Genetic differentiation over a short water barrier in the Brazilian tanager, Ramphocelus bresilius (Passeriformes: Thraupidae) an endemic species of the Atlantic forest, Brazil

Atlantic forest is a biome of high diversity that is increasingly under threat from habitat clearance. This habitat contains a large number of endemic species, many of which are currently endangered. Here, we consider the genetic population structure of a Brazilian endemic bird restricted to Atlantic forest, the Brazilian tanager, Ramphocelus bresilius. While currently not threatened, this narrow range endemic faces future habitat reductions, potentially serving as a model species for other birds reliant on this biome. We sampled six populations, three on the mainland (Rio de Janeiro state), and three on a neighbouring continental island (Ilha Grande), to compare the genetic diversity and population structure in these two regions. Using microsatellite data from eight loci generated for 177 individuals, we found that the short ocean crossing significantly restricted gene flow (20–150 km between populations, FCT = 0.057 P < 0.001). Structure analyses, partial Mantel tests and AMOVA all demonstrated a significant split into two groups separated by the ocean. This result was unexpected as the ocean barrier was only 2 km. While island populations were isolated from the mainland, their levels of genetic diversity were similar (AR 4.04 vs 4.59). This represents the first genetic study of an Atlantic forest species, contrasting mainland and populations from the island reserve, Ilha Grande. Similar levels of genetic diversity support the importance of this reserve in the conservation of Atlantic forest species, however the potentially limited exchange by bird species should be taken into account in future conservation planning.