Does fragmentation of wetlands affect gene flow in sympatric Acrocephalus warblers with different migration strategies?

Abstract

Wetlands are naturally patchy habitats, but patchiness has been accentuated by the extensive wetlands loss due to human activities. In such a fragmented habitat, dispersal ability is especially important to maintain gene flow between populations. Here we studied population structure, genetic diversity and demographic history of Iberian and North African populations of two wetland passerines, the Eurasian reed warbler Acrocephalus scirpaceus and the moustached warbler Acrocephalus melanopogon. These species are closely related and sympatric in our study sites, but the reed warbler is a long-distance migrant and widespread bird while the moustached warbler is resident or migrates over short distances, and breeds across a more discontinuous range. Using microsatellites and mtDNA data, we found higher population differentiation in moustached than in reed warbler, indicating higher dispersal capability of the latter species. Our results also suggest that the sea limits dispersal in the moustached warbler. For both species, we found evidence of gene flow between study sites, indicating the capability of compensating for habitat fragmentation. However, in most cases gene flow was restricted, possibly because of the large distances between study sites (from c. 290 to 960 km) or breeding site fidelity. Haplotype diversity was higher for the reed warbler, possibly because of a more important contribution of dispersal from different populations and the higher population size. Studying demographic history, we obtained signs of postglacial population growth for both species, and evidences of a recent colonization or re-colonization of the Mallorca Island by the moustached warbler