Invasive alien species pose a greater risk to Europe's biodiversity, economy and human health than previously thought, a report has concluded.
The European Environment Agency (EEA) has compiled a list of 28 invaders that highlight the range of threats facing ecosystems in the continent.
Non-native species, such as food crops, can also be beneficial, the study adds.
The reports have been published ahead of a high-level meeting at the European Parliament to discuss the issue.
It is estimated that there are more than 10,000 non-native species in Europe, of which at least 15% are deemed to be "invasive", which are organisms that are known to have negative ecological or economic impacts.
Invasive alien species (IAS) are considered to be one of the main threats to biodiversity, explains EEA executive director Prof Jacqueline McGlade.
"In many areas, ecosystems are weakened by pollution, climate change and [habitat] fragmentation," she says.
"Alien species invasions are a growing pressure on the natural world, which are extremely difficult to reverse."
Japanese knotweed
The report, The Impact of Invasive Alien Species in Europe, lists the various impacts.
"Competition, predation and transmission of diseases between alien and native species are frequent and can pose a major threat to native species," the authors observe.
"Alien species may also affect ecosystem services, which in turn can have an impact on human wellbeing."
One species whose spread and impact has been well documented is Japanese knotweed (Fallopia japonica).
It can reach 4m in height, growing up to 30cm in a day. Its powerful root system can reach depths of three metres into the soil and spread up to 20m, making it almost impossible to eradicate once it becomes established.
The report says the plant forms dense stands and squeezes out other plant species and outcompetes native plants, resulting in a botanical "monoculture".
The publication adds: "The rhizome system of knotweeds can seriously damage infrastructure, such as buildings, river bank stabilisations and water channels, railway tracks and roads, and construction land.
"By disrupting the integrity of flood defence structures, the risk of flooding is increased."
The Asian tiger mosquito is a more direct threat
An IAS that poses a direct threat to human health is the Asian tiger mosquito (Aedes albopictus), which has been linked to the transmission of more than 20 human pathogens, including yellow fever and dengue.
The species is an "aggressive daytime‑biting insect" and its distribution has spread rapidly in western and southern Europe over the past two decades.
On Thursday, Czech MEP Pavel Poc is hosting an event at the European Parliament in Brussels that will look at ways to tackle the threat posed by IAS.
Organised by conservation groups IUCN and Birdlife, the high-level debate at the European Parliament in Brussels will consider measures that could be taken within the EU policy framework to mitigate the present and future threats from invasive alien species.
The EEA report warns that, given the increase in both goods and people moving around the globe, the "number and impact of harmful IAS in Europe may grow significantly in the future".
It adds that changes to the climate may provide opportunities for IAS to proliferate and spread.
"In this situation, some IAS might initiate complex, unpredictable cascades of effects," it warns.
The EEA suggests that the best way to tackle the threats posed by invasive species was through a "combination of preventative measures, early detection and rapid response to incursions, with permanent management only as the last option".
