Invasive Species Mammals

Two species of mammal have been identified as 'High risk' in Essex, muntjac deer and mink.

Muntjac deer - Muntiacus reevesi

Description and identification

Muntjac are small, approximately 50 cm high at the shoulder and chestnut brown with an arched back. They are usually solitary. The male has small antlers and canine 'tusks'. Bucks have a distinctive bark and hold their tail erect when alarmed.

Problems caused
Muntjac are selective browers of shrubs and woodland ground flora. This leads to loss of flora such as oxlip, bluebell and dog's mercury. Loss of shrubs results in less habitat for nightingales, some butterflies and for dormice. Damage to shrubs can be seen from ground level up to about 60 cm high, the level at which the deer browse. Females are fertile for 12 months of the year, enabling the populations to increase rapidly. Biodiversity is reduced by the presence of many muntjac.

Muntjac, native to China, have now spread to cover most of southern and eastern England. They originally escaped from Woburn Abbey in Bedfordshire in the nineteenth century. Most woodland and thick scrub now harbour the species, which is continuing to increase its range at around 8% per year. Numbers are predicted to increase by 30% by 2015.

Control measures
For small areas, vegetation can be protected from muntjac by fencing. However, the deer are quick to find rabbit burrows under the fences and can use these to gain access to the plants. The best control is culling by professional marksmen. Muntjac, if captured, cannot legally be released into the wild, as they are officially designated as a pest species. To be effective, culling should be carried out by deer management groups.

Action that the public can take
a) Tell relevant landowners if there is an issue.
b) Inform The Deer Initiative of any road casualities.
c) Support local initiatives to control muntjac, such as fencing and culling.

Sources of information and references
Cooke, A.S. (2006) Monitoring Muntjac deer Muntiacus reevesi and their impacts in Monk's Wood National Nature Reserve. English Nature Research Report 681.
Rackham, O. (2006) Woodlands. Harper Collins
The Deer Initiative (Eastern Region). David Hooton, PO Box 465, Bury St Edmunds, Suffolk IP28 6XD. email:

Mink (North American mink)  Mustela vison

Description and identification

The North American mink is a member of the stoat family. They are usually a dark brown chocolate colour, appearing almost black when wet, but most have a small white patch under the chin. More exotic colours, such as silver grey, are also possible, but these are much less common.

Males are typically a maximum of 60cm from the tip of the nose to the end of the tail and weigh approximately 1kg. Females are smaller and usually weigh less than 750g. Large mink are often mistaken for otters, however fully grown otters are twice the length, can weigh ten times as much and are much more robust looking animals.

Key features
Dark brown fur
White chin patch
45-60 cm body and tail (ferret sized)
Elongated stoat-like shape

Problems caused

In their native North America the chief prey item is muskrat, an animal similar in size and habits to our native water vole. Female mink are small enough to enter the burrow systems of water voles, and in many cases have wiped out an entire colony in one season to feed their young. Mink also take ground nesting birds, young chicks, eggs of water fowl and sometimes even adult birds as well as other small mammals, fish, lizards and reptiles. Their impact on native fauna along water courses and lakes can be dramatic, with water voles and water fowl often the first to disappear.


Mink are native to North America but were brought to the UK in the late 1920s to farm for fur. Escapes were regularly recorded and the first verified record of feral breeding mink was on the River Teign in 1956. A wave of releases in the 1960s led to further colonisation and by the early 1970s it was believed that they were present in every county. Since the late 1980s numbers and distribution have increased dramatically within all counties, but these are now the result of feral breeding pairs as mink farming is no longer licensed in the UK. In Essex, mink have been recorded in most catchments although they remain largely absent from the coastal grazing marshes, with just occasional transient individuals appearing sporadically.

Control measures

Catchment scale trapping is now considered to be the only viable method of control. The newly developed Game Conservancy Trust mink rafts are used to track the presence of mink on water courses. Where mink are discovered, live capture traps are installed allowing any non target species, such as otters or water voles, to be released unharmed. Captured mink should be humanely dispatched on site by a competent individual, with a rifle or pistol. Mink must not be drowned, and there are no approved methods of killing them by gas or injection. Hunting mink with hounds is also currently illegal.

Actions that the public can take

a)     Inform Essex Wildlife Trust of any possible sightings of mink. Essex has a dedicated Water for Wildlife Officer who is responsible for water vole recovery projects, including liaising with the Eastern Region Mink Control Group.

b)     Report any rapid declines in native species such as water voles, moorhens, mallards and coots as these are often indicative of the arrival of mink on a water course.


Darren Tansley, Water for Wildlife Officer

Essex Wildlife Trust -

Water for Wildlife -

The Mammal Society -