For all groups of organisms on all continents, 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.
Quite a sobering statement. However, it summarizes the results of a new study in which a large international group of researchers analysed alien species accumulation during the last centuries. They used more than 45,000 first occurrence records for more than 16,000 alien species.
The colleagues found that 37% of all recorded alien species have been introduced between 1970-2014 and thus recently. At its peak 585 new species were recorded within one year. This corresponds to more than 1.5 new alien species per day globally. This is likely an underestimate as the date of first record is not available for most alien species.
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.
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. Well, it clearly hasn't.