Hybridization and low numbers in isolated populations of the natterjack, Bufo calamita, and the green toad, B. viridis, in southern Sweden: possible conservation problems

Abstract

Abstract Two isolated rock pool localities in southern Sweden with breeding green toads (Bufo viridis) and natterjacks (B. calamita) were studied, one on the mainland coast (Vik) and one on an island in the Baltic (Utklippan). Both localities are isolated by at least 30 km from other known populations of either species. A new character, dark throat patches present in B. calamita, aided in species separation on Utklippan. Body length was larger (9-41 % ) than reported from continental Europe. Individuals intermediate between the two species in size, morphologv and mating calls were found on Utklippan, suggesting the occurrence of viable hybrids. In Vik, neither of these characters nor electrophoretic data indicated hybridization. We suggest that the number of suitable breeding ponds may explain why the two species hybridize on Utklippan since only one sympatric breeding pond could be found on this site and a number of suitable ponds were found on Vik. Thus the species may be spatially more scparated at the Vik site making hybridization less likely to occur. However, few green toad males could be found on Utklippan making the sex ratio highly skewed. Accordingly, the low number of male green toad may explain why the species hybridize. Hybridization may be a potential threat to the persistence of endangered populations already in low numbers, if a large proportion of female gametes are wasted in hybrid matings. Thus this possibility is important when considering the conservation of endangered species.