ABSTRACT
Amphibian populations have been declining globally over the past decades. The intensification of agriculture, habitat loss, fragmentation of populations and toxic substances in the environment are considered as driving factors for this decline. Today, about 50% of the area of Germany is used for agriculture and is inhabited by a diverse variety of 20 amphibian species. Of these, 19 are exhibiting declining populations. Due to the protection status of native amphibian species, it is important to evaluate the effect of land use and associated stressors (such as road mortality and pesticide toxicity) on the genetic population structure of amphibians in agricultural landscapes. We investigated the effects of viniculture on the genetic differentiation of European common frog (Rana temporaria) populations in Southern Palatinate (Germany). We analyzed microsatellite data of ten loci from ten breeding pond populations located within viniculture landscape and in the adjacent forest block and compared these results with a previously developed landscape permeability model. We tested for significant correlation of genetic population differentiation and landscape elements, including land use as well as roads and their associated traffic intensity, to explain the genetic structure in the study area. Genetic differentiation among forest populations was significantly lower (median pairwise FST = 0.0041 at 5.39 km to 0.0159 at 9.40 km distance) than between viniculture populations (median pairwise FST = 0.0215 at 2.34 km to 0.0987 at 2.39 km distance). Our analyses rejected isolation by distance based on roads and associated traffic intensity as the sole explanation of the genetic differentiation and suggest that the viniculture landscape has to be considered as a limiting barrier for R. temporaria migration, partially confirming the isolation of breeding ponds predicted by the landscape permeability model. Therefore, arable land may act as a sink habitat, inhibiting genetic exchange and causing genetic differentiation of pond populations in agricultural areas. In viniculture, pesticides could be a driving factor for the observed genetic impoverishment, since pesticides are more frequently applied than any other management measure and can be highly toxic for terrestrial life stages of amphibians.
ABSTRACT
Field margins are important seminatural habitats in agro-ecosystems, but they can be negatively affected by pesticide inputs via direct overspray and spray drift. In Germany, risk mitigation measures (like buffer zones) to reduce pesticide inputs in terrestrial noncrop habitats do not have to be put in place by farmers next to narrow field margins (<3 m width). Because data on structure, size, and width of field margins are scarce, we identified field margins in 2 German agricultural landscapes (Rhineland-Palatinate [RLP], Brandenburg [BB]; 4000 ha each) using digital orthophotos and geographical information systems. In RLP, most of the field margins were less than 3 m wide (85% of margin length and 65% of the margin area), whereas in BB narrow field margins account for 45% of the margin length and 17% of the margin area. Hedgerows were only occasionally recorded. Hence, narrow grassy field margins can represent a large part of the available seminatural habitats adjoining agricultural sites and potentially act as corridors between further habitat patches. For this reason, these margins should be protected from pesticide inputs, at least in landscapes under intensive agricultural use. Field margins are also the main, so-called nontarget habitat protected by the terrestrial risk assessment for plants and arthropods. With many (narrow) margins not considered relevant for risk management, the current practice for protecting the biodiversity from negative effects of pesticides seems questionable. More data on field margin constitution in Germany and other European countries is necessary to critically assess the current practice of pesticide risk assessment and management on a larger scale.
