Water vole research conducted at Loch Leven National Nature Reserve, in Perthshire

The water vole is the UK’s most endangered, and most rapidly declining, mammal. They have a small body size of just 35 cm and secretive nature, so sightings are very rare. They inhabit watercourses throughout the country in a series of colonies, between which the voles move through dispersal and colonisation. This movement maintains a larger population over a wider area, called a metapopulation.

 

Yet with numbers falling dramatically, this system is failing as colonies of water voles are being wiped out, resulting in increased isolation of the remaining individuals. With no other colonies close enough to allow an influx of new individuals, water voles have now disappeared from approximately 95% of the sites they occupied early in the 20th Century. This was around the time when numbers began to decline. Land use changes, including riverbank management and drainage, have caused the loss and fragmentation of riparian and wetland habitat.

Yet it has been within the past 50 years that the most dramatic decline has occurred, caused by the introduction of the American mink to breed on fur farms. Escapes were inevitable, and throughout the 1950s and 1960s, mink were breeding and thriving in the wild. Against these fierce predators, water voles have no escape. While their usual anti-predator strategies of diving into water and the soft mud underneath, or into their burrows, are effective against native avian predators, American mink are fast runners and highly effective swimmers. Female mink are even slim enough to fit through narrow burrow entrances, leaving water voles with no place to hide.

 

The rapid spread of American mink coincided with a rapid decline of water voles and, combined with habitat change, resulted in just 16% of sites within the UK being occupied by water voles in 1996, when a nationwide survey took place. Earlier in 2006, a study was conducted at Loch Leven National Nature Reserve, in Perthshire in Scotland. This is an extremely important site for local wildlife as it is Scotland’s largest lowland loch, covering 1,824 hectares.

Its eutrophic (nutrient rich) water allows plentiful plant and invertebrate life, which in turn supports numerous fish species including brown trout and pike. It is also the home of thousands of birds including cormorants, osprey and 10% of the world’s population of pink-footed geese. This has given Loch Leven the designations not only of a National Nature Reserve, but also a Site of Special Scientific Importance, a Special Protection Area and a RAMSAR wetland site of international importance. However, this site is not a stronghold for water voles. Even though there is an abundance of ditches and burns flowing into the loch, surveys to locate water vole field signs such as burrows and latrine sites revealed that numbers here are just as low, with only around 0.3 individuals per 100 metres. The footprints of American mink were also discovered, so can be confidently stated as the cause of their downfall at Loch Leven.

 

One ray of hope is that, even with the presence of mink, water voles have not been completely eradicated. Scientists now recognise the positive effect that another species of riparian mammal has on water voles. Otter are also present at Loch Leven, and previous studies have shown that in areas where otter are thriving, mink are often in decline. Otters, with a larger, more streamlined body, out-compete mink for aquatic prey, such as fish and crustaceans. When alternative terrestrial prey is limited, mink have been forced out of areas within as little as four years.

Trapping has traditionally been considered the best way to control mink, but to have much impact must be extensive and consistent throughout the year. However, as mink are so virulent and can travel over great distances, trapping alone has not always been effective. Otters are a more natural solution to increasing water vole numbers, and encouraging their growth and spread into new areas will also help reverse this native species’ own recent decline.

Regardless of the presence or absence of American mink, without adequate habitat, water vole recovery is not possible. More surveys are required to identify the specific features preferred by water voles which will allow more suitable habitats to be managed or created.

At Loch Leven, for example, it has been shown that narrow and shallow watercourses are preferred by water voles due to the lower risk of burrows becoming flooded; and flowing water is preferred to static water as it brings nutrients and promotes dense vegetation growth used for food and cover. It was also shown that cattle grazing is detrimental as trampling leaves banks unsuitable for their extensive burrow systems and cattle also limit food availability, a serious issue as water voles consume approximately 80% of their own body weight daily. Future management at Loch Leven will therefore include fencing off ditches to reduce disturbance to water voles.

This is just one example of how surveys can provide a detailed picture of the status of the water vole in a particular area. The more sightings of this mammal, or of the field signs they leave behind, that are reported, the more knowledge scientists will have to identify the aspects of habitats that need to be modified in order to maximise population growth. With a greater the awareness of local areas where this extremely endangered animal still exists, there is a greater chance of the survival of the water vole.

This paper was written by Anna Taylor, to contact Anna, please click here.