Image copyright PA Image caption Brown rats opportunistic behaviour helped the species to spread quickly

Brown rats are among the most invasive mammals in Europe, according to a wide-ranging assessment.

Swiss researchers found that the creatures, along with sika deer and muskrats, were having the greatest ecological and economic impact.

The team considered a range of measurements, including the threats to native species and how widely the alien species had become established.

The findings have been published in the journal Conservation Biology.

The scientists said they had developed a scoring system that compared the impact of non-native species across the taxonomic group of mammals.

"This scoring can be used to identify the most harmful alien species, so that conservation measures to ameliorate their negative effects can be prioritised," they wrote.

"Alien invasive species are a large threat to biodiversity and the economic damage that they cause exceeds 5% of the global gross product."

Scoring impacts

To develop the "scorecard", the researchers looked at impact reports for all of the known invasive mammals found in Europe, before classifying the recorded impacts as either environmental or economic.

A better understanding of the relationship between impact scores and species traits may provide a novel method with which to predict the potential impact of a new alien mammal species Research team writing in Conservation Biology

Within the two classifications, there were a number of sub-divisions of impacts, including predation, hybridisation, transmission of diseases, and the species' impact on agriculture.

For each criterion, the researchers awarded "impact scores", which ranged from zero (no known impact) to five (maximum possible impact) to reach a "potential impact" score.

This was then multiplied by the percentage of area within Europe that was occupied by the invasive species to give the researchers an "actual impact" score.

The team applied the scoring system to 34 "true alien" mammal species, all of which had a native range outside of Europe. Brown rats, sika deer and muskrats had the highest overall scores.

The brown rat (Rattus norvegicus) is found across Europe in all habitats except high mountain ranges. It was believed to have been introduced in the 18th Century as maritime traffic increased.

Once introduced into an area, there is a recorded fall in other small rodent species, as well as marine and land bird species. The main economic impact is the result of damaged crops and food stores, and damage to people's homes.

According to the EU 's database of alien species, sika deer are described "a serious forest pest, causing significant damage to broadleaved and conifer plantations".

The deer "ring" trees, in which they strip bark around the base of trees, causing them to die. There have also been recorded cases of the animals carrying bovine and avian TB.

Muskrats are stocky aquatic rodents, weighing up to 1.8kg. Once imported for fur farming, they are found throughout continental Europe and cause extensive damage to aquatic vegetation habitats, crops, irrigation systems and dams.

The researchers said they decided to develop the scorecard because it had been proving difficult to get an overall view of what species were causing the most harm to native habitats.

"It is obviously challenging to compare the damage caused by different species, such as the carnivorous American mink and the herbivorous sika deer," they observed.

"To overcome these obstacles, a general system of impact categories was needed, which allowed scoring and comparison of all potentially relevant types of environmental and economic impacts caused by alien species.

"A better understanding of the relationship between impact scores and species traits may provide a novel method with which to predict the potential impact of a new alien mammal species."

The team added that the system could be applied to other taxonomic classes of species, such as birds or fish, and allow resources to be targeted towards the species that cause the most damage.