The ability to maintain small populations in quasi-natural settings is an issue of considerable importance in biodiversity conservation. The genetic structure of urban common frog (Rana temporaria) populations was determined by allozyme electrophoresis and used to evaluate the effects of restricted intersite migration. Despite the lack of any absolute barrier to movement between ponds, substantial genetic differentiation was found between sites separated by an average of only 2.3 km. Genetic distances between these town ponds correlated positively with geographical distances and were almost twice as great as those found between rural sites separated by an average of 41 km. Measures of genetic diversity and fitness were always lowest in the town, where the degree of subpopulation differentiation (FST =0.388) was high. Population decline was not evident in the town, but molecular and fitness data indicated the presence of genetic drift and inbreeding depression. The long-term survival of artificially restricted populations, particularly of relatively sedentary species, may require molecular monitoring, if genetic diversity is not to be lost by chance when facets of the species niche prove to be poorly understood.

The natterjack toad (  Bufo calamita) is an endangered species in Britain and has been legally protected since 1975. This amphibian suffered a major decline during the first half of the twentieth century, due partly to habitat destruction but mostly to successional changes in its specialized biotopes and anthropogenic acidification of breeding sites. In addition to site and species protection, extensive autecological research over the past 25 years has provided the foundations for an intensive, 3‐year species recovery program funded by the statutory nature conservation organizations (English Nature and the Countryside Council for Wales). This program was based on habitat management and reintroductions to restored sites and followed similar but less intensive efforts. Management of heath and dune habitats focused on restoration and maintenance of early stages of seral succession, initially through physical clearance of invasive scrub and woodland vegetation and later by the reestablishment of grazing regimes similar to those prevalent in earlier centuries. In some cases extra breeding pools were provided to either increase or stabilize natterjack toad populations that had become reliant on one or very few pools at small sites or to promote range expansion within large habitat areas. By 1995 proactive conservation work had been carried out at 29 (69%) of the 39 sites with extant native populations, including 8 during the recovery program. Twenty reintroductions also had been attempted, including nine during the program. At least six reintroductions resulted in the foundation of expanding new populations, and an additional eight have shown initial signs of success. Conservation methods developed for Bufo calamita should provide a useful precedent for long‐term conservation of early successional habitats and species.Programa de Recuperación para el Sapo Natterjack (Bufo calamita) en Gran BretañaEl sapo Natterjack (Bufo calamita) es una especie en peligro de extinción en Gran Bretaña y esta sujeto a protección desde 1975. Este anfibio sufrió una declinación importante durante la primera mitad del siglo veinte, debido en parte a la destrucción del hábitat pero principalmente a los cambios sucesionales en sus biotopos y a la acidificación antropogénica de sitios de reproducción. Además de la protección de sitios y de la especie, extensas investigaciones autoecológicas en los últimos 25 años proporcionaron las bases para un programa intensivo de recuperación patrocinado por las organizaciones de conservación de naturaleza estatutarias (English Nature y Countryside Council for Wales). Este programa se basó en el manejo del hábitat y reintroducciones en sitios restaurados, como continuación de esfuerzos similares pero menos intensivos. El manejo de brezales y dunas se enfocó en la restauración y mantenimiento de las primeras etapas sucesionales, se inició con la remoción de hierbas y vegetación leñosa invasoras y posteriormente se restablecieron regímenes de pastoreo similares a los prevalecientes en siglos anteriores. En algunos casos se proporcionaron charcas para reproducción para incrementar o estabilizar las poblaciones o para promover la expansión del rango en áreas con hábitats extensos. Para 1995 el trabajo de conservación se había desarrollado en 29 (69%) de los 39 sitios con poblaciones nativas, incluyendo 8 durante el programa de recuperación. También se habían intentado veinte reintroducciones, incluyendo 9 durante el programa. Por lo menos 6 reintroducciones resultaron en la fundación de nuevas poblaciones y 8 más han mostrado señas iniciales de éxito. Los métodos de conservación desarrollados para Bufo calamita deben constituir un precedente útil para la conservación a largo plazo de hábitats en sucesión inicial y de especies.

The ability to maintain small populations in quasi-natural settings is an issue of considerable importance in biodiversity conservation. The genetic structure of urban common frog (Rana temporaria) populations was determined by allozyme electrophoresis and used to evaluate the effects of restricted intersite migration. Despite the lack of any absolute barrier to movement between ponds, substantial genetic differentiation was found between sites separated by an average of only 2.3 km. Genetic distances between these town ponds correlated positively with geographical distances and were almost twice as great as those found between rural sites separated by an average of 41 km. Measures of genetic diversity and fitness were always lowest in the town, where the degree of subpopulation differentiation (FST =0.388) was high. Population decline was not evident in the town, but molecular and fitness data indicated the presence of genetic drift and inbreeding depression. The long-term survival of artificially restricted populations, particularly of relatively sedentary species, may require molecular monitoring, if genetic diversity is not to be lost by chance when facets of the species niche prove to be poorly understood.

Measures of genetic diversity (including heterozygosity), survival and developmental homeostasis were found to be significantly lower in small, urban populations of the Common Toad (Bufo bufo) than in larger, rural populations of the same region. The autecology and genetic analysis of this relatively sedentary species suggested that the causal mechanism was genetic drift, arising from barriers to migration created by urban development. The pre-metamorphic survival of larvae cultured in identical conditions increased positively with the mean number of alleles at a locus and the percentage of polymorphic loci. Observed heterozygosity in urban garden and rural populations was correlated inversely with the number of observed physical abnormalities (used as a measure of developmental homeostasis) in the developing tadpoles. Genetic distances between town sites of mean 2.2 km separation were significantly higher than those between rural sites of mean 37 km separation. Genetic data were based on allozyme analysis of 27 loci in 8 urban and 4 rural populations. A subset of these sites (3 urban, 2 rural) were also assessed at 3 minisatellite loci and a positive correlation found between the average number of alleles per locus detected by the two methods. Estimates of Nei's 1972 genetic distance, derived separately from the DNA and protein data, were not, however, correlated. The reduction in genetic diversity and fitness observed in these urban toads provides an example of the effect on population persistence that longer term depletion in numbers and habitat fragmentation can have in the wider environment.

Like other amphibians native to Britain, the natterjack toad Bufo cahmita must have colonized the islands during the relatively short period between the end of the last glaciation and the separation of Britain from mainland Europe by rising sea levels. Unlike the other native amphibians, however, E. cahmircl is a habitat specialist at the north-westerly edge of its biogeographical range and for most of the 8000-10000 years since its colonization has probably been restricted to open dunes, heathlands and upper saltmarshes, as isolated populations in a few discrete areas of the country. We have investigated the genetic diversity and relatedness of six widely separated British natterjack populations by allozyme analysis, and shown that all have very low diversity (Overall Pss% = 2.7%, H = 0.004) by comparison with other anurans, including natterjack populations in mainland Europe and common frogs (Rum 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 significance of these findings to the persistence of small isolated populations at range edges is discussed.