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 (N-c). 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 (N-e) or surrogates such as effective breeding population size (N-b) that are of primary importance for long-term viability. Although N-c 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 N-b to N-c 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 N-b for R. temporaria was larger than that for B. bufo. Differences in adult sex ratios at breeding sites probably contributed to lower N-b values in B. bufo populations compared with those of R. temporaria. The relationship of N-b to N-c can therefore vary dramatically even between similar species, to the extent that just monitoring N-c can give misleading impressions of relative effective breeding sizes and thus of population viability. It will be increasingly important to estimate N-e or N-b in wildlife populations for assessment of conservation priorities. (c) 2006 The Linnean Society of London.