The increase in numbers of alien species does not show any sign of saturation at a global level, an international team of 45 researchers with participation of the University of Fribourg has discovered. They found that during the last centuries the number of new introductions has continuously increased worldwide, with more than a third of all first introductions recorded between 1970 and 2014. Although individual trends differ among taxonomic groups, the ongoing increase in alien species numbers is still visible for all groups of organisms. The study is published in Nature Communications.

Although it was known that the number of alien species increased during the last decades, it remained unclear whether or not the accumulation of alien species has already reached a point of slow-down. Sven Bacher from the University of Fribourg, co-author of a new study on the topic comments: «For all groups of organisms on all continents, we found that the number of alien species has increased continuously during the last 200 years. For most groups, even the rate of introduction is highest recently. Barring mammals and fishes, there are no signs of a slow-down and we have to expect more new invasions in the near future.»

This outlook comes at the end of a large collaborative effort in which 45 scientists from all over the world established a database of the date an alien species was first detected in a region outside the species’ native range. Using more than 45.000 of these first records of more than 16.000 alien species, they analysed the development of alien species accumulation during the last centuries.

They found that 37% of all recorded alien species have been introduced between 1970-2014. At its peak, 585 new alien species were recorded within one year. This corresponds to more than 1.5 new alien species per day globally. «As the date of first record is not available for most alien species, these numbers are clearly underestimating the full extent of alien species introductions», says Franz Essl from the University of Vienna, Austria, senior author of the study.

The trends of increase vary among taxonomic groups, which can be attributed to human activities. «We observe a distinct increase in first record rates of vascular plants in the 19th century, probably as a result of the intensification of horticulture. The rates of new introductions of other organisms such as algae, molluscs or insects increased steeply after 1950. This is most likely a consequence of the ongoing globalisation of trade,» explains Hanno Seebens from Senckenberg, Germany, first author of the study.

The unprecedented increase in alien species numbers can lead to an increase in regional species numbers. However, these come at the cost of a variety of negative impacts on native ecosystems, the global homogenisation of floras and faunas, and the global extinction of native biota. Therefore various legislations are currently in force globally attempting to mitigate the introduction of new alien species. «However our results show that the past efforts have not been effective enough to keep up with ongoing globalisation. There is an urgent need to implement more effective prevention policies at all scales», concludes Bacher.

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Image: Asian longhorned beetle