From plague-harbouring crustaceans, to sprawling superweeds and allergy-inducing insects, invasive species are one of the greatest threats facing biodiversity today.
And they are not only impacting ecology - the economy and human health are suffering too.
Invasives are a significant threat to a large proportion of the world's biodiversity
Graham Madge, RSPB
An alien invasive species is a plant, animal or microorganism that is not native to an area, but has been introduced, either accidentally or deliberately, by humans.
All this week, BBC News will be taking a closer look at some of the alien invaders that are in the UK.
While the movement of species around the world is certainly not new, recent horror stories that highlight the damage that some invasive species can cause have made ecologists, politicians and industry sit up and take note.
David Roy, from the Nerc Centre for Ecology and Hydrology, has recently co-ordinated the Delivering Alien Invasive Species Inventories for Europe (Daisie) project, which is the most comprehensive inventory of invasive species ever undertaken for Europe.
He told BBC News that the number of invasive species had been chronically underestimated.
He said: "We've identified almost 11,000 alien species in Europe and the trend of new arrivals is showing no signs of levelling out."
But while the majority of these are not troublesome, a significant proportion can cause problems.
Dr Roy explained: "We found that approximately 15% of the aliens within Europe are known to have some impact on the environment or economy - and this problem goes across all taxonomic groups."
The problems that alien species can cause are well documented in the UK.
Some have caused the dramatic loss of native flora and fauna - like the catastrophic collapse of the population of red squirrels thanks to the introduction of greys, and the damage caused to ecosystems by species such as the zebra mussel and signal crayfish.
Their impact on the UK economy has also been considerable; the minister for biodiversity recently estimated that invasive species cost the British economy approximately £2bn a year.
Programmes to control plants like the Japanese knotweed or floating pennywort, which clogs up rivers causing potential flood risks, are time intensive and expensive.
And other species have proved costly to agriculture, forestry and aquaculture industries.
Some invaders can even affect human health.
Giant hogweed, which was introduced from Asia but is now widespread across the UK, can cause painful blisters and burns if its sap comes into contact with skin. Oak processionary moth caterpillars can cause rashes or asthmatic attacks.
Globalisation has been a key factor for the spread of alien invaders, says David Roy.
He explained: "Trade is the biggest source of movement of species - and new transport routes are being opened up all the time."
But is the UK doing enough to hold back this tide of alien invaders?
Ruth Waters, who co-ordinates invasive species work for Natural England, said: "Places like Australia and New Zealand are really streets ahead of the UK in terms of understanding the issues, co-ordination and getting out there and doing something."
But, she says, this is down to the fact that these countries have suffered economic and ecological damage from invaders on a much greater scale than the UK.
Contact with giant hogweed can cause huge blisters
She added: "But the UK can really learn from what they have been doing."
Ecologists say that controlling invasive species is complicated by the fact that there are so many of them.
Responsibility for dealing with them is also shared by many government and non-government bodies.
In May, the Department for Environment, Food and Rural Affairs (Defra), the Scottish Government and the Welsh Assembly Government launched a draft strategy for dealing with invasive species in Great Britain.
The Northern Ireland Assembly and the Republic of Ireland Government are also working on a joint strategic approach to the problem.
Grey squirrels have had a dramatic effect on reds
The aim is to find new methods to control existing species and faster and more co-ordinated ways of dealing with new species, as well as improving surveillance, monitoring and recording programmes.
While the governments' new focus on invasive species has been broadly welcomed, conservationists are warning that swift action is necessary if the threat of invasives is to be contained.
Graham Madge, from the Royal Society for the Protection of Birds (RSPB), said: "Government documents are long on rhetoric but very thin when it comes down to well-defined actions and accountable responsibility.
"Invasives are a significant threat to a large proportion of the world's biodiversity."
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