Denmark has recorded approximately 2,600 non-native species, i.e. plants, animals, and fungi that do not originate from the Danish natural environment. The vast majority of these species pose no problems, and some will not even survive our climate in the long term. But according to the Danish Environmental Protection Agency, 77 of the non-native species are invasive. This means that they have successfully established themselves in the Danish environment with consequences for the native flora and fauna or the economy. In other words, an invasive species can become so dominant that it severely affects other animals or plants in our environment.
The UN Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) has identified invasive species as one of the five drivers threatening biodiversity worldwide. In the autumn of 2023, the panel published the 'Invasive Alien Species Report', which further emphasized the seriousness of the problem. The report was made by 86 researchers from 49 countries, and it shows that invasive species are estimated to have played a role in 60 per cent of cases where a plant or animal has become globally extinct. In 16 per cent of cases, the invasive species were the sole reason why animals or plants became extinct. Invasive species spread through human activity, and IPBES expects the problem to grow in the coming decades.
Introduced through ballast water
On land, it is easy to spot invasive species, which in Denmark include raccoon dogs, Rosa rugosa, and killer slugs. It is much harder to see what is happening below the surface in our seas, fjords, lakes, and streams, but there are also invasive species here. A total of 85 alien marine species have been recorded in Denmark. Four of them are considered invasive, i.e. they can cause damage to our ecosystems. Some may remember the comb jelly Mnemiopsis leidyi from the media, or the round goby - a fish originating from the Black and Caspian Seas. They are both marine species that were introduced into Danish waters through ship ballast water 15-16 years ago and immediately settled in.
At DTU Aqua, Senior Researcher Jane Behrens helped keep track of the round goby from 2014 - six years after the 10-20 cm long fish first appeared in the waters of southern Denmark.
"By studying the round goby, we learned about its spread and the characteristics of a successful invasive species," says Jane Behrens.
Successful invasive species - animals and plants alike - often share the same characteristics: They multiply rapidly, they have good conditions for spreading, they're competitive, and they are capable of withstanding varying environmental conditions. This also applies to the round goby.
"It's extremely tough. It thrives in water with both a high and low salt content and in different temperatures, which means that it can spread in many types of aquatic environments. It's also slightly larger than the native gobies and very aggressive when defending its territory, so it ends up dominating food sources and breeding grounds. And it's not picky and will eat many types of food," says Jane Behrens about the fish, which is a perfect example of a successful invasive species.
