Gene-flow in the clouds: landscape genetics of a viviparous, montane grassland toad in the tropics

Anthropogenic habitat alteration often increases fragmentation and isolation, which decreases population sizes and increases extinction risk for species. Extrinsic threats may be buffered or enhanced by intrinsic factors. Within amphibians, the influence of different environmental and intrinsic factors on the population structure is not yet fully understood. Four factors were found to be important for population connectivity: life history traits, recent (anthropogenic) land use history, habitat, and topography, but the direction of their influence differed between studies. Here, we examine the genetic population structure and interpopulation connectivity within the complete distribution of Nimba toads ( Nimbaphrynoides occidentalis ), a toad from montane tropical West Africa. The Nimba toad is the only known viviparous, matrotrophic (foetuses are nourished during the gestation by their mothers) anuran on Earth. It occurs in three regions, the smallest is situated in disturbed, the largest population in partly disturbed habitat and the third was not yet impacted. We found small, but significant population differentiation, no indication of a recent bottleneck in the smallest population, but an indication of a reduction in population sizes in the more distant past in all three populations and no sex-biased dispersal. Correlations with landscape classifications indicate that high elevations, due to their high humidity levels, are the most important landscape characteristic facilitating dispersal. This underscores desiccation risk as an important landscape characteristic for amphibian population connectivity. We found indication that life-history traits (viviparity), land use history (mining-related activity) and topography (elevation) have an influence on Nimba toad population differentiation and gene-flow.