War on alien predators
November 2008
A project officer explains to a group of volunteers how to use a mink raft.
by Rosalind Bryce
A North-East project is using science to inform conservation in a bid to protect native species from an invasive predator, the American mink.
The inexorable spread of the grey squirrel in this area is a visible example of a non-native gradually displacing a native; the charismatic red squirrel in this case. Removing established invaders is an onerous task. Those who have tried to halt the spread of rhododendrons can testify to the huge effort required.
A more elusive non-native that is bad news is the American mink. This unwelcome member of the mustelid family was brought over to the UK to stock fur farms. The practice is now banned, but releases and escapes during the fur-farming era means that mink are now established across Britain, as they are over large areas of Europe, South America and Iceland.
Attempts to eliminate invasive species have often been confined to island ecosystems where the chances of reinvasion are very low. However, it is essential to tackle the problem on mainland soil, where the highest numbers of vulnerable native species need protection.
American mink tend to forage in aquatic habitat, in a linear fashion along coastlines and river systems. They consume a wide range of prey including rabbits, ground-nesting birds, fish, eggs, rodents and amphibians. They have caused havoc in tern colonies in the Western Isles where an eradication programme has begun.
Water vole under threat
The species perhaps most threatened by American mink – and the most rapidly declining mammal in the UK – is the water vole, Arvicola terrestris. The water vole is a large rodent that exists in colonies in riparian habitat. It lives along slow-flowing watercourses, usually where there is good vegetative cover and banks of peat or clay where it creates extensive burrow systems. Signs of their presence include obvious burrow entrances, networks of runways through vegetation, piles of clipped plants in feeding areas and latrines of droppings by the water’s edge. Water voles in Scotland are often jet black, making them distinctive from their brown-furred English counterparts.
Water voles used to be abundant across the UK, but they have suffered a massive 90 percent decline in recent decades. In some areas they are a victim of man’s degradation of their habitat, but the primary agent of decline here is predation by American mink. Colonies of water voles tend to be small, as they occupy discrete patches of suitable habitat. They are good dispersers and travel between colonies in search of mates and food.
Unfortunately mink are also capable of covering large distances; rapidly moving along rivers and burns, they wipe out any water vole colonies they encounter.
The North-East, however, is an extremely important place for this fascinating creature. Here colonies of water voles thrive in upland areas. In the Cairngorms there are still strongholds of them high in the headwaters of river systems. They are found in rich flushes of grasses and reeds along small mountain burns, surrounded by what is fairly hostile habitat. There are fewer mink in such areas as there is less prey to support them.
A decade of research has been carried out on the ecology of these water voles by scientists at the University of Aberdeen, led by Prof. Xavier Lambin. These scientists are fostering partnerships with conservationists, land managers, sporting estates and communities.
The Cairngorms and the Aberdeenshire Water Vole Conservation Projects constitute an ambitious attempt to control American mink on a large scale to protect water vole and other vulnerable species. The Cairngorms project alone spans an area of 2,123 square miles and is the largest mink control effort on the UK mainland. Building on the lessons learned and success of the North-East Scotland project, the two neighbouring projects now cover an area of around 2,700 square miles.
The objective is to create large mink-free areas. Mink control is in the interests of many and has been carried out in certain areas for many years. It is a priority for grouse estates, keen to protect eggs and chicks from predation. Trapping is also carried out by fishing bailiffs and ghillies concerned about the impact of the predator on salmon, another important industry. However, unless the species can be controlled over a large enough area, we are faced with an interminable fight against animals recolonising from uncontrolled areas.
It is important to take both a short term and long term view. The resources available for conservation projects eventually run out and we must develop a strategy that can be implemented by local stakeholders into the future.
To increase the chances of long-term success, the projects use an ‘adaptive management’ approach. The initial clue that mink were a problem in the North-East was the decline of water voles, but there was little knowledge of the mink – for example the population size, breeding habitat, and distances travelled by dispersing juveniles were unknown. We need to know these things to understand what level of control is necessary to bring about an overall population decline.
Mink rafts
There is significant scientific research to ascertain the effectiveness of mink control. Information about the mink population in the area is being constantly gathered and analysed, allowing the scientists to alter strategies as the work progresses; hence the term ‘adaptive management’. One element of the research involves genetic finger printing technology to determine how far siblings or parents and offspring disperse from one another. It was found that mink can move great distances between river catchments, with members of some families being found 100 yards apart!
Mink rafts, designed by the Game & Wildlife Conservation Trust, help to monitor progress. These floating wooden structures contain a clay tracking cartridge within a tunnel. They are anchored to the banks of rivers, burns, lochs and ponds.
Mink, being inquisitive creatures, investigate the rafts and move through the tunnels leaving their prints on the clay. They can usually be caught within a few days of their presence being confirmed, in traps placed on the rafts. The traps are then removed and the rafts returned to monitoring mode until prints reappear.
This technique has many benefits over traditional trapping. It is time efficient as the rafts need only be checked weekly or fortnightly, and only daily while traps are operating. Effort is targeted towards areas where mink are known to be.
The long term goal of clearing mink from such large areas depends upon the involvement of communities, and there are now over 100 volunteers from diverse backgrounds monitoring rafts in North-East Scotland. Many people – such as gamekeepers, fishing bailiffs and ghillies – use the rafts as part of their jobs, whilst others volunteer out of an interest in conservation, and enjoy checking rafts while out on a regular walk. Schools participating in the ‘Ecoschools’ programme monitor nearby rafts and report results to the project. Volunteers are supported by four area officers who provide materials, advice and feedback on progress.
The story so far has been one of success. The Ythan river catchment in Aberdeenshire is now believed to be clear of mink. Efforts are being expanded downstream in other major rivers, including the Dee and the Spey, safeguarding water vole populations in the headwaters. There has been no evidence of mink upstream of Aboyne on the Dee this year and they have been substantially reduced on the Spey.
There is evidence of water voles recolonising former haunts in some areas, and we hope to see their reappearance along many of our rivers.
If you live in the vicinity of the Cairngorms or Aberdeenshire and would like to participate in the project by monitoring a raft in your area, please contact Ros Bryce on 07899 060 569. Further information is available on the project website www.watervolescotland.org
Dr Rosalind Bryce of the University of Aberdeen is project manager of the Cairngorms Water Vole Conservation Project. She has worked extensively in the Scottish uplands carrying out research on water vole ecology and now enjoys working with a diverse range of volunteers in the Cairngorms.
This is an article from the November 2008 edition of Leopard Magazine. To read much more like this every month, see our subscription details.