Non-native mammals, including grey squirrels, North American beavers and chipmunks, are invading Europe at a rate higher than previously thought, a new study has revealed.

The report, written by scientists at the International Union for Conservation of Nature (IUCN) and published in the journal Integrative Zoology, also confirms the severe impact of the alien invaders, which directly threaten a significant number of native species, including several that are at high risk of extinction.

It found that 71 mammals alien to Europe have been introduced to the continent over the past 5,000 years, and that 38 mammals native to at least one European region have been introduced to other European regions.

The study, the most extensive yet made on the subject, states: Results confirm that mammal introductions in Europe have always occurred since Neolithic times, but a marked increase in the rate of invasions has been noted since the beginning of the 20th century.

Some 27 non-native mammals listed in the report were found to have damaging direct effects on native European species.

The most damaging of all the invaders is the American mink, which affects 47 native species, including six that are classified as ?threatened? by the IUCN.

Dr Alastair Leake, director of policy at the Game and Wildlife Conservation Trust, told Shooting Times that a warming climate made it likely that non-native species would continue to become naturalised within the UK.

He said: ?Some will have made their own way here and, if benign, may well become accepted as new elements of our fauna and flora. Some, however, might create problems for our native species, in the way that grey squirrels, signal crayfish or mink have already done.?

?Too often, policies and actions have only been developed years after an invasive alien has been identified as a problem, and this action is then often too late to prevent a crisis.?

?We must not forget that wildlife carries on reproducing while people sit around talking ? decisive action at an early stage is crucial.?

The most recent invasions noted by the report are the introductions of the Canadian beaver to Belgium and Luxembourg in 2010; the spreading of the raccoon to Scandinavia, recorded for the first time in Sweden in 2010; and the illegal release of the same species in Ireland in 2011.

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