Even under normal circumstances, red squirrels struggle to find food in the spring and summer months and this can result in breeding failure and even death. When the larger, more aggressive grey squirrel is added to the equation, the situation becomes intolerable. Living in higher densities than the reds and able to exploit food sources more efficiently, it is easy to understand why the greys are taking over. In addition, the greys carry a disease, the squirrelpox virus, to which they are immune, but which ensures a painful lingering death for any red which becomes infected. That is why the Trust agrees wholeheartedly with Doctor Craig Shuttleworth in his report, 'Grey Squirrel control on the Island of Anglesey 1998 - 2004', which begins with the statement, ‘Grey Squirrel Sciurus carolinensis control is the single most important factor in the conservation of the red squirrel Sciurus vulgaris populations within woodlands where the two species of squirrel occur together. Research has demonstrated, and regional red squirrel extinctions illustrated, that it is paramount that contact between red and grey squirrels is minimised.’.
Over time prey species develop strategies to evade predators. The water vole is no different and had a number of ways it would escape from the native predators. If it was feeding on land, it could escape a fox by going down its burrow. When threatened in its burrow by a weasel, it could avoid attack by diving into the water and, when in the water, if a heron became a threat it could retreat again to its burrow. The arrival of the American mink changed all that overnight. The mink can hunt and kill water vole everywhere, on riverbanks, in their burrows and in the water. Evasion tactics developed over thousands of years no longer worked. The water vole now has nowhere to hide. Sir David Attenborough summed up the situation in the BBC’s ‘Wildlife on One’, “…When mink invade a stretch of river, they quickly reduce the number of vulnerable creatures such as duck and water voles….Since their invasion of rivers and streams across Britain, mink have wiped out many water vole communities and will continue to do so unless their own numbers are controlled”.