Latitudinal patterns in body size of Roesel’s bush cricket Metrioptera roeselii within and outside the continuous area of its distribution

Over time, species colonize new areas naturally or through humans. The colonizers are often few and founded populations represent only a fraction of the genetic diversity existing in the original population. If the established population becomes isolated after the colonization event genetic diversity will remain low and if the population, in addition, remains small in size for an extended period this will lead to increased inbreeding and possibly inbreeding depression. Metrioptera roeselii is a common species with a widespread distribution in continental Europe. During the last century, it has expanded to the Scandinavian Peninsula which is separated from continental Europe by the Baltic Sea. My aim was to test the hypothesis that individuals in isolated populations in Scandinavia including Denmark and the Baltic Sea islands are smaller than individuals in the continuous part of its distribution due to low genetic diversity and lacking incoming gene flow. I measured and analyzed ten morphological traits and found no support for my hypothesis. Rather, no trait in the isolated populations was smaller than in continuous ones but was instead on average larger. To control for the influence of regional climate, distance to coast and latitude were included as factors and both explained a large part of the trait variation in the isolated populations while they did not in continuous ones. My conclusion is that M. roeselii manages the isolated situation well because it is a generalist and good competitor. Differences in gene flow among the two groups of populations lead to different latitudinal pattern with increasing body mass at higher latitude in isolated populations while in continuous populations, they were of equal size