Wood ants on the edge: how do the characteristics of linear edges affect the population dynamics of an edge specialist?

Abstract

Landscape structure modulates species dispersal by presenting barriers or opportunities. Slow-dispersing edge specialists, e.g. the northern hairy wood ant (Formica lugubris), are likely to be most affected by topography and land management practices, because they require adjacent contrasting habitats, e.g. to access both food and sunlight. In managed forests, canopy gaps are often linear and anthropogenic, such as paths, firebreaks, and roads (collectively rides), and their orientation determines shade distribution. Using data spanning 10 years, we ask how ride orientation and width affect the distribution and dispersal of three F. lugubris populations in the North York Moors, UK. Ride orientation clearly affected nest abundance, with a higher nest density on rides oriented north-south (N-S) or east-west (E-W) (cardinal directions) than on those oriented NE-SW or NW-SE (intercardinal directions). Conforming to predictions based on sunlight availability, N-S oriented rides were occupied more symmetrically than E-W ones, where the north side was used predominantly. Nests were generally larger on narrower rides. Ride orientation also clearly affected dispersal: wood ants dispersed c15 m/year along rides oriented in cardinal directions, compared with only c5 m/year on rides oriented intercardinally. Our results show that ride characteristics (width, orientation), resulting directly from forestry practices, influence the distribution and dispersal of an ecosystem engineer woodland species; this may also apply to other forest-edge specialists. As wood ants can suppress defoliating pests, these findings could benefit forest management; forest planners could encourage wood ant colonisation of plantation forest by ensuring linear features contribute to north-south and east-west connectivity.