Genetic structure of island populations of the anurans Rana temporaria andBufo bufo

Amphibians have traditionally been considered to have low dispersal ability and they have become a model for studies on the effects of man-made habita t fragmentation on genetic variation and population differentiation. This s tudy examined the genetic population structure in the common frog (Rann tem poraria) and the common toad (Bufo bufo) in an archipelago of the northern Baltic Sea. Heterozygosity was not correlated with distance of the island f rom the mainland nor, in R. temporaria, with effective population size base d on census estimates. Generally, no inbreeding was detected in island popu lations. The overall differentiation among islands was weak, but the F-ST v alues were significantly larger in R. temporaria (F-ST = 0.068) than in B. bufo (F-ST = 0.019). Most of the differentiation was a result of difference s among groups of islands, differentiation within them playing a minor role . Thus, assuming Wright's island model of migration, gene flow was rather h igh among closely located islands, but longer distances seemed to form a sl ight dispersal barrier for R. temporaria. Strong gene flow within the study area was confirmed by lack of isolation by distance. The estimated effecti ve population sizes in R. temporaria were small, the average being 32 breed ing females per island. The results indicate that gene flow between island populations across the matrix of open, brackish-water sea is extensive and suggest that the anurans are well able to disperse in this natural metapopu lation system.