Amphibians in drained forest landscapes: Conservation opportunities for commercial forests and protected sites

Forest drainage has led to the loss of a considerable number of wetlands, especially in the northern hemisphere. In order to find ways to mitigate the loss of biodiversity in drained forests, we investigated two distinct conservation measures: (i) leaving drained peatlands for natural succession, i.e. protecting without active restoration, and (ii) constructing a variety of mitigation pools during ditch maintenance work in commercial forests. We tested the effectiveness of these conservation approaches for wetland biota, selecting brown frogs (Rana arvalis, R. temporaria) as our focal species. We found that ditches do not substitute natural floods as breeding habitats for brown frogs in protected peatlands. One of the main reasons was the reduction of sun exposure due to drainage-induced forest growth. However, secondary wetlands formed on ditches impounded by Eurasian beaver (Castor fiber) offered high quality reproduction sites for brown frogs. In commercial forests, the number of natural pools decreased due to ditch maintenance work, but the colonisation rate of brown frogs increased in cleaned ditches. The reproduction site selection of the two frog species differed - R. arvalis bred mainly in natural and constructed pools, while R. temporaria bred more frequently in ditches. Among mitigation pools, those with a shallow littoral zone were primarily used for breeding. Thus, the conservation measures are effective only if certain key assumptions are met. The success could stand on the considering of the habitat requirements of target species before creating mitigation habitats or presence of a restoration agent (like beaver) in protected sites modified by human activities.