Postmetamorphic dispersal and recruitment of first breeders in a Bufo calamita metapopulation.

The migratory capacity of metamorphs and their later integration into local breeding populations was studied in a Bufo calamita metapopulation inhabiting the northern Rhineland, Germany (1991-1994). A new tagging method allowed unequivocal distinction among metamorphs (n = 1,709), which originated either from the spring-breeding cohort in 1991 and 1992 or from the summer-breeding cohort in 1991. The life history of batch-marked individuals was followed until adulthood, focusing on postmetamorphic dispersal, choice of breeding locality and timing of adult reproduction. Almost all metamorphs left the patches used for breeding within a few weeks after emergence. They were found at about the same density in all favourable microhabitats in a radius of up to 2 km around their native patch. This demonstrates that even small toadlets (6-11 mm snout-to-vent length) migrate distances of several hundred metres within a short time and that they significantly help prevent local extinction in sink populations (rescue effect). Recaptures of reproductive adults originating from tagged metamorphs were limited to males belonging to the 1997 spring and summer cohorts. The absence of recaptured females was probably due to their greater vagility and later sexual maturation. First-breeding males (2 years old) without exception joined the breeding cohort from which they originated: early breeders originated from spawn of the spring cohort, mid-summer breeders from spawn of the summer cohort. This result is in accordance with the hypothesis that the integration of first breeders to a specific breeding cohort is genetically controlled in central European natterjacks. The exchange of individuals between different cohorts in the same patch is considerably less than the migratory exchange between the same breeding cohorts among local populations, as predicted from the allozyme data.