UBC Theses and Dissertations
Tailed frogs (Ascaphus truei, Stejneger) in natural and managed coastal temperate rainforests of southwestern British Columbia, Canada Wahbe, Tanya Rhoda
Ten headwater creeks were surveyed for tailed frog tadpoles in the summers of 1994 and 1995 to assess possible effects of timber harvest on their population density and structure. Three watersheds were selected to replicate three treatments: old-growth (250+yrs), mature second growth (60-80yrs) and young clearcut (~5yrs). One buffered clearcut creek was located and sampled but not replicated. Creek and site characteristics modified the response forest harvest had on tailed frog tadpole populations. Effects on tailed frog tadpoles associated with forest harvest depended on elevation, creek size, percent sand cover and percent riffle cover. Higher tadpole biomass per unit area and density were associated with high elevation sites, small creeks and low percent cover of riffles. Both average tadpole mass and snout-vent length were found to increase with increasing percent sand cover. Tadpoles appear to select microhabitats of pool, run or riffle depending on their body size or stage of development. Higher tadpole densities are also associated with lower creek temperatures. Old-growth and buffered clearcut creeks showed highest biomass per unit area and density of tailed frog tadpoles. Old-growth creeks also exhibited more tadpole cohorts (5 'year-classes'), higher tadpole biomass per unit area but lower average mass than clearcuts. Low (3 m maximum) tadpole movements were detected in clearcut creeks, while longer distance (65m maximum) movements were detected within old-growth creeks. Harvesting history appears to be an indirect measure of the factors influencing the population dynamics of tailed frogs, whereas aspects of microhabitat, which can vary in parallel with forest age, are probably the true limiting factors. Structural features of old-growth forests are also influenced by gradient, elevation and disturbance history. In my study, more than 85% of the variation in tadpole biomass per unit area and tadpole density was associated with creek and site characteristics. History of forest disturbance only had a major impact under specific conditions. When managing for tailed frogs, the variability of creek and site characteristics must be considered prior to making harvesting decisions.
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