Invasive alien animal species can seriously disturb the balance of the natural environment

Ljubljana, 2 August - Invasive alien animal species harm biodiversity and the natural environment. Slovenia is home to quite a few of such species, with the most well-known being the pharaoh ant, harlequin ladybird and tiger mosquito. The Ljubljana Marshes Nature Park mostly deals with the pumpkinseed, the pond slider and the nutria. These animals were introduced into new ecosystems by humans, so every individual should bear in mind that they share responsibility for preserving the natural balance through their actions.

The invasive alien red-eared slider or pond slider species.
Photo: Anže Malovrh/STA

Ana Tratnik of the Ljubljana Marshes Nature Park holding the invasive alien red-eared slider or pond slider.
Photo: Anže Malovrh/STA

The invasive alien red-eared slider or pond slider species.
Photo: Anže Malovrh/STA

The invasive alien red-eared slider or pond slider species.
Photo: Anže Malovrh/STA

Ana Tratnik, an ecology and biodiversity specialist with the Ljubljana Marshes Nature Park, told the STA that not all animals that had been imported into Slovenia are invasive species. "By definition, invasive alien animal species are those that harm our biodiversity and the natural environment they inhabit," she explained.

The introduction of such animals into the local environment can be deliberate or accidental. "Humans are deliberately releasing certain game fish species into nature, whereas animals can be introduced accidentally as stowaway species through transport or food. Some species suitable as pets are introduced after humans get bored of them and release them into the wild."

Invasive animal species affect biodiversity in different ways: "They are predators of native species, rivals for food or habitat, or they transmit certain diseases to which native species are not immune." When it comes to invasive species in the Ljubljana Marshes Nature Park, the the pumpkinseed, pond slider and nutrias are the biggest cause of concern.


Gregor Lipovšek, an agronomist and crop scientist who works for the nature park, explained that the pumpkinseed had arrived in Europe as a fish tank species and was then released into nature. Now it is mostly spreading when fishers move fish from one water environment to another.

The pumpkinseed is a predator that preys on the spawns and fries of other fish, frogspawn as well as insects such as dragonfly larvae. "This invasive alien fish has had a direct impact on protected species in our environment." The pumpkinseed may grow up to 30 centimetres long in the warm South American waters, however in Slovenia the maximum size is ten centimetres. According to Lipovšek, the pumpkinseed reproduces very fast with females spawning already in the first year.

In addition, pumpkinseed overpopulation is hard to curb as the Draga ponds in the Ljubljana Marshes Nature Park are mostly managed by holders of permits for extensive fish farming. In the three ponds where this is not permitted, measures will be taken this year to remove the pumpkinseed by means of traps or by draining the ponds in autumn. That way, the pumpkinseed population will be curbed at least to a certain extent.

Pond slider

The removal of alien turtles that were mainly introduced into the natural environment after people got bored of them is already under way in the entire area of the Ljubljana Marshes as part of the PoLJUB project. These turtles are mostly members of the pond slider aquarium species, the most famous one being the red-eared slider.

The pond slider species features on the list of invasive alien species in the EU, so its import and sale have been banned for a few years now. However, this has not fixed the problem since shops are still selling turtle species whose original habitat is North America or Asia. "So far, we have already detected nine alien turtles that are usually kept as pets in Slovenia," Lipovšek said.

These turtles are considered a problem in the Ljubljana Marches mostly because they are crowding out Slovenia's only indigenous species of fresh water turtles - the European pond turtle. Alien turtles are competing with the indigenous species for habitat, places where they can sunbathe and food, or they can transmit various diseases, thus accelerating the decline in the European pond turtle population. Some alien turtles may also be predators killing other protected animal species with a number of studies indicating their direct impact on ecosystems.

One of the aims of the 2018-2023 project PoLJUB is the removal of at least 50 turtles from the Ljubljana Marshes. So far, more than 40 have been removed. The action plan, prepared by the Institute for Nature Conservation, also involves setting up a shelter for the captured turtles so that they can be put up for adoption after the removal, Lipovšek said. "This solution also involves microchipping the adopted animals, so that if they find themselves back in nature, either by accident or due to man-made decisions, their adopters would cover the cost of the capture, which is not low."

As long as the adoption of an alien invasive turtle is not possible, the only other option in Slovenia is euthanasia. "There is no official shelter for turtles, and other animal shelters do not accept these animals as they cannot provide the right conditions for reptiles. Another problem is that turtles live very long, up to 20 or 30 years, and they keep growing until they die. Owners getting bored of their turtles after five or ten years and releasing them into nature is definitely not a solution. People think that they are returning the animal back into their natural environment, where it will get on well, but that is not true. Since we are talking about an invasive species, sooner or later someone will capture it and the animal will end up at the vet's," Lipovšek warned.


The third animal which the nature park deals with is the coypu or nutria, which is spreading even in urban areas. The nutria comes from South America and has been present in Slovenia ever since nutria breeding, primarily for fur farming but also for food, was abandoned. Unlike the other alien species, the nutria is a game species that can be hunted throughout the year. However, its monitoring it not systemically regulated.

According to Lipovšek, hunters may hunt nutria in hunting areas that usually start 50 metres from the last building in an urban area. Hunting is of course banned in cities. "As a result, we achieve nothing by eradicating nutria in a certain area by the Ljubljanica river when the population is maintained in the city along the river by people intentionally feeding the adorable animals. From these locations nutria can spread to the natural environment."

Nutria cause the most damage by digging riverbank tunnels in the Ljubljana Marshes, Lipovšek warns. "The marshes are a system of ditches and nutria cause additional soil erosion by digging riverside tunnels. Nutria also have a major impact on bigger aquatic plants or macrophytes, which they eat. These plants are a hiding spot or food source for protected species. Fish and amphibians lay their spawn there as well and the plants play an important role in water filtration." Nutria also cause agricultural damage.

Currently, there is no plan for nutria eradication. Culling by shooting may be ordered, but the practice is "banned in cities and would not be appropriate there anyway". There is also an option of removing them by live trapping them and later euthanasing them in a humane way. "Such systems have been used abroad and turned out to be very successful," Lipovšek said.

Legislation on invasive alien species in the works in Slovenia

The issue of invasive alien species has been addressed by the EU, which adopted the first regulation to that effect only in late 2014, according to Lipovšek. The regulation currently deals with 66 plant and animal species. National legislation is still in the works in Slovenia, however action plans to eradicate certain species have been implemented, including in the case of the red-eared slider. The European Commission has urged a total of 18 countries, including Slovenia, to start implementing various provisions of the regulation. "Both the European Green Deal and the EU's biodiversity strategy for 2030 highlight the importance of the EU stopping the loss of biodiversity by preserving natural areas and rehabilitating damaged ecosystems," the Commission has said.

Alien species are indeed a systemic issue, one that affects every individual. Lipovšek warned that releasing pets into nature is not a solution, instead such actions only exacerbate the problem and shift the responsibility for the animals' removal onto somebody else. Efforts to raise public awareness about this are of great importance. Any removal of alien species means spending funds that could be allocated for improving the situation of protected species instead.

Janez Kastelic, director of the Ljubljana Marshes Nature Park, said: "There should be no sloppy attempts when it comes to alien invasive species. One needs to be very rigorous. Of course, ethical or moral dilemmas always arise in relation to animal removal. But I always say that nature is not pretty at all times, nor ethical or moral. This is a fight for survival. And if this is unacceptable, we as society must accept that the ecosystem will change in the future too."