Large variations in the ratio of effective breeding and census population sizes between two species of pond-breeding anurans

The viability of wild populations is frequently assessed by monitoring adult census sizes (Nc). This approach is particularly useful for pond-breeding amphibians, because assemblages during the breeding season are relatively easy to detect and count. However, it is the genetic effective population size (Ne) or surrogates such as effective breeding population size (Nb) that are of primary importance for long-term viability. Although Nc estimates of one anuran amphibian (Bufo bufo) in Britain were much larger than those of another (Rana temporaria) at the same sites, the ratios of Nb to Nc were much smaller in B. bufo than in R. temporaria. These differences were sufficiently great as to reverse the effective size order at one site, such that Nb for R. temporaria was larger than that for B. bufo. Differences in adult sex ratios at breeding sites probably contributed to lower Nb values in B. bufo populations compared with those of R. temporaria. The relationship of Nb to Nc can therefore vary dramatically even between similar species, to the extent that just monitoring Nc can give misleading impressions of relative effective breeding sizes and thus of population viability. It will be increasingly important to estimate Ne or Nb in wildlife populations for assessment of conservation priorities.