When American swamp crayfish were sighted for the first time on the paths and meadows of the Tiergarten in Berlin in 2017, it caused a stir. Presumably, the offspring of abandoned animals initially reproduced unnoticed before hunger or lack of space drove them out of the park waters and into the public eye.
For the past five years, the invasive species, which is actually native to the southern United States and northern Mexico, has been taken out of the waters year after year in the summer. This is to prevent it from spreading further. The voracious and migratory animals are considered a threat to native species and ecosystems – not only in Berlin, but throughout the European Union.
In addition to the American red swamp crayfish (Procambarus clarkii), there are numerous other immigrant species in Berlin that cause problems for people and/or nature: giant hogweed, tree of heaven and narrow-leaved waterweed as well as Egyptian goose, raccoon or nutria.
According to the Federal Agency for Nature Conservation, around 900 species have settled here permanently throughout Germany since 1492 – the year 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 unchecked spread once they have arrived.
“For 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. “With species that are already established, such as the swamp crabs or the giant hogweed, it is no longer possible to eliminate them. Then it’s a matter of containing the stocks and keeping the species away from particularly sensitive areas such as nature reserves.”
In the case of the red American swamp crabs, this means above all: catching animals. In Berlin, the senate administration has commissioned a fisherman to do this. The caught animals are sold to Berlin restaurateurs, among others. “We see this as an opportunity to find an ecological and sustainable solution to a problem,” says Lukas Bosch, co-founder of the company HolyCrab, which takes over the organization from catching to sale.
“In terms of regenerative fishing, you can reduce the stock to a tolerable level and at the same time market a regional, exotic product economically.” In a nutshell, the concept is reflected in the company motto: “If you can’t beat it, eat it” – eat what you can’t get rid of.
However, not all invasive species can be fought with knife and fork, and in many cases there is a risk of massive economic damage if they spread. For example, combating giant hogweed, which can cause severe skin damage when touched, incurs considerable costs. “Ash dieback can be problematic for forestry,” says Kühn, giving another example. “The disease is caused by an introduced Asian fungus and can lead to the death of the trees.”
Combating invasive species is often like a Sisyphus task: Mammals are hunted down, insects are killed with targeted poisons, or plants are painstakingly uprooted. Uncompromisingly relying on the expulsion of an invasive species is often not expedient, says Sebastian Kolberg, species protection officer at the German Nature Conservation Union (Nabu). It often makes more sense to strengthen the ecosystem as a whole.
“You have to take a close look at which species you want to protect on a case-by-case basis,” says Kolberg. “And then you have to think about what this species needs to survive, how you can help it and, for example, create natural shelters.”
Important in this context: Not all newcomers are a danger to people or nature. Experts put the proportion at around ten percent. “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.”
On the other hand, there are also numerous native species that can become problematic if they disturb the sensitive ecological balance through excessive reproduction or threaten human health, such as the example of the oak processionary moth, which is often fought over a large area.
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 been cherished and cared for as a beautiful city tree for a long time,” says Ehlert. “For about 80 years, the species has been spreading massively because the winters have become warmer and the frost-sensitive young trees are increasingly surviving.” Today, the tree of heaven is officially undesirable, also because it can get stuck in every crack and damage infrastructure such as roads or walls.
In order to prevent foreign species from being introduced in the future, it is also necessary to create and educate people about the problem, says Kühn, for example in gardening companies or road maintenance depots. “They need to know, for example, that equipment should be cleaned before driving from A to B to prevent seeds from being transported.” Pet shops and nurseries are also not allowed to sell potentially problematic species, or at least only with a corresponding warning.
With climate change, the situation is unlikely to ease in the coming years. According to UFZ researcher Kühn, there could be fewer frost-loving species, but most of the species introduced come from warmer countries and will benefit from the expected changes.