All this week, BBC News has been taking a closer look at some of the invasive species that have hit the UK.
Harlequin ladybirds are one of the species that have invaded the UK
These alien invaders are causing huge problems for some of our native species. They are a costly economic problem, and some are even causing health problems.
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. Here, he answers your questions.
Is there an acceleration rate of new invaders and is it linked to global warming, and/or the effects of global warming?
Sue Haswell, Dawlish, Devon
Dr Roy: There is a clear trend for an accelerating invasion rate across most species groups. Our recently-completed inventory of alien species in Europe has shown, for example, that in the 1970s around eight new insect species were arriving each year, whereas the number arriving per year since 2000 has been around 18 species. A similar trend is shown in plants and mammal too. In the majority of cases there is a clear link between invasions and pathways of arrival associated with trade (e.g. importing garden plants, aquarium species, pets and zoos) so this is likely to be a more important factor that global warming.
What can the average person do about invasive species? For example, with signal crayfish and Chinese mitten crabs, would it be legal for a passer by who sees either of these to pick them up and destroy them?
Ben Glazier, Wendover, Bucks
Dr Roy: I think it is inadvisable for the general public to take these matters into their own hands. In many cases identifying an alien species is not straightforward.
What measures do other countries have to keep out invasive species that the UK does not? I gather that Austrailia is well ahead of us, for example. Why are these not in place here?
Dorothy, Loughborough, UK
Dr Roy: Yes, Australia does have a government agency dedicated to tackling the issue of invasive alien species. Similarly, New Zealand, has very strong biosecurity systems in place. Most of the rest of the world is trying to catch up. Great Britain is making progress through a co-ordinating body to tackle the issue across all parties concerned - including a number of government department, agencies, research organisations and non-governmental organisations.
Starlings from the UK are causing problems in other countries
Over the last few years we have heard lots of different reports referring to foreign species invading the UK. Are there any native UK species that have taken over and become a danger to local flora and/or fauna in other parts of the world?
Sam, Camberley, Surrey
Dr Roy: Most certainly. There are countless examples of UK native species becoming problems in other parts of the world. For example, stoats are problem in New Zealand predating on ground, and tree-nesting birds. The common starling has been introduced to South Africa, Australia, New Zealand and North America, with large economic costs to fruit and grain crops.
Can you tell me why do we have so many alien species in the UK and why don't we try to stop them at source instead of letting them into the country and then having to pay millions of pounds in trying to get rid of them?
Terry Cross, Norwich
Dr Roy: Being an island means that many species only arrive at our shores with the assistance of man. We import a large number of goods which act as a vector of new species arriving.
Signal crayfish are difficult to eradicate
I live in Cheshunt, just by the New River. It has been overrun by American signal crayfish that are wiping out everything in their path. The obvious question has to be why isn't anything done about it?
Alec Cap, Cheshunt
Dr Roy: The Environment Agency has looked into the feasibility of eradicating the American signal crayfish. There are many features of its biology which make it a difficult species to control and so-far no successful approach has been found. Trapping only catches a small proportion of the population and numbers can quickly build up.
Hitting the UK? Invading the UK? What's the big deal? Surely it's just evolution at work - why should we be concerned?
Carl, Jarrow, UK
Dr Roy: Over evolutionary history timescales, species evolve in different parts of the world because they are separated from each other, for example, by physical barriers such as oceans, mountain ranges, deserts etc. Man is breaking down these barriers by the mass movement of species around the globe. If you are concerned about preserving the world's natural resources then this mixing is a '"big deal". Putting conservation concerns aside, many species introduced into the UK have a major economic cost or human-health impact.