Does pond quality limit frogs Rana arvalis and Rana temporaria in agricultural landscapes? A field experiment.
When assessing the cause of population absence or decline, it is important to understand the relative effect of local, regional and global factors. In this study we evaluated the first of these factors for two frog populations. Amphibians are often absent from intensively farmed areas. There could be several reasons for this, one of them being the quality of the aquatic habitat available for reproduction. In order to test why common frogs Rana temporaria and moor frogs Rana arvalis are absent from most ponds in the intensively agricultural areas of southern Sweden, we performed a field experiment. Spawn of both species were introduced into 18 ponds surrounded by intensively cropped fields. Tadpole performance generally did not differ from that in a set of reference ponds in various other habitat types where one or both of these frog species occurs naturally. In the same experimental ponds and in a number of reference ponds, we also introduced tadpoles of the two species into enclosures that protected them from predation and thus increased recapture rate. This experiment revealed that the water quality of farmland ponds is rarely unsuitable for successful frog reproduction. Having measured abiotic and biotic variables in the experimental and reference ponds, we assessed the importance of different parameters to tadpole performance. While farmland ponds generally had higher pH, higher conductivity and higher nitrate and nitrite concentrations than our reference ponds, these factors had no discernible effects on tadpole performance under the ranges found across all pond types. None of the other parameters differed between the two groups of ponds, nor did they have any strong or obvious effects on tadpole performance or survival. Synthesis and applications. The results indicate that water quality alone is not responsible for the scarcity of amphibians in farmland areas of southern Sweden. To understand better the cause of their rarity, future studies should also focus on the quality of the terrestrial habitat surrounding the ponds and the metapopulation structure.