Does landscape and habitat limit the frogs Rana arvalis and Rana temporaria in agricultural landscapes? A field experiment

Frogs are often rare or absent from intensively farmed areas. Here we explore the possibility that the landscape and the quality of the terrestrial habitat are unsuitable for these populations. Spawn of Rana arvalis and R. temporaria was introduced into ponds in a south Swedish agricultural landscape in 2003 (eight ponds) and 2004 (ten ponds). Metamorphs emerged from nine (R. a.) and 12 (R. t.) of these. In years following the introduction, spawn was found in five (R. a.) and eight (R. t.) of these 18 ponds. The number of spawn clumps peaked two years after the introduction. Three or four years after the introduction, breeding persisted in only two of the ponds (where both species were breeding). One year later also these populations had also become extinct. In control ponds (ponds within 750 m of the introduction ponds), spawn of R. temporaria was occasionally found but there was no trend, nor any temporal peak in frequency or quantity of spawn in these ponds. There were calling males of both species already one year after the introduction at a few ponds, indicating an unusually early maturation for some individuals. We suggest that the terrestrial habitat in this region is not suitable for the continued presence of populations of R. arvalis and R. temporaria. This may be related to the habitat per se or to the isolation of the populations.