The European Environmental Agency estimates that more than 10,000 known and unknown species have been artificially released into Europe's natural environment - among them the non-native cane toad, pictured here. Such exotic species are considered to be a primary threat to biodiversity, while they also cause a number of problems for humans.
Originally bred at fur factories, raccoons have quickly reproduced in Germany over the past several years. The nocturnal omnivores make their way in the wild - or come to pay residential areas a visit, like this individual at a vacation home on Berlin's Müggel Lake.
The tiger mosquito, an aggressive species that spreads illnesses such as dengue fever, is an unwelcome guest in Germany. Originally from Asia, it has long established itself in the Americas and Europe. Researchers believe that as a consequence of climate change, the Asian tiger mosquito could encounter ideal living conditions in broad swaths of Europe some time between 2030 and 2050.
The so-called Iberian slug, which actually comes from France, is a plague for gardeners and farmers in Germany. The species can rapidly reproduce in muggy weather, and has become among the most common and widespread slug species in Germany.
The Common Ragweed was accidentally introduced to Europe from North America. The seeds are often contained in bird seed - one reason why the plant has spread so quickly. The invasive plant grows particularly on the sides of roads, in ditches, on construction sites, and dumps. Its pollen often causes strong allergic reactions.
The Ring-necked Parakeet made a new home along the Rhine in the late 1960s. There are now several thousand of them in Germany. The consequences of the arrival of this African bird are as yet unknown. It is feared that the parakeet provides too much competition for other birds that brood in caves.
The red swamp crawfish is often kept in aquariums. It is considered a low maintenance animal, nibbling at plants and occasionally eating fish. But in the wild the crawfish can cause great damage. In Spain, where it was first introduced in 1973, it has already decimated a number of amphibian species, which is why it is one of the most feared invasive species.
The Egyptian Goose is often to be found near rivers and lakes. The African birds were once kept in captivity, but some escaped, and have since multiplied rapidly in the past ten years. They are a particular nuisance to farmers, since they eat crop seed and often reduce harvests.
There are around 5,000 different species of weevil, or snout beetle, in the world. Some of these cause massive damage in Europe - for instance by destroying palms in the Mediterranean. But they are also particularly feared in agriculture.
The Japanese knotweed has been spreading through Europe since the 19th century, threatening biodiversity in many countries. The plant is native to Asia and was deliberately introduced to Europe to serve as food for red deer - unsuccessfully. Now special permission is needed to plant the knotweed in Europe.
Innumerable non-native plant and animal species have become established in Europe. Arriving through commerce or tourism, such exotic species are often invasive to local ecosystems, and can cause various problems.