Habitat fragmentation selects for low dispersal in an ant species

Increased habitat fragmentation is one of the major global changes affecting biodiversity. It is characterised by a decrease in habitat availability and by an increase in the isolation of suitable habitat patches. The dispersal capacities of species may evolve in response to increased habitat fragmentation. Spatial heterogeneities and/or costs of dispersal, which are directly linked to habitat fragmentation, tend to select for lower dispersal abilities. We studied the effects of habitat fragmentation on dispersal using an ant species that exhibits a marked dispersal polymorphism. Myrmecina graminicola produces winged queens dispersing by flight over long distances, or apterous queens dispersing on foot over short distances. We sampled queens in 24 forests around Paris and 25 parks within Paris, representing varied levels of habitat fragmentation and habitat size. We identified the queen morphotypes in each environment and used it as a proxy of dispersal. Winged queens predominated in both environments. However, apterous queens were comparatively more common in parks than in forests, suggesting that high fragmentation counterselects dispersal in this species. We argue that this is because dispersing within urban environments is very costly and discuss the factors favouring each queen morph or resulting in their co-occurrence (maintenance of polymorphism).