of snails, such as roof tiles, live plants, vegetables and fruits. This means that for similar climates, the stronger the trade of these products between two countries, the more similar the species communities in those countries become.

The new study is the first global analysis of how invasions are reorganizing biogeographic patterns that took millions of years to form, and it provides evidence that major biodiversity changes are currently underway. The study also confirms the homogenization hypothesis and adds the influence of climate as an additional parameter. Geographic barriers to dispersal seems to disappear, instead climate limits species colonization in new areas. This suggests that globally, communities with similar climate conditions will become increasingly homogeneous.