The majority of ladybird beetles in Central Europe are immigrants, since the Asian ladybird beetle is now the most common species found in our latitudes and original native species have been pushed back more and more. Another species introduced by humans is the western corn rootworm (Diabrotica virgifera). Once native only to Central America, this pest has now spread not only within the North American continent, but also to Europe, causing considerable damage to agriculture in all of these regions. The box tree moth (Cydalima perspectalis), a small moth from East Asia, is now also seen in many European gardens. The caterpillars of this alien species have the unpleasant habit of eating all the leaves off the eponymous boxwood trees.
A common aspect of these three bio-invaders is the fact that they are quite new to Europe. But although they were detected in larger numbers only from the 2000s onwards, they have already caused considerable impact. Franz Essl from the Department of Botany and Biodiversity Research at the University of Vienna considers them to be just the tip of the iceberg: “In the last 20 years, we have seen a rapid increase in alien species introduced by human activity in Central Europe, but also in most other parts of the world”, the ecologist notes. In the FWF project “AlienScenarios”, Essl is working with an international team to develop prediction models that allow us to estimate the extent to which the number of invasive species will increase worldwide in the course of the 21st century. The resulting scenarios are intended to serve as a basis for future UN targets in the area of biodiversity – the so-called “Post-2020 Biodiversity Framework”.
Freight transport and climate change as influencing factors
There are a number of reasons for the rapid increase in bio-invaders over the past few decades. “Cargo traffic has increased immensely, including air travel, which allows many more species to survive the journey – the corn rootworm being one example. Then there is climate change, which is impacting ecosystems. The spread of many new species is also favoured by changes in land use,” Essl says, listing factors of influence. In 2017, the ecologist Essl was a member of an international team of researchers and, in a study published in the journal Nature Communications, he was able to demonstrate that the number of documented alien species worldwide has been steadily increasing since 1970 – with an average of up to 1.5 new sightings per day.
While there are detailed forecasts for other factors of environmental change with potentially far-reaching consequences, there is still a lack of corresponding model calculations when it comes to the development of the number of alien species. “There are scientifically sound scenarios for biodiversity conservation, land use and, of course, climate change. Thanks to the IPCC reports regarding global warming, we know very well that we can limit warming to 1.5 degrees if we take ambitious action, and that it will be beyond 4.5 degrees Celsius in the worst case,” Essl explains. “For alien species we do not yet have such scenarios, although the data situation has improved greatly in recent years.”
Analysis of the context around introduced species
How can one extrapolate the increase of alien species “into the future”? Essl explains that in order to create appropriate models the researchers start by analysing in detail existing relationships in a step called parameterisation. Expert knowledge and existing data on traffic and the flows of goods, economic structures or climate changes, as well as the previously observed numbers of bioinvaders, are combined to describe the status quo in a model. “In order to create forecasts, the parameters that influence the number of alien species are replaced in the model by estimated future values,” Essl notes. “This serves to calculate the number of bioinvaders that is the result of future changes in the climate or in economic and travel activities.”
As one basis of these calculations, the researchers are using the “Alien Species First Record Database”, a worldwide database of alien species sightings with more than 50,000 entries to date, which Essl himself has also helped to compile. The models are intended to provide plausible estimates of how the number of bio-invaders will develop by 2050 or even 2100. In an initial study published in 2020 in the journal Global Change Biology, Essl and his colleagues were able to show that the number of alien species is expected to increase globally by 36 per cent within four and a half decades (2005 to 2050). The data for the creation and validation of the model cover the years 1950 to 2005.
Europe ranks first on the list of alien species
The study also provides information on how differently the continents will be affected by alien species: with a 64 per cent increase in bioinvaders between 2005 and 2050, or a total of 2,543 species, Europe is the frontrunner, followed by the temperate zones of Asia (50 per cent/1,391 species) and North America (23 per cent/1,484 species). Australia brings up the rear with an increase of 16 per cent or 1,286 alien species – and it is the only continent where the growth curve is predicted to flatten out.
These predictions are intended to provide an initial basis for more detailed model calculations that may, for instance, address different regions or take into account different degrees of countermeasures. The tangible consequences will also be investigated by future studies. “Only some of the invaders, pests that harm crops for example, also have economic consequences,” says Essl. “However, given the high growth rates, we still expect considerable ecological and economic repercussions.”
Franz Essl studied biology and botany at the University of Vienna. Having specialised in biological invasions science, he did research at the Umweltdachverband as well as at the Federal Environmental Agency in Austria before returning to the University of Vienna in 2013 as an associate professor at the Department of Botany and Biodiversity Research. The “AlienScenarios” project, which is co-funded by the Austrian Science Fund FWF together with other national funding bodies in Europe, will run from 2019 to 2022 and will receive EUR 241,000 in funding from the FWF.
Yang Q., Weigelt P., Fristoe T.S. et al.: The global loss of floristic uniqueness, in: Nature Communications 12, 2021