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Aquatic and terrestrial movements of tailed frogs (ascaphus truei) in relation to timber harvest in Coastal British Columbia Wahbe, Tanya Rhoda

Abstract

In British Columbia, Oregon, and California, coastal Ascaphus populations are designated 'at risk.' Local extirpation is a concern because recolonization may be slow, even if post-logging habitats recover quickly. In coastal BC streams flowing through clearcuts, second growth, and old growth, I investigated movements of larval Ascaphus and associations with stream parameters using area-constrained stream surveys. In old growth, larvae moved seven times farther than in clearcuts. Embedded logs, abundant in clearcut streams, may explain shorter larval movements. Using pitfall traps, I examined terrestrial movements of Ascaphus in clearcuts and old growth. More juveniles were trapped in clearcuts but more mature adults were trapped in old growth, suggesting fewer effective migrants in clearcuts. Many frogs moved at least 100 m from streams, and exhibited weaker stream affinity compared to inland Ascaphus that moved at least 12 m. In fall, I trapped frogs farther from streams in old growth than in clearcuts, and more frogs were trapped within 25 m of streams in clearcuts. Long distance overland movements appear more likely where forested stands are present. Using RAPDs, I examined population genetic structure of Ascaphus in an old growth and clearcut stream. In the clearcut, larvae were less diverse than in the old growth and exhibited no relationship between physical distance and genetic relatedness. In the old growth, larvae decreased in genetic similarity with increasing physical distance. Lower heterozygosity in the clearcut suggests that Ascaphus may be less able to adapt to environmental fluctuations and more susceptible to disease than larvae in the old growth. Maintaining viable populations throughout the range of Ascaphus is an underlying assumption of my thesis. My results suggest reduced recolonization potential and lower genetic variation where forest cover is absent. Aggregations of Ascaphus at individual streams may not represent distinct populations, and should not be managed as distinct units. Connectivity between multiple streams within a watershed will be a more meaningful unit of management than individual streams with forested buffers. Conservation measures more likely to promote long-term population persistence should be considered, such as the retention of a partial forest matrix between streams.

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