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

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 widespread long-distance migrant while the moustached warbler's breeding range is patchier and it is resident or migrates over short distances. Using microsatellite and mtDNA data, we found higher differentiation in moustached than in reed warblers, indicating higher dispersal capability of the latter species. Our results also suggest that the sea limits dispersal in the moustached warbler. However, we found evidence of gene flow between the study sites in both species, indicating a capability to compensate for habitat fragmentation. In most cases, the gene flow was restricted, possibly because of the large distances between study sites (from ca 290 to 960 km) or breeding site fidelity. The reed warbler had higher haplotype diversity, likely due to dispersal from different populations, past admixture event and a larger population size. We found also signs of postglacial population growth for both species and evidence of a recent colonization or re-colonization of the Mallorca Island by the moustached warbler.