Forest canopy closure and juvenile emigration by pool-breeding amphibians in Maine

For amphibians that breed in temporary pools, juvenile emigration is an important life-history movement linking the aquatic habitat of larvae to the surrounding upland habitats occupied by maturing animals and adults. However, little is known regarding the habitat preferences and sensitivity to disturbance of newly metamorphosed amphibians. We examined whether selection occurs for closed-canopy forest conditions during emigration by using pitfall traps and drift fences to sample naturally occurring populations of wood frogs (Rana sylvatica) and spotted salamanders (Ambystoma maculatum) along recently created forest-clearcut edges in central Maine. Habitat preferences during emigration were further investigated via an experimental approach that quantified movements of newly metamorphosed wood frogs cultured in artificial pools located along an abrupt forest-power line edge. Among natural populations, the abundance of juveniles and adults of both species declined sharply across a gradient running from relatively mature forest-interior habitat (70-90 yr old) to recently clearcut habitat (2-11 yr old). Similarly, in the power line experiment, juvenile wood frogs showed an emigration preference for closed-canopy habitat immediately upon metamorphosis, with the highest capture rates occurring in microhabitats characterized by dense foliage in both the understory and canopy layers. Our results suggest populations of wood frogs, spotted salamanders, and other amphibian species breeding in temporary pools are likely to benefit from efforts to maintain connectivity between upland forest habitats and aquatic breeding sites. Seminatural field experiments provide a valuable manipulative approach for studying the potential effects of habitat fragmentation on populations of migratory amphibians.