Population genetics of Formica aquilonia wood ants in Scotland: the effects of long-term forest fragmentation and recent reforestation

Formica aquilonia wood ants are forest specialists which play a key role in the ecology of forests in Europe. Many of the Scottish populations at the edge of the species distribution range occur in highly fragmented landscapes. We used ten microsatellite loci to study the genetic diversity and structure of populations from two contrasting regions (Inverpolly and the Trossachs) to set the Scottish populations in the context of conspecific populations in mainland Europe. Historically, both study regions have experienced extreme habitat loss and fragmentation over several centuries. Inverpolly has remained fragmented whereas large scale reforestation over the last century has greatly increased the forested area in the Trossachs. Despite the long history of fragmentation, genetic diversity in the Scottish populations was greater than in the populations in mainland Europe. Genetic diversity was similar in the two Scottish regions and no evidence of inbreeding was detected. However, the populations in Inverpolly showed more evidence of genetic bottlenecks, possibly due to more frequent stochastic events such as moorland fires. The ant populations in individual forests were genetically distinct and we detected no contemporary gene flow between forests. The most intensively studied forest where non-native conifer plantations now occupy the matrix between the remaining ancient woodland fragments showed evidence that admixture and gene flow between nests was reducing the past differentiation. This may reflect a dynamic response to the reconnection of previously isolated populations in forest fragments by recent reforestation.