1. Terrestrial dispersal by aquatic insects increases population connectivity in some stream species by allowing individuals to move outside the structure of the stream network. In addition, individual survival and reproductive success (as well as dispersal) are tightly linked to the quality of the terrestrial habitat.
2. In historically forested catchments, deforestation and altered land use have the potential to interfere with mayfly dispersal or mating behaviours by degrading the quality of the terrestrial matrix among headwater streams. We hypothesised that loss of tree cover in first-order catchments would be associated with an increase in population substructure and a decrease in genetic diversity of mayfly populations.
3. To test this hypothesis, we investigated spatial patterns of genetic variation in the common mayfly Ephemerella invaria across a gradient of deforestation in the central piedmont region of eastern United States. Intraspecific genetic diversity and population substructure were estimated from data obtained using fluorescent amplified fragment length polymorphism (AFLP) markers.
4. We found that mayfly populations had low population substructure within headwater stream networks and that genetic diversity was strongly negatively correlated with mean deforestation of the first-order catchments. The large-scale pattern of population substructure followed a pattern of isolation by distance (IBD) in which genetic differentiation increases with geographical distance, but assignment tests placed a few individuals into populations 300 km away from the collection site.
5. Our results show that loss of genetic diversity in this widespread aquatic insect species is co-occurring with deforestation of headwater streams.
6. Most arguments supporting protection of headwater streams in the United States have centred on the role of these streams as hydrological and biogeochemical conduits to downstream waters. Our work suggests that headwater stream land use, and specifically tree cover, may have a role in the maintenance of regional genetic diversity in some common aquatic insect species.