The weasel (Mustela nivalis) is Britain's smallest carnivore, weighing as little as 25g. Despite its small size, it is a highly effective predator, able to pursue prey into the smallest burrows. Weasels have declined significantly in the UK — possibly by over 50% since the 1970s — driven primarily by the collapse of small mammal prey populations.
Weasels specialise in hunting small mammals, primarily field voles, mice, and shrews. Their slender body shape allows them to pursue prey into burrow systems and vole runs. They hunt by scent, locating prey corridors and waiting at entrances. Female weasels (significantly smaller than males) can enter vole burrows directly. Weasels are active throughout the year, requiring a high food intake relative to body mass to maintain their metabolic rate.
Weasel populations are highly dependent on prey availability. In vole peak years, weasel reproduction increases dramatically; in vole crash years, weasel populations collapse. This "numerical response" means weasel populations are closely tied to the 3-4 year field vole cycle. Long-term declines in weasel populations therefore closely track declines in rough grassland habitat and field vole availability.
Weasel conservation is primarily habitat-based: maintaining and creating rough grassland with high vole populations directly benefits weasels. Weasels are included in Vincent Wildlife Trust monitoring and in the UK's farmland mammal tracking schemes. Increasing awareness of weasel decline and its ecological significance is an important conservation challenge.