PERSISTENCE OF BRITISH NATTERJACK TOAD BUFO-CALAMITA LAURENTI (ANURA,BUFONIDAE) POPULATIONS DESPITE LOW GENETIC DIVERSITY

Like other amphibians native to Britain, the natterjack toad Bufo cala mita must have colonized the islands during the relatively short perio d between the end of the last glaciation and the separation of Britain from mainland Europe by rising sea levels. Unlike the other native am phibians, however, B. calamita is a habitat specialist at the north-we sterly edge of its biogeographical range and for most of the 8000-1000 0 years since its colonization has probably been restricted to open du nes, heathlands and upper saltmarshes, as isolated populations in a fe w discrete areas of the country. We have investigated the genetic dive rsity and relatedness of six widely separated British natterjack popul ations by allozyme analysis, and shown that all have very low diversit y (Overall P-95% = 2.7%, H = 0.004) by comparison with other anurans, including natterjack populations in mainland Europe and common frogs ( Rana temporaria, L) in Britain. Eighty percent of loci were fixed for the same allele in all six British natterjack populations and genetic differentiation between colonies was extremely low. The possible signi ficance of these findings to the persistence of small isolated populat ions at range edges is discussed.