Dutch Turtle/Tortoise Society
Nederlandse Schildpadden Vereniging (NSV)
Trachemys s. elegans in The Netherlands; a threat for Dutch flora and fauna?
By Jaco Bruekers & Harrie van der Keijlen
The following English summary is from a rapport made by the NSV members mentioned above, who offered the rapport to the NSV for publication. If you have any questions concerning this rapport please contact the writers.
Introductions of non-native fauna is common in many parts of the world. A small number of these introductions consists of reptiles and amphibians. Information concerning the introduction of these animals in Europe is relatively rare. Most documented cases deal with ONLY two species: Rana catesbeiana and Trachemys. These species were frequently mentioned in the media in The Netherlands during the last few years. Before going into the details of the research, the authors give some general information about the genus Trachemys..
To give a reliable picture of the introduction of Trachemys species in the wild in The Netherlands it was essential to gather as much field observations as possible. A special questionnaire was designed, based on the form used for similar purposes by the National Terrapin Project in England. Almost 200 institutions, societies and government authorities were approached and asked to report observations on turtles. Forms were published in several national journals, so it is expected that a vast number of people have been reached.
A large number of the returned forms referred to the observation of one species of turtle, Trachemys scripta elegans (red-eared slider). Also important are the reports of authorities, working in water management, which claimed not to have seen turtles over a period of several years. Although the form had room to report other turtle species, no other species were observed. The investigation also made clear that the sightings of turtles correlated with the concentration of human settlements. It is true that Trachemys is hardy enough to survive most winters in The Netherlands, even extreme winters (extended periods of -17°C [about -5° F] Nevertheless, these cold winters MUST have had an impact on some of the overwintering turtles in the wild. Reports about dead turtles in garden ponds substantiate this assumption. These harsh winters probably prevent Trachemys to really establish populations in The Netherlands. Of course the increase of the turtles by new released turtles should be avoided. Expansion of feral populations of Trachemys by successful breeding is very unlikely, if not to say unrealistic. This species cannot breed successfully under Dutch climatic conditions (USDA zone 7/8).
One of the assumptions refers to a threat toward our native amphibians. Fact is that most water bodies mentioned above are avoided by amphibians (Bufo) bufo may be an exception) because of the presence of fish. Next to this hypothesis we think that the (illegal) distribution of fish in small water bodies should be considered a more serious threat to amphibians than turtles.
Only the juvenile turtles are mainly carnivorous and hunt for snails, fish, amphibians and waterbugs. A protein-rich diet ensures a rapid growth. Research in the USA revealed that adult Trachemys are not specialised fish hunters although sometimes they will try to get them. Mature turtles generally feed on water vegetation and water insects. Turtles also feed on sick or dead fish or other animals.
Trachemys s. elegans is an adaptable reptile which is able to survive even outside its natural range in other climate zones with harsh winters and relatively cool summers. The species does occur in the wild in The Netherlands but can’t be considered common. Additionally, Trachemys is not capable of breeding successfully here (USDA zone 7/8). In The Netherlands feral Trachemys should not be considered a serious threat to flora and fauna. Fortunately the European Community just executed a legislation which prohibits the importation of red-eared sliders in European countries. The motive for this act was that this turtle is considered a threat for native flora and fauna. Despite the fact that the import stoppage is supported, the present authors do not accept the motive used to introduce this regulation. The import stoppage means that, in time, the number of feral turtles in The Netherlands will reduce naturally (old age or death caused by the unfavourable climatic conditions).