Movement and habitat use of the pool frog (Pelophylax lessonae) in Sweden: gaining ecological insights to improve forest management practices

Abstract

The pool frog (Pelophylax lessonae) has a limited distribution in Scandinavia and is red-listed in Sweden. Most of the ~120 Swedish localities are concentrated along a limited coastline in south-central Sweden. The major threat to the species is loss of suitable habitat, e.g. due to forest drainage ditching and clear cutting. Understanding important pathways between local subpopulations (ponds) is important for conservation and landscape management. To improve forest management practices around populated ponds, knowledge on habitats used for dispersal and hibernation is crucial.

We conducted a landscape connectivity analysis for the main region of the Swedish populations, based on presence/absence data on populated ponds from a survey in 2009; and a cost raster based on habitat data, forest data and a wetness index. To verify these theoretical dispersal routes, we performed radio-telemetry tracking of 43 individuals around four ponds, during the summer and autumn of 2017. As most frogs did not move away from the pond, translocations 500 m into localities connected with the ponds were conducted for 20 individuals. Six individuals were followed to their hibernation sites. Home range of individuals was estimated by utilization distributions which were calculated with the assumption of biased random walks. We determined habitat use preference with GLMMs of usage/available habitat using forest data and wetness indices.

Most individuals moved according to theory, in distinct moves along the wet habitat strings predicted from connectivity analysis. Translocated individuals moved quickly to suitable water bodies and remained there, often showing homing behavior. Habitat use of these individuals showed preference for open water bodies and open wetlands. Pine forests were avoided, while proximity to streams and distance from forest favored habitat usage. The theoretical dispersal routes (in low cost areas from the connectivity analysis) were used more often than high cost areas, verifying their relevance for conservation planning and landscape management. Hibernation sites were not located in burrows, as previously suggested, but instead directly in litter on the forest floor. All individuals hibernated on solid ground, within 250 meters from breeding ponds.

To maintain functional connectivity we suggest that also stream networks that interconnect neighboring ponds and have low presence of pine, should be preserved as dispersal corridors. Around the breeding ponds, hibernation sites at up to 250 meters should be considered during management activities. This project exemplifies the potential of combining theoretical analyses and practical field studies to improve our understanding of the requirements of a threatened species. Furthermore, it shows how an adaptive management approach and cooperation between forestry owners and managers, authorities, nature conservation consultancies and researchers may improve management and conservation practices.