Thursday, August 6, 2009

PS 62-16: Multi-year study of adult and juvenile Wood Frog and American Toad migration orientations

Rebecca N. Homan, Meredith Atwood, Ashley Dunkle, and Sarah Karr. Denison University

Background/Question/Methods Wood Frogs (Rana sylvatica) and American Toads (Bufo americanus) are pond-breeding amphibian species that are commonly syntopic, but are thought to have different levels of habitat specificity. Understanding amphibian preferences among habitats that are all suitable but are of unequal quality may facilitate developing best management practices when some habitat loss is inevitable. To that end, we conducted a four-year study of Wood Frog and American Toad migration orientations at a breeding site surrounded by habitats of different presumed quality, but equal accessibility, to investigate whether adults and juveniles of these two species had non-random migration orientations, how any non-random patterns mapped onto the different habitats, and whether those orientations differed among species, life stages, and years.

Results/Conclusions We found that in all years, Wood Frog adults and juveniles had non-random migration orientation patterns (all p<0.001). Adult Wood Frogs were relatively consistent in their pattern of orienting with the greatest frequency from or toward the habitat consisting of relatively mature, extensive secondary forest with moderate canopy cover and moderately well drained soils, and orienting with the lowest frequency from or toward the habitat consisting of relatively younger, successional, more restricted forest, with high soil moisture, reduced canopy cover, and a road edge. Juvenile Wood frog patterns were inconsistent across years. American Toad adults and juveniles also had clear non-random migration orientation patterns in each year (all p<0.005), with the exception of adult inbound migration in 2005 (p>0.10), but the patterns were not consistent across years. Forest extent appears to be an important factor influencing adult Wood Frog migration patterns and abundance. By contrast, soil moisture appeared to be less important than we predicted as a factor influencing adult Wood Frog distribution. It remains unclear why juvenile Wood Frogs and juvenile and adult American Toad distributions displayed inconsistent patterns over time. Based on the significant, but variable, non-random migrations we found for three of the four life-stage/species combinations we studied, we suggest that multi-year studies may be crucial to fully understanding the migration patterns and habitat preferences of particular life stage and/or particular species.