By Becky Oskin, LiveScience Contributor:

Whether they escaped from zoos or accompanied migrating nomads, invasive species from giant Himalayan bats and porcupines to house mice now account for 22 percent of mammals in Europe, a new study finds.

If bats and marine mammals are excluded, the researchers found aliens make up some 28 percent of terrestrial mammals in Europe.

Yet despite growing awareness of the economic and ecologic costs of invasive species, the number of alien mammals across Europe continues to rise, the researchers report.

"These findings confirm that invasions are still increasing, with no sign of a saturation effect," lead author Piero Genovesi, a senior scientist at the Institute for Environmental Protection and Research in Rome, Italy, said in an email interview. The research is detailed in the September issue of the journal Integrative Zoology.

The study will help conservation agencies prevent new invaders, Genovesi told LiveScience. "Introductions of animals carried by people, sometimes accidentally, sometimes intentionally, are a severe and growing threat to biodiversity, requiring urgent action, he said. "The data can be used to detect the main pathways of introductions."

Reports of aliens

As predators, mammals have played an out-size role in past extinctions — for instance, rats have caused 40-60 percent of all seabird and reptile extinctions. "Mammals are one of the most — if not the most — harmful group of invasives," said Genovesi, chair of the Invasive Species Specialist Group in Valby, Denmark, part of the International Union for Conservation of Nature.

The researchers documented 117 mammal species imported into European countries since the Neolithic, or about 10,000 years ago. Some came from faraway continents, while others were transported from one part of Europe to another. With some reports ambiguous, and some colonies extinct, the current count is 85 invasive mammal species now in Europe, many of them wreaking economic and ecologic havoc. [In Photos: The Peskiest Alien Mammals]

Mammals that live in close quarters with humans were among the earliest invaders, including the house mouse, dog and cat. Genovesi and his colleagues also found a few quirky cast-offs: the escape of giant fruit bats from an enclosure in the Canary Islands and Himalayan porcupines from a zoo in the UK. And apparently, Beluga whales were introduced to the Black Sea in the 1990s.

Islands held the most invasions of any European regions, and included the United Kingdom, Sardinia and Corsica. But the United Kingdom is also the country with the highest number of extinct alien species, as a result of either eradication programs or failures in early attempts to deliberately introduce species, the study found.

How they wreak havoc

Domestic cats, feral goats and the American mink are the worst invaders. Cats are highly effective hunters, threatening 16 endangered species, four of them critical. Feral goats trample habitats on islands, imperiling 15 endangered native species. The American mink is both a predator and a habitat competitor, impacting 47 native species.

Introduced mammals also spread diseases to native species or to humans. The twee Siberian chipmunk, brought to France as a pet during the 1960s, now plays host to ticks that carry Lyme disease.

The European Union is developing universal legislation on invasive species and individual countries have banned import of offenders like the American mink. Controlling invasive species isn't cheap: European countries spend at least 12 billion Euros a year, and mammals account for 1.1 billion of those costs, according to a 2009 report prepared for the Institute for European Environmental Policy. The United States spends at least $1.2 billion per year fighting invasives.

But despite the bans, invasive species continue to spread, said Riccardo Scalera, a study co-author and conservation biologist with the Invasive Species Specialist Group. For instance, a raccoon — listed as one of the world's 100 worst invasive species — was first sighted in Ireland in 2011. The demand for exotic animals as pets or as hunting prey plays a key role, along with escapes and deliberate releases, the researchers report. For example, in 2010, 5,000 American mink were released from a fur farm by animal rights activists in Donegal, Ireland. In 2009, American red squirrels were found in Denmark, likely escapees from a commercial pet-breeding facility.

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