From eating crops to threatening native wildlife, a series of "invasive" mammals introduced to the UK over the centuries are damaging the countryside, a report said yesterday.
Some are well-known "villains", such as the American mink or muntjac deer, while others including rabbits and rats have been here so long that they are regarded as natural.
Two of our fastest declining native species, the red squirrel and the water vole, are both threatened by mammals which were introduced by humans in the last two centuries.
The American mink, released from fur farms since the 1920s, eats water voles, which have seen numbers decline by 90 per cent in recent years.
And the grey squirrel, which was brought to this country from the US in the 19th century, out-competes its smaller red cousin for food and habitat and carries the squirrelpox virus which is fatal to the native species.
Such is the impact of the grey squirrel, which also damages trees and forestry plantations, that it features on the global list of the "worst 100" invasive species around the world.
Joining the grey squirrel among the top 100 invasive species globally are rabbits and house mice, both of which are non-native species and were introduced to the UK around 1,000 years ago.
Rabbits, which have been introduced more recently to places such as Australia with devastating effects, are a major agricultural pest in the UK, as well as carrying disease.
Other mammals which can cause problems in the countryside range from muntjac deer to the red-necked wallaby, which could do potential damage to capercaillie on Loch Lomond island, the study from the Peoples' Trust for Endangered Species said.
Impacts include from predating on native species, competing for food and habitat, carrying diseases which can affect wildlife, livestock and humans, breeding with species to produce "hybrids" - as in the case of red and sika deer - or altering the landscape and damaging crops or woodlands.
And a changing climate, along with the increase in global trade, could facilitate more species establishing themselves here, the report said.
In total there are 14 invasive mammals which have an impact on UK wildlife, landscape and agriculture, most of which were introduced by humans - with arrivals in the countryside dating as far back as the Romans or occurring more recently as zoo escapes in the 20th century.
Some introduced species are now thought of as "natural" in the UK, including fallow deer and brown hares.
The hare, brought to this country by the Romans, is now on our list of conservation priorities under the UK Biodiversity Action Plan, and it is illegal to hunt it with dogs.
The UK now has around 10 per cent of the world's population of Chinese water deer, which have minimal impact on the landscape in this country and are considered "vulnerable" to extinction globally.
And while it is mostly seen as a pest, the rabbit can have conservation benefits in areas where its grazing benefits the landscape, according to the report produced for the Trust by Prof. David Macdonald and Dr Dawn Burnham from the University of Oxford Wildlife Conservation Research Unit.
Controlling invasive species, whether it is attempts to stop the spread grey squirrels or hedgehogs introduced to Scottish islands where they eat seabird eggs, can be controversial.
The People's Trust's chief executive officer Jill Nelson said: "Our campaign to conserve Britain's native mammals is rooted in finding more about each animal's behaviour in response to the various threats they face and translating that knowledge into practical conservation action.
"The incursion of non-native mammals into Britain is a major threat to water voles and red squirrels, two of our fastest declining species, and resolving the complex issues around how we deal with this problem is a vital component in preventing their extinction."