The role of macropters during range expansion of a wing-dimorphic insect species

Marked changes in distribution in consequence of global warming have been observed not only for highly mobile insect taxa, which are capable of flight, but also for wing-dimorphic species with predominantly short-winged individuals. In the special case of wing-dimorphic species, it is likely that the rarer long-winged (macropterous) morph plays an important role in the dispersal process, but little is known about how and to what extent it is involved. The aim of our study was to provide more information on the mechanisms behind dispersal processes in wing-dimorphic insects at expanding range margins. As solitary individuals are believed to play an important role in the range expansion of wing-dimorphic species (potential dispersers), we recorded the number of long-winged and short-winged solitary males at the local range margin of our model organism Metrioptera roeselii (Orthoptera: Tettigoniidae) in NW Germany. To investigate differences in dispersal capability (% macropters) between populations with different colonisation histories, we studied 43 populations of M. roeselii. Our results show that about 2/3 of the solitary males were long-winged and these long-winged individuals were significantly more frequent in recently colonised areas. Moreover, M. roeselii had a significantly higher dispersal capability (% macropters) in high-density populations and in recently established populations at the expanding range margin compared to populations characterised by medium- or long-term establishment nearer to the range core. Our study is the first that quantifies the importance of macropters for the recent range expansion of a wing-dimorphic species and it provides for the first time detailed insights into the complex dispersal processes that take place at the expanding range margin. It is likely that density stress and a changed genetic predisposition to become macropterous, and thus a combination of both ecological and evolutionary effects, leads to a high percentage of macropters in recently colonised areas.