Invasive animal species are a global problem. Biologists have now proven for the first time that three of these alien species have gained a foothold here in Germany. These are freshwater turtle species originally from North America that occur in water bodies in Baden-Württemberg and apparently also breed there, the team found. This is the first evidence of successful reproduction of non-native tortoise species in Germany.
The introduction and immigration of non-native animal species causes ecological problems worldwide and often also economic damage in the billions. Such bioinvaders can displace native species and throw ecosystems off balance. Among the problematic new arrivals here in Germany are exotic reptiles that originally came from the pet trade and were released by humans.
Turtles as an invasive species
The North American lettered slider turtle (Trachemys scripta) is one of the worst invasive reptile species in the world. This freshwater turtle was often kept as a pet and imported into Europe in the 1980s and 1990s. In many regions it was thus able to establish itself outdoors and spread rapidly. In order to prevent a further advance, the import of this turtle species was banned by the EU in 1997, and since 2016 the sale of specimens born here has also been prohibited. So far, however, it was unclear whether this invasive turtle could already establish itself in Germany - the climate in this country is actually considered too cool.
In order to provide clarity, Benno Tietz from the University of Freiburg and his colleagues have now examined three alien freshwater turtle species in more detail, which had already been sighted several times in lakes near Freiburg and Kehl. In addition to the lettered slider turtle, the common slider turtle (Pseudemys concinna) and the false map turtle (Graptemys pseudogeographica) belong to it. They are also originally from North America and are popular with pet owners. "We wanted to find out whether the tortoise species are to be regarded as invasive - i.e. whether they reproduce here independently and regularly in nature," explains senior author Melita Vamberger from the Senckenberg Natural History Collections in Dresden.
Reproduction despite cool climate
For their study, the researchers temporarily caught almost 200 turtles in baited traps, took blood and tissue samples from them and then released them back into the wild. With the help of DNA analyses, they then examined the family relationships of the specimens and, in connection with their age, were able to draw conclusions as to whether the animals came from captivity or had reproduced themselves in the wild. The analyzes showed that all three invasive species have reproduced in the waters, so they have already established themselves in Germany. "We have now been able to show for the first time that all three species have become native to Baden-Württemberg. This is the first evidence of successful reproduction of non-native turtle species in Germany,” says Vamberger.
In the case of the lettered slider turtle, this is the northernmost evidence of outdoor reproduction in Europe, and for the other two species it is the first ever. "It's surprising that the invasive species have established themselves so far north," says Tietz. “Successful breeding and self-sustaining populations of Trachemys scripta in Europe have so far been known from the Mediterranean regions and the continental climate zone of Slovenia. Until recently it was assumed that these tortoises could not reproduce in Central Europe, mainly because of the cooler climate. The false map turtle in particular is actually more sensitive to cold.”
Potential hazard to aquatic ecosystems
This is not good news for aquatic ecosystems and especially for native turtle species such as the European pond turtle (Emys orbicularis). Because it is considered threatened and only occurs in parts of Brandenburg in Germany. If the alien species continue to spread, they could crowd out the native species. Experiments have already shown that European pond terrapins kept with Trachemys scripta lost weight and were more likely to die. Biologists suspect that the larger new arrivals crowd the native turtles out of warming sunbathing spots and snatch their food away. In addition, the invasive aquatic turtles can introduce and transmit viruses and parasites.
The effects that invasive turtles have on other members of aquatic ecosystems such as amphibians, fish or aquatic plants have not been studied sufficiently. It is also unclear whether the non-native species could possibly take over ecosystem services in damaged ecosystems by occupying vacant niches. "All of these questions urgently need further research," says Vamberger. "At the same time, we need to educate the general public so that no more animals – of any kind – are abandoned in the future. It would also make sense to offer mandatory training for keeping certain animals based on the principle of 'proof of competence'.”
Source: Senckenberg Research Institute and Natural History Museums; Specialist article: NeoBiota, doi: 10.3897/neobiota.82.87264