Recent introduction or ancient ancestry? Use of genetic evidence to investigate the origins of range edge populations in natterjack toads (Bufo calamita)

The ability to distinguish between native and translocated populations is important in conservation biology, but can be problematic, especially near range edges. We describe here the use of variable genetic markers as an approach to resolving this issue. Natterjack toads Bufo calamita have a very restricted distribution in south-west Ireland. Populations in the main distribution area are well documented, but there are also some recently discovered ones at a widely separated site. We used genetic evidence to investigate whether the recently discovered populations are newly established or of ancient origin, using three criteria: genetic diversity, genetic structure and estimated time since divergence. The results highlighted some difficulties of using genetic data from range edge populations, where such issues are most likely to arise but where diversity (and thus statistical power) is often low. On balance the genetic data weakly supported the hypothesis that the newly discovered populations are not recent translocations. It will always be important in such investigations to use genetic inferences in combination with other independent lines of evidence (such as fossils) wherever possible.