Muskrat, swamp crab, tree of heaven: some introduced species are spreading rapidly in Berlin. Other cities are also struggling with invasive species. What to do?

When American swamp crayfish were sighted for the first time on meadows and paths in the Berlin Tiergarten in 2017, it caused quite a stir. Presumably, the offspring of abandoned animals initially reproduced unnoticed before hunger or lack of space drove them out of the park waters.

Every summer, specimens of the invasive species, which is actually native to the southern United States and northern Mexico, are now caught from the waters. Further spread should be prevented and the proliferation should at least be slowed down.

The voracious and migratory animals are considered a threat to native species and ecosystems – not only in Berlin, but throughout the European Union.

Some cause problems

Many other species that were not originally native to Berlin have spread to Berlin and other cities, and some of them are causing problems: giant hogweed, tree of heaven and narrow-leaved waterweed, as well as Egyptian goose, raccoon and coypu. According to the Federal Agency for Nature Conservation, around 900 species have settled permanently in Germany since 1492 – the year of the discovery of America, which scientists use as the boundary for distinguishing between foreign and native.

66 animal and plant species are on a list drawn up by the EU Commission, the so-called Union list of invasive species. The member countries must prevent the introduction of these species or stop their uninhibited spread once they have arrived.

“In the case of species that are not yet native here, you have a very good chance of keeping them away,” says Ingolf Kühn from the Helmholtz Center for Environmental Research (UFZ) in Halle. “It is no longer possible to eliminate species that are already established, such as swamp crabs or giant hogweed. Then it’s a matter of containing the stocks and keeping the species away from particularly sensitive areas such as nature reserves. »

Fighting is like a Sisyphus task

In Berlin, the senate administration has commissioned a fisherman who empties the pots laid out at least twice a week during the high season. The animals are sold to Berlin restaurateurs, among others.

However, knife and fork can rarely be used against invasive species – and fighting them is often tantamount to a Sisyphus task. “Often there is not enough capacity for this,” says Sebastian Kolberg, consultant for species protection at Nabu. “The lower nature conservation authorities simply lack the financial and human resources.”

Uncompromisingly relying on the expulsion of an invasive species is often not effective, says Kolberg. Focusing all efforts on managing one type of conflict is not a sustainable strategy. It often makes more sense to strengthen the health of the ecosystem as a whole.

A constant change

“Especially with the plants, many of the neophytes do not cause any problems, on the contrary,” says wildlife expert Derk Ehlert from the Berlin environmental administration. “Our parks would probably have far fewer species if there were no neophytes.” In general, nature is constantly changing – and so is the assessment of animal and plant species.

“The tree of heaven, which originated in China, has been planted here for around 250 years and has long been cherished and cared for as a beautiful city tree,” says Ehlert. “The species has been spreading massively for around 80 years because the winters have become warmer and the frost-sensitive young trees are increasingly surviving.”

Today, the tree of heaven, which can neither be fought well with cuts nor poisons, is officially undesirable, also because it can get stuck in every crack and damage streets and walls.

Many of the species that are considered problematic today were once deliberately introduced: the raccoon, for example, as a source of fur, the Asian lady beetle for biological pest control. Today they are one of the species that can no longer be driven out.

consequences of climate change

With climate change, the situation is unlikely to ease in the coming years. According to UFZ researcher Kühn, frost-loving species could become fewer. However, the majority of the introduced species come from warmer countries and will benefit from the expected changes.

“Fend off the beginnings,” says wildlife expert Ehlert. “Once a species has established itself, there is often little opportunity to get rid of it.”