Unwanted Americans – an invasive immigrant’s tale

Water voles are believed to have lost 90-95% of their historic numbers in the UK.  Although they may seem to be flourishing if you see them, that is only because it is a local group doing well.  These small groups of voles, seemingly doing well one year, can blink out of existence.  Why would a local group disappear even if they have great habitat, with plenty of lush grasses and other vegetation to munch on?  Maybe they just moved out to find an unrelated mate. But there may be another more dire reason for their disappearance.

Enter the Unwanted American: the American mink (Neovision vison).  Imported in the 1930-50’s, these mustelids were farmed for their fur.  “Unwanted” is a slight misnomer for then, but now, now the fur farms have been shuttered and in the decades from their inception, mink escaped and established a population in the wilds.  Mink are super good at what they do, and that happens to be reproducing and hunting.  The mink likes the same type of habitat that water voles like.  This habitat overlap makes for bad neighbors.  The mink slip right into the vole burrows and carry off vole after vole to the waiting teeth of its young.


An American Mink. https://www.flickr.com/photos/ericbegin/ (CC BY-NC-ND 2.0)



Mink aren’t the only thing that the vole gets predated by.  Other predators include foxes, birds of prey, and this guy: A stoat!  https://www.flickr.com/photos/derek_p/ (CC BY-NC-ND 2.0)

Research has shown that water voles are disappearing wherever these mink are cropping up (and they impact important economic species like salmon).  So far the worst hit areas have been the lowland populations in the southern half of the UK, but the mink is moving north.  Right now the Assynt area seems to be free of them, but if a couple of mink wander in the right direction, that may change.  There are currently efforts underway to trap and remove the mink but the best way to do it is to keep them from establishing a presence in the first place.  Monitoring for early “pioneers” is the best way to keep them from creating permanent settlements and drive out the natives (they eat more than just water voles too).

There are several less dramatic, but equally bad-ending reasons for the disappearance of your local water vole group too. Sometimes, when an animal lives in small groups they can have something happen out-of-the-blue that could cause a local extinction.  These could be large storms or fires or other natural disasters that kills or removes the animals homes.  Other times, it can be something more subtle.  What if one year, all the female voles gave birth to only female offspring?  There would be no one to mate with!  The smaller the population, the more likely that something like these freak events could have serious consequences for a population of animals.

Water voles give birth spring to summer in the year in the Scottish Highlands. The young of the year eventually decide to try and leave their natal (birth) patch and try to find a new nice little patch of earth to call home.  They have to travel across terrain that is hard for large animals like Ph.D. students to walk around in.  So if it can’t find a new place to call home quickly (or at least a vole equivalent of a hotel), it may never end up finding a home.  When there are fewer good spots left because humans built a house or put barriers up on streamsides, it can make it hard for a vole with a limited number of options to survive.  It can create a spiral that can lead to large areas going completely extinct.


Although this is a picture of a stream that lost it’s natural banks (courtesy of @mtbogan).  No animal is gonna burrow through those concrete banks.

It’s not all doom and gloom though.  Because of the hard work of many dedicated people, there are many local conservation efforts to reverse habitat and invasive species damage in the UK.  Read more about them here:




And more about mink in Scotland in general:

Article from the Scotsman: http://www.scotsman.com/news/invasive-mink-killing-native-wildlife-in-highlands-1-3482305







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