Graduate Thesis Or Dissertation

The influence of forest management on headwater stream amphibians at multiple spatial scales

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  • To effectively manage for biodiversity at broad, ecosystem scales, the influences of habitat structure at multiple spatial scales on vertebrate species must be understood. There are few studies on the broad-scale habitat requirements of stream amphibians despite their importance in streams in forest ecosystems in the Pacific Northwest (PNW) as predators and prey, and potentially as indicators of ecosystem health. In particular, studies on the influence of forest structure at landscape scales on stream amphibians are lacking. I examined stream amphibian-habitat relationships for Pacific giant salamanders (Dicamptodon tenebrosus), larval and metamorphosed tailed frogs (Ascaphus truei), and torrent salamanders (Rhyacotriton spp.) at four spatial scales (2-m sample unit, intermediate, sub-drainage, and drainage). Over two field seasons (1998 and 1999), I captured 1,568 amphibians in 702 sample units in 16 randomly chosen drainages in the Oregon Coast Range. I used an information theoretic approach of analysis to rank sets of a priori candidate models that described habitat relationships at each spatial scale. At the 2-m sample unit scale, all species of interest were negatively associated with fine sediments and were positively associated with either stream width or elevation. At the intermediate spatial scale, Pacific giant salamanders, metamorphosed tailed frogs, and torrent salamanders were positively associated with the presence of a 150-ft. forested band on each side of the stream, and larval tailed frogs were positively associated with the presence of forest >105 years old on at least one side of the stream. At the sub-drainage and drainage scales, all species were positively associated with the proportion of stream length in a sub-drainage or drainage with a 150-ft. forested band on each side of the stream. Heat load index (aspect) was also important for Pacific giant salamanders and larval tailed frogs at the intermediate and sub-drainage scales. Results at all spatial scales suggest that Pacific giant salamanders and larval tailed frogs occur lower in the drainage network, and metamorphosed tailed frogs and torrent salamanders occur higher in the drainage network. This study demonstrates the importance of examining headwater stream amphibian habitat at multiple spatial scales, provides insights on linkages between amphibian responses across spatial scales, and shows that broad-scale variables (e.g., the presence of forested bands or the percentage of stream length with forested bands) can be used to assess management approaches for stream amphibian communities. Geophysical characteristics such as stream aspect may also help identify areas that should not be harvested if protection of amphibian habitat is an objective.
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