Mammals not native to Europe that invaded the continent are posing a big threat to the native biodiversity, finds a new study.

Researchers from the Institute for Environmental Protection and Research in Rome, Italy, along with  Riccardo Scalera of the Invasive Species Specialist group of the International Union for the Conservation of Nature (IUCN) in Valby, Denmark, studied the invasion of alien species since the Neolithic age.

Experts continued their research as part of the previous database from Delivering Alien Invasive Species Inventories for Europe (DAISIE) that took the details from 1500s only till 2007. The new study takes into account the years before 1500s, that is the Neolithic age, and years after 2007. They remodeled the pattern of alien mammals' expansion and invasion by considering various data on introductions of new species, extinctions and eradications.

They found that there has been a consistent increase in the arrival of invasive species in Europe with no sign of reduction. Besides, the researchers also revealed that the invasive species are posing a big threat to the native species, mainly the threatened species.

The new report revealed that 740 mammals were introduced corresponding to 117 species that were introduced since the Neolithic times in at least 52 European countries. In all 740 records, experts were successfully able to reconstruct the date on introduction for 635 cases. They found that the wild boar were introduced into Sicily, an island in the Mediterranean Sea, during the Neolithic era.

Of the 117, eight species are said to have an uncertain status. With the remaining 109 mammals, 71 mammals are not native to the European continent, while  the remaining 38 are native to at least one European region that have invaded other European countries.

The report stated that there are 22.2 percent of the alien mammal species in Europe (47 invasive mammals from 71 excluding species that are either eradicated or extinct). While mammal invasions have taken place since the Neolithic times, significant increase in the rate of invasions is said to have started since the beginning of the 20th century.

Alien mammals like racoons, muskrat, Siberian chipmunk, coypu American mink, beavers and wild boar have invaded the European continent in large extents.

Experts found that 68.2 percent of the alien species are either harmful to health or cause enivronmental damage. While 55.3 percent of these are known to cause ecological impacts, 62.3 percent have been found to caused harmful impact on livelihood like damaging crops and livestock.

According to a report in BBC, UK is one of the three European nations where there is an increased rate of invasion with at least 18 alien mammals invading the region.

The findings of the study are published in the journal Integrative Zoology.