Although it has been long presumed that population genetic variability should decrease as a species' range margin is approached, results of empirical investigations remain ambiguous. Sampling strategies employed by many of these studies have not adequately sampled the entire range. Here we present the results of an investigation of population genetic diversity in a vertebrate species, the Italian agile frog, Rana latastei, sampled comprehensively across its entire range. Our results show that genetic variability is not correlated with population location with respect to the range periphery. Instead, the model that best explains the genetic variation detectable across the range is based on an east-to-west gradient of declining diversity. Although we cannot state definitively what has led to this distribution, the most likely explanation is that the range of Rana latastei expanded postglacially from a Balkan refugium.

heterocigosidad) están bien reconocidas y establecidas empíricamente. Sin embargo, no está bien reconocido que una disminución repentina en el tamaño poblacional efectivo inducida por tal desconexión produce un desequilibrio temporal en las distribuciones de frecuencias alélicas que es comparable con la firma genética de un cuello de botella demográfico. Para documentar este efecto, utilizamos simulaciones basadas en individuos y datos empíricos de la riqueza alélica y diversidad genética en seis pares de poblaciones aisladas de Hyla arborea versus poblaciones bien conectadas (núcleo). En nuestras simulaciones, la desconexión poblacional deprimió la riqueza alélica más que la heterocigosidad y por lo tanto resultó en un exceso temporal de diversidad genética en relación con el equilibrio por deriva mutacional (i. e., firma de un cuello de botella genético). Observamos un exceso similar en la diversidad genética en poblaciones aisladas de H. arbórea. † †Current address: Team "Diversity and Connectivity in Coastal Marine Landscapes," Roscoff Biological Station, UMR 7144 CNRS -Pierre and Marie Curie University, 29682 Roscoff, France, email thomas.broquet@sb-roscoff 1597Nuestros resultados muestran que la desconexión de la población puede crear un cuello de botella genético en la ausencia de un colapso demográfico.

Functional connectivity among fragmented populations depends on the landscape matrix between occupied habitat patches and its effect on the frequency of animal movement and gene flow. The quantification of landscape effects on gene flow should therefore be scale‐dependent. Here, we explored the impact of different spatial scales in a landscape genetic analysis of the European tree frog Hyla arborea in a fragmented landscape in Switzerland. We examined the effects of landscape elements and geographic distance on genetic differentiation at three distance classes reflecting varying frequencies of tree frog movement. We calculated pairwise FST‐values and assembled 16 landscape elements within 1 km wide corridors between all pairs of tree frog breeding sites. Per distance class, we computed a multiple regression model with stepwise backward elimination and permutation testing. At distances of<2 km, only a larger river acted as a barrier to gene flow. At distances>2 km, geographic distance had a negative effect on gene flow as had landscape elements such as forests and roads. In general, hedgerows and various structure‐rich landscape elements positively affected gene flow. As we found distinct scale‐dependent landscape effects on gene flow, future landscape genetic studies should analyse the effects of landscape variables at different spatial dimensions relevant for the movement and dispersal of the study organisms. Corresponding studies should also carefully consider relevant correlations among the landscape elements tested and should preferentially replicate their analysis at the landscape‐level in order to avoid idiosyncratic results owing to the particular scale and landscape studied.

Summary1. Governmental authorities in many countries financially support the implementation of habitat connectivity measures to enhance the exchange of individuals among fragmented populations. The evaluation of the effectiveness of such measures is crucial for future management directions and can be accomplished by using genetic methods. 2. We retraced the population history of the European tree frog in two Swiss river valleys (Reuss and Thur), performed comprehensive population sampling to infer the genetic structure at 11 microsatellite markers, and used first-generation migrant assignment tests to evaluate the contemporary exchange of individuals. 3. Compared with the Thur valley, the Reuss valley has lost almost double the number of breeding sites and exhibited a more pronounced genetic grouping. However, similar numbers of contemporary migrants were detected in both valleys. In the Reuss valley, 81% of the migration events occurred within the identified genetic groups, whereas in the Thur valley migration patterns were diffuse. 4. Our results show that the connectivity measures implemented in the Reuss valley facilitated effective tree frog migration among breeding sites within distances up to 4 km. Nevertheless, the Reuss valley exhibited high genetic differentiation, which reflected the impact of barriers to tree frog movement such as the River Reuss. By contrast in the Thur valley, a larger number of breeding sites have been preserved and high admixture indicated exchange of individuals at distances up to 16 km. 5. Synthesis and applications. We show that genetic methods can substantiate the effectiveness of connectivity measures taken in conservation management at the landscape scale. We urge responsible authorities from both river valleys to continue implementing connectivity measures and to create a dense network of breeding sites, as spatial gaps of 8 km are rarely traversed by tree frogs.