Water vole reaching up for blackberries.

Since the 1960s water vole numbers have crashed due to a perfect storm of threats © Mark Bridger/Shutterstock

Water voles return to Exmoor for the first time in 30 years

Water voles populations have declined dramatically in recent times by a swirling combination of threats. The National Trust is now trying to reverse that trend on one of their estates in southern England.  

After over three decades of silence, the streams and rivers of a Somerset national park will once again be filled with the gentle 'plopping' sound of water voles taking the plunge.

The National Trust has begun a project to reintroduce the rotund rodent to the waterways of Holnicote Estate, found within Exmoor National Park. They are releasing 150 water voles in the hope of establishing a breeding colony of the threatened creatures, which have suffered heavily in the last few decades.

It is thought that water voles have been removed from 94% of their natural range in the UK since the 1960s, when it is estimated there were as many as eight million of them. Even in just the last decade they have lost 30% of their habitat. 

In fact, the water vole has the dubious title of the fastest declining mammal in the UK, as numbers continue to tumble despite conservation efforts. 

Water vole swimming.

Water voles live in the burrows they dig into the banks of streams and rivers © Erni/Shutterstock

Alex Raeder, the National Trust's South West Conservation Manager, says, 'I remember being enchanted by these creatures as a child, and hugely welcome their return. They were once a vital part of the Holnicote ecosystem, and could be again. 

'This ambitious project not only brings back to its rightful home a much-loved small animal, which sadly became locally extinct due to human activity, but also adds to the whole wealth of wildlife and enjoyment of this wild and stunning estate.'

The perfect storm

The voles are thought to be disappearing from the nation's countryside for a number of reasons, all combining to devastating effect.  

One major issue is the simple fact that they have fewer places to live. As floodplains are built upon and riverside vegetation removed, they are being pushed out of their key areas. Poor farming practices can also lead to river banks being trampled and water vole homes destroyed.

The introduction of American mink into the UK is thought to be one of the biggest causes for the decline of the voles. Initially brought here in the 1920s to be bred for fur, the predators quickly began escaping from the fur farms and were established and breeding in the countryside by the 1950s.   

As the voles had never encountered American mink before, their natural defences against such predators were useless. The mink are incredibly efficient, slender enough to fit into the voles burrows the animals can work their way down the banks of rivers wiping out entire colonies. In an attempt to help the voles, mink are frequently controlled along many important waterways.

There is also a concern that agricultural run-off in the form of pesticides and farm waste may also be playing a role in the decline as they pollute the rivers and streams. This has been addressed in many regions, and the result has been the return of otters, which outcompete the American mink and so may inadvertently help the water voles recover.

All of these threats feed into another: population fragmentation. As habitat is destroyed and colonies ruined, individual groups are becoming more isolated. This only hastens their decline, as they become vulnerable to loss of genetic variation, so populations are less able to adapt to any further changes. 

An American mink sitting in grass.

After escaping from fur farms, American mink have decimated the native water voles © Holly Kuchera/Shutterstock

Continental colonisers

This is not the first time that water voles have been pushed to the brink of extinction within the UK.

A genetic study of water voles carried out by Museum researchers previously found that there are two distinct lineages of the creatures living in Great Britain.

It turns out that the voles living in Scotland are markedly different genetically from those in England and Wales. This suggests that they came from different colonisation events, and hints at environmental changes that have occurred over the last few thousand years.

It appears that the original Celtic water voles that once inhabited all of the British Isles were pushed north at some point during the last ice age. Then, while Britain was still attached to the rest of Europe between 12,000 and 8,000 years ago, European voles made the crossing to England.

This is why there are now two distinct populations in the UK, with the southern voles being more closely related to those on the continent.

It also raises interesting questions about the rodent's conservation, particularly when it comes to reintroduction.       

The voles being released at this particular site are from a British breeding programme. But this study raises the question of whether more consideration should be taken to introduce European genes into the population in an attempt to maintain the genetic diversity that historically existed in the English and Welsh vole populations.

The National Trust will continue to monitor the voles they release, listening for their plopping and checking for feeding signs, in the hope that they will eventually establish a breeding population on the estate.