Draining wetlands to increase timber production is a common practice in Asia, Europe, and North America. It results in widespread, permanent bodies of water being replaced by a linear ditch system – a unique spatial arrangement that creates a homogenous forest with a dense network of water corridors. Aquatic and semi-aquatic species in the drained forest often have no other options but to utilize the ditches, even though the ditches are prone to summer drying.
As a result, maintaining species diversity in such a forest, particularly amphibian diversity, means active management of the ditch network. New research by Soomets et al. gives insight into ways to improve habitat quality for amphibians in drained forests through simple brush removal.
The study focuses on two species of brown frog (Rana arvalis and Rana temporaria) that frequently breed in drainage ditches in a pine wetland-open mire forest of southwestern Estonia. Across 32 overgrown ditch sites, half were cleared of overgrowth and nearby trees that blocked the ditch and the other half were left unmanipulated. Ditch sections were sampled for frogs and habitat structure pre-manipulation as well as the spring following manipulation.
The results show a clear pattern: sites where ditches had been cleared increased the numbers and recruitment of breeding brown frogs. The reason adults were more likely to select cleared ditches is almost certainly because the increased sunlight helped warm the water in the ditches. The frogs switched their preferred habitat in response to the removal of shade rather than any changes in water levels, and this switch was followed by successful breeding.
Since linear ditches in drained forests represent the only reliable source of breeding habitat for frogs, improving their quality is the best way to keep frogs around. Keeping ditches sun-exposed may mitigate the negative effects of forestry drainage to a small extent, although continued management is necessary to keep the ditches clear. Even though the ditch corridor system is not a natural way to keep amphibians connected and breeding, simple management can ensure that individuals still have options in a heavily modified landscape.
Soomets, E., A. Lohmus, and R. Rannap. 2017. Brushwood removal from ditch banks attracts breeding frogs in drained forests. Forest Ecology and Management 384: 1-5.