Attention: Terrapin! Invasive pond slider on the move

The pond slider (Trachemys scripta) is the world's most widely distributed species beyond its native range. These turtles with a shell length of up to 30 centimeters are native to the Southeastern U.S. - but today, they are found on all continents, except for Antarctica and a few oceanic islands. "The turtle can be found in the wild in practically all European countries." explains Dr. Melita Vamberger of the Senckenberg Natural History Collections in Dresden, and she continues, "These reptiles owe their wide-spread distribution to the captive animal trade."

The species is considered a threat to native turtles, since it is in direct competition with them regarding food as well as nesting and basking sites. Moreover, the introduced reptiles are potential carriers of parasites and other pathogens. Since the 1990s, the import of these popular pets with their vividly orange to red head stripes has been outlawed in Europe. "However, in some countries, in particular in the Balkan states, the illegal trade continues to flourish," adds Vamberger, and she goes on to say, "But for a long time, it was not clear whether the species could become invasive in Europe." Until now, the successful reproduction and establishment of these animals had only been documented in the Mediterranean region.

The Slovenian-German team of researchers around Dr. Vamberger and the director of Senckenberg Dresden, Professor Dr. Uwe Fritz has now been able to demonstrate by means of genetic studies that the turtles also reproduce in Slovenia. The biologists took samples of 77 turtles from three sites and could show that they reproduce in all areas examined in Slovenia. "We selected the sites based on climatic differences," explains Fritz, and he adds, "Unfortunately, the pond slider also reproduce and spread in the vicinity of Ljubljana - a temperate, continental climate."

For the first time, the researchers were thus able to offer genetic proof that Trachemys scripta can also reproduce outside the Mediterranean region with its mild climate. Around Ljubljana, the capital of Slovenia, fewer animals were found that were related to each other than in the warmer regions, which could indicate that the turtles reproduce less frequently here. "However, it is likely that more animals with new genetic material are being released near the city, which necessarily leads to fewer related animals," cautions Vamberger.

Due to the potential for expansion beyond the Mediterranean region and the potential threat to native species, the sliders should be classified as invasive, according to the biologists from Dresden. "In addition, we recommend to intercept the pond slider, at least in habitats occupied by native turtle species, in order to prevent the spread of the invasive turtle and the displacement of the native inhabitants," says Fritz in closing.