Welcomed - or feared and hated: Europe is home to hundreds of nonnative animal and plant species. They affect ecosystems and harm biodiversity - and cause millions in damage. Now, the EU is acting to stop their spread.
"It's hotly controversial," says Claus Mayr. The director of European affairs at the German Nature and Biodiversity Conservation Union (NABU) is all too familiar with the invasive species debate.
"A list of species to take action against dates back to the year 1992," he adds. But lobby groups for the Scandinavian fur industry and the flower industry in the United Kingdom have been hindering action for many years, preventing the addition of more species to the list - to his regret.
"The current list is a beginning - but 37 species are too few," Mayr judges.
The goal is to keep nonnative wildlife, such as raccoons or nutrias safe and controlled. But that increases the cost of fur, fur traders complain.
In addition to mammals like the raccoon, muskrat, grey squirrel, and nutria, several species of birds, mollusks, fish, and crayfish are also included in the official EU invasive species index.
Globalization boosts spread of invaders
Eighteen plants - including the Himalayan balsam, Japanese knotweed, and Canadian pondweed - are also included in the list. Their names indicate where these plants originated.
Some neophytes (invasive plant species) and neozoans (invasive animal species) came to Europe by accident, as stowaways in ocean-going ships. Many Eurasian species have made their way to other continents over recent centuries in the same way.
Other species were deliberately introduced to Europe - like the Canadian goldenrod in the 19th century. The nectar of this perennial shrub attracts bees in droves.
But the plant also has a downside: Without predators while on the European continent, it spreads quickly on disturbed soils, or "brownfields" - particularly along railway tracks - and displaces native species.
The Himalayan balsam has also become an annoyance for many communities. Originally imported from the Himalayas, it was cultivated as an exotic garden plant, welcomed for the pleasing aroma of its elegant pink flowers. Today, it blooms wild along riverbanks - and has become so dominant that other species there are crowded out.
The North American Robinia - commonly known as black locust tree - fixes nitrogen from the air. That impedes the growth of some rare species that prefer nitrogen-poor soils, including certain native grasses, herbs and orchids. Moreover, the bark, leaves, and wood of black locust are toxic to both humans and livestock.
The hogweed plague
Every year, community workers and volunteer environmentalists try to fight against the Caucasian giant hogweed plague in many different locations across Europe.
This monster among plants was intentionally brought to Europe for use in forestry - to offer greater cover for deer, as a shield-hedge along roads, or as a silage plant. Unfortunately, all parts of the plant contain the intensely toxic allergen furanocoumarin.
Even a few drops of the plant's juice can cause photosensitivity and burns. It's very difficult to eradicate, because each plant produces thousands of seeds that remain viable for years. Moreover, it can re-sprout from the roots even after it has been cut down or doused with herbicides.
According to the German Federal Nature Conservation Act, Germans require special permits to plant species like the giant hogweed, which are part of a neophyte plague. Yet the giant hogweed is not on the EU's invasive species list.
"It is an ornamental plant in some EU countries, and remains very popular amongst beekeepers despite its toxicity," Mayr said, describing the difficulties behind attempts to reach an agreement.
"Another argument is the immense cost of pushing them back - never mind the potential costs of trying to exterminate them," Mayr added.
According to a study from the German Federal Environment Agency (UBA), damages caused in Germany by just 20 invasive species amount to 167 million euros annually (as of 2002).
However, about 170 species are known to have negative impacts. The European Commission has estimated the cost to repair all damages from harmful invasive species in the EU at 12 billion euros a year.
Stowaways from the Far East
Many organisms have been ferried between continents by ocean-going ships. For more than a hundred years, it has been standard practice to pump water from ballast tanks into the harbor - to compensate for lightening a ship's load after its cargo has been removed.
Germs, disease-causing organisms, and marine animals are released along with the ballast water, which is discharged in the destination port when the ship is re-loaded with cargo. Thus, the biotic stowaways end up in the bays, waterways, and shorelines of other continents.
Which is why the Chinese mitten crab has been living in northern German waters for the past hundred years. The omnivorous Chinese mitten crab, also known as the da zha freshwater crab or Shanghai hairy crab, has spread all over Europe.
While it's harvested for food in China, mitten crabs are considered a plague in Europe, where the hand-sized arthropod has hardly any natural enemies.
In order to manage the invasive species problem, and to reduce the damage they cause to the maritime environment, after lengthy negotiations, the International Maritime Organization adopted an agreement for the treatment of ballast water in 2004.
However, this won't enter into force until September 2017.
Arduous struggle against sneaky invaders
By 2050, the European Union wants to counter the loss of native plant and animal species "to protect, safeguard, and adequately restore its natural capital for the wellbeing of the people and for economic prosperity."
This goal will necessarily include achieving proper control of invasive species, which "endanger biodiversity, change habitats and displace naturally occurring species," argued German environmental minister Barbara Hendricks (SPD) at the 2016 national nature conservation conference day in Magdeburg.
"They can also cause economic damage and endanger human health," Hendricks continued.
EU Member States are now required to draw up management plans detailing how they plan to regulate the handling of non-native species. "Raccoons and nutria are so widespread in Germany that controlling them will be difficult," says NABU manager Claus Mayr.
And some species are now so well-established, that it is probably no longer possible to remove them from European ecosystems, Mayr added.
One example: As a result of climate change, a species of Asian mosquito that transmits a dangerous sleeping sickness parasite has crossed the Alps to southern Germany. That mosquito is not, however, on the EU invasive species list.
Danger averted: With invasive goats - their competitors for habitat - to be eliminated, maybe Galapagos giant tortoises will survive
Other continents and regions also face major problems with unwanted invaders. On the Galapagos Islands off the coast of Ecuador in the Pacific Ocean, 140,000 goats are to be killed in the context of the world's largest invasive mammal extermination program.
The goats were introduced around 80 years ago, and multiplied rapidly in the absence of natural enemies. As they multiplied, they ate the grass and herbs on which the native Galapagos giant tortoise depended for food.
The goats have destroyed the tortoirses' nests and eggs, denuded the landscape of vegetation, and caused erosion. In the end, the Ecuadorian government is resolved to eliminate them.
In the European Union, there are no such rigorous plans as yet for dealing with invasive species.