The influence of landscape structure on occurrence, abundance and genetic diversity of the common frog, Rana temporaria

Abstract

Human-caused habitat destruction and modification constitute one of the largest threats to population persistence and biodiversity, and are also suspected to be the major cause behind the global decline of amphibian populations. We assessed the potential role of agriculture-related habitat fragmentation on population size and genetic variability in the common frog (Rana temporaria) by recording the occurrence, population density and genetic diversity in three geographically disparate regions in Sweden - each containing landscapes of high and low agricultural activity - and related these to landscape variables extracted from digital maps. We found a highly significant region-by-landscape interaction in occurrence, population density and genetic diversity revealing a reversed response to agriculture from south to north: while the effects of agriculture on R. temporaria populations were negative in the south, there were no effects in the central region, whereas positive effects were observed in the north. Spatial autocorrelation analyses of genetic data revealed that populations in high agricultural activity areas were more isolated than populations in low activity areas both in the southern and central regions of Sweden. Landscape diversity showed a strong positive correlation with both density and occurrence of frogs in Sweden as a whole, as well as in the southern region. Also, negative effects of roads and positive effects of ditches on genetic diversity were found in the south. Overall, these results suggest clear but regionally opposite effects of habitat structure on the population size and genetic diversity of amphibian populations. This means that the management strategy aiming to maximize the size and genetic diversity of local common frog populations, and perhaps also those of other amphibian populations, should account for regional differences in existing land-use patterns. © 2005 Blackwell Publishing Ltd.