Are the otter and polecat combining to reduce mink numbers?
PUBLISHED: 15:37 31 March 2019 | UPDATED: 15:47 31 March 2019
The recent re-colonisation of East Anglia by polecats combined with a growing population of otters could be causing a decline in the number of mink in the region.
Essex Wildlife Trust’s river catchment officer Darren Tansley, who co-authored the most recent edition of the book Mammals of Essex, has come up with the theory after receiving a number of reports that mink numbers are decreasing.
American mink is a non-native species of mustelid (the same family as stoats and weasels) that was bred in fur farms in the UK from the 1920s onwards and, according to the Game and Wildlife Conservation Trust, began to establish in the wild after the animals escaped or were deliberately released into the countryside.
The semi-aquatic predator is regarded as an alien pest in the UK - a ruthless killer of birds and widely considered responsible for decimating the native water vole population that has not co-evolved with the mink and has little defence against it.
Mr Tansley says at the time the mink population was establishing itself in the East of England it had no native carnivorous rivals who might compete with it for territory and food, and check its expansion.
“Mink started to populate when otters were in decline and polecats had been wiped out - mink had a free rein,” he said.
But even after otters started to appear in greater numbers in East Anglia during the 1990s and 2000s, Mr Tansley said there was no noticeable drop in mink numbers, as some had predicted.
As part of a project to re-introduce water voles to the River Colne in north Essex in 2009, 70 mink were caught in traps in an attempt to make the waterway mink-free.
Mr Tansley said; “There have been otters in the Colne to a varying degree since 1991 and by 2009 there was a good breeding population of otter there but they seemed to have had little impact on mink numbers. The finding flew in the face of the theory that otters might have an impact on mink.”
But now another potential threat to the mink, the polecat, is back in East Anglia after decades of absence, having expanded eastward from the west of Britain over the past half century.
Mr Tansley said polecats first started to appear in the northwest of Essex at the millennium with sightings near Saffron Walden, Braintree, Kelvedon and Coggleshall.
He continued: “They have colonised Essex north of Chelmsford, having hopped over from Hertfordshire. A lot of the sightings of polecats in Suffolk tend to be along the western border of the county as well. They can cover some distance and spread fairly quickly.”
Getting an idea of the exact number of polecats in north Essex is virtually impossible though, with most evidence of the mammal’s presence in a location gleaned from dead animals killed on the region’s roads or images captured via camera traps. Mr Tansley estimates there are several hundred polecats in the area, some of which are likely to be hybrid ferret-polecats that have cross-bred.
He added: “I’ve recently spoken to a number of people who have told me they are not seeing mink in the same numbers compared with a few years ago. Three people have said this to me - independent of each other. If polecats are now re-established, is the combination of both the otter and polecat bringing the mink to a tipping point? The mink is now having to deal with two native predators that they have not encountered at the same time before.”
Mr Tansley is at pains to emphasise that this is only a theory based on anecdotal evidence and that further research is required. More conclusive evidence could be provided by environmental DNA analysis - an innovation that gives researchers the ability to analyse water samples to find out what creatures have been present.
Essex Wildlife Trust is hoping to launch a field trial in the summer in partnership with scientists from Salford University although any meaningful results aren’t expected for at least two years.
Should it turn out that mink numbers are going down, it will be good news for the water vole, which has suffered in the face of the mink invasion for many years.
“Polecats will take water voles but only the odd one when they are outside their burrows,” continued Mr Tansley.
“The polecat and the water vole have co–evolved over millennia and can survive side-by-side whereas the mink is not native and the female mink is small enough to get inside the water voles’ burrow system and move down and kill all the young.
“Co-evolution gives you a balance which can be upset by species that are introduced.”