The ecology of anurans in a wetland system in north-east Scotland and implications for predation by otters (Lutra lutra)
The present study is an investigation into Eurasian otter (Lutra lutra) predation on the adult phase of the life-cycle of two amphibians, the common frog (Rana temporaria) and the common toad (Bufo bufo). The study aimed to determine whether otters showed preys selection when foraging on these species. The implications for the population dynamics of these species were examined. Initially, frog spawning population size and habitat use during the breeding season were examined. Although the number of females using each individual spawning site varied between years, the overall spawning population size appeared relatively stable over the course of the study. A number of interacting habitat variables were found to influence spawning site choice, the most important of these being the water body surface area, presence of toads, presence of loch edge and water flow. The migrations to and from the spawning sites and length of stay at the spawning sites were also examined in detail. Common frogs were found to have an earlier breeding season than common toads. Males were present at the breeding site for longer periods than females in both species. In general, a female's body length did not influence length of stay at the breeding site, but individuals which arrived at the site earlier tended to be present for longer. Habitat use within the summer home range was also examined for the two species of anuran. Both species were found in all habitats but relative abundance of the two species varied between habits. Toads occurred most frequently in birch woodland and rough-grazed fields, while frogs were most abundant in bogs. The species and sexes captured were strongly dependent on the capture techniques used. Key bones for identifying the remains of both species in otter spraints were then determined to enable the study of otter diet.