Water vole
Arvicola amphibious
Llygoden y dwr
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Status:
The water vole is a native species, formally widespread in Snowdonia but now declined across most of its range. A predominately riparian species it occurs in areas of tall reed, rush and tussock forming grasses beside streams, ditches, lakes and ponds as well as open moorland on high ground. An expansive population remains in the ditches and streams associated with the Mignient at the headwatwers of the Afon Conwy, Afon Dwyryd and Afon Tryweryn.
The species has fully protected legal status in the UK under the Wildlife and Countryside Act 1981 (as amended 2008) and is a priority species with its own Biodiversity Action Plan. Since the 1970s, the species has declined by 94% in Wales due to habitat loss, habitat fragmentation and predation by mink. This is one of the most rapid and serious recorded declines of any British wild mammal during the 20th century.
Water vole population density varies seasonally, highest in autumn and lowest post winter before breeding. A typical rodent they are short lived (rarely surviving more than 2 years) but are highly fecund with a single female producing up to 5 litters in a year each of 5-6 babies. The population therefore has an annual increase to peak numbers each October. Persistence at any one site is dependent on immigration and emigration between adjacent colonies as water voles disperse at the end of winter and as juveniles. Best sites involve a cluster of 8 or more colonies no more than 1.5km apart, individual sites blinking on and off due to local losses and chance events such as local floods, poor breeding or excessive predation.
The water vole is another key species and favoured prey of many predators from weasels, stoat, polecat, mink, fox, and occasionally otter as well as kestrel, buzzard, harrier, heron, barn owl and short-eared owl. Areas of dense vegetation may give the best protection from predators, but normal anti-predation strategy involves diving into the water and darting underground into their burrows (some especially excavated as bolt holes within the voles home range). This strategy is effective against the suite of native predators but proves ineffective against the non-native mink that can catch it on the bank, in the water and underground (the stoat-sized female mink will readily hunt the burrow system)
Isolated and highly fragmented sites in Snowdonia are the most vulnerable to extinction and the species has been lost from the lower and middles reaches of most river catchments.