Should I Stay or Should I Go? Seasonal Fluctuations of Wood Mouse Populations in Fields Surrounded by Woodlands

Abstract The wood mouse Apodemus sylvaticus is common in woodlands and open areas of Western Palearctic. Despite extensive research, little is known about its population ecology in fields in the Mediterranean area, where climate involves great seasonal changes in environmental features. Here, we investigated wood mice seasonal fluctuations in abundance and population structure by sampling long-fallow fields and two woodland types, i.e. oak forest and conifer plantation, in a heterogeneous landscape of central Italy. Mice were live-trapped in three sites per habitat type, every other month for three years (23.814 trap-days). Population abundance, age structure, mice body weight and proportion of residents and breeding individuals were analysed. Sampling effort resulted in 350 captures of 245 individuals. Mice dynamics changed across seasons and habitats. In fields, we recorded more captures, more reproductive individuals, and fewer immature and resident individuals in warmer than in colder months; mice were heavier in warmer months. These results suggest that fields may better suit the species ecological requirements (i.e. food and cover resources) in spring-summer, representing a seasonally suboptimal habitat for mice. Population abundance in fields was lower than in woodlands during the cold season, when immatures were more frequent in fields than in resource-rich woodlands. Conversely, breeding and non-resident mice were more abundant in fields than in woodlands in warmer months. Overall, the seasonal demographic variations we recorded evidence that fields can represent a suboptimal habitat in Mediterranean heterogeneous landscapes and can nonetheless act as a source of resources, mates and space for mice in spring-summer.