Short-term relationship of timber management and Pacific giant salamander populations, and the response of larval stream amphibian to predators under differing sediment levels Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/gf06g712v

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  • In the Pacific Northwest, multiple studies have found negative effects of timber harvest on stream amphibians, but the results have been highly variable and region-specific. In this collection of studies, I examined the short term effect of timber harvest using a field study, and used lab work to examine a potential mechanism for timber harvest effect. Over the last 30 years forest management practices have changed substantially, yet little work examines how modern forest management relates to the abundance or density of stream amphibians. I examined the influences of contemporary forest practices on Pacific giant salamanders as part of the Hinkle Creek paired watershed study. I used a mark-recapture analysis to estimate Pacific giant salamander density at 100 1-m segments spread throughout the basin and then used extended linear models that accounted for correlation resulting from the repeated surveys at sites across years. Density was positively associated with substrate, negatively associate with upstream area drained, and had a weak positive association with fish density, but I found no evidence of an effect of harvest. A Monte Carlo analysis suggested that our results were not sensitive to missing captures at sites with no captures. Pacific Northwest stream amphibians are often negatively associated with sedimentation, but the mechanism underlying this relationship is not clear. One hypothesized mechanism is that the reduced interstitial space that results from sedimentation increases susceptibility of amphibians to predation. I used laboratory mesocosms to test this hypothesis and examine the response of larval Pacific giant salamanders and tailed frogs to cutthroat trout and adult Pacific giant salamander presence under three different levels of sediment. I found amphibian larvae were more visible as sediment level increased and some evidence that larvae were less visible in the presence of fish. Movement decreased in the presence of cutthroat trout, though for tailed frog larvae this effect was marginally significant (p = 0.066). Larvae did not respond to presence of adult Pacific giant salamanders. These patterns are consistent with the hypothesis that sediment affects larval stream amphibians by increasing vulnerability to predation. While both species of larvae actively sought cover in response to fish, I found little evidence that this behavior mitigates the effects of increasing sediment.
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