Recent large-scale range expansion and outbreaks of the common vole (Microtus arvalis) in NW Spain

Irruptive populations of rodents cause damage to agriculture worldwide. By the end of the last century, the distribution range of Microtus arvalis in NW Spain greatly expanded to encompass agricultural habitats, with the appearance of crop damaging population outbreaks. The absence of long term vole monitoring data has so far precluded outbreak forecasting, which might help mitigating associated bioeconomic costs. We used non-standard and diverse sources of information, including newspaper and national technical reports, to describe the vole expansion and outbreak dynamics in NW Spain since the late 1960s. We illustrate a rapid (<20 years) and large scale (ca. 5 million ha) colonisation of agricultural lowlands, and suggest a pattern of westward expansion emanating from the peripheral mountains. Crop damaging outbreaks directly followed range expansion and our analyses indicate that they have occurred at approximately 5-year intervals since the early 1980s. This is the first description of long term (>40 years) regional scale vole dynamics reported for the Iberian Peninsula. We suggest that expansion from (humid) mountains to (dry) plains may be related to recent changes in land use. If confirmed at a local scale, the apparent cyclicity of outbreaks would provide a basis for forecasting outbreak risk in NW Spain and may help managers adjust current control strategies.