Riparian vegetation and larval Pacific Giant (Dicamptodon tenebrosus) and adult Western Redback (Plethodon vehiculum) salamanders in the Oregon Coast Range

Graduation date: 2007Riparian areas in the Pacific Northwest provide important biotic and abiotic\ud features, such as down wood, moist microsites, and abundant invertebrate prey that\ud benefit aquatic and terrestrial amphibians. Reported high densities of amphibians from\ud streams and riparian areas in the Pacific Northwest highlight their importance in riparian\ud food webs. Amphibians provide an important trophic link between terrestrial and aquatic\ud systems because they may exploit both terrestrial and aquatic prey and in turn they are\ud prey for other vertebrates. In the Oregon Coast Range, riparian vegetation is often more\ud diverse than upland vegetation and there is typically a considerable deciduous\ud component. However, forestry regulations in Oregon require landowners to promote the\ud growth of conifer over deciduous trees in riparian areas to benefit salmonids and other\ud fishes. The goal of our research was to examine associations between the distribution\ud and diets of aquatic and terrestrial amphibians and vegetation in second- and third-order\ud Oregon Coast Range streams and riparian areas. Sites reflected overstory conditions in\ud managed forests of the central Oregon Coast Range, ranging from primarily red alder\ud (Alnus rubra) in the riparian zone to mostly Douglas- fir (Pseudotsuga menziesii). The\ud two most common amphibians detected were the aquatic Pacific Giant Salamander larvae\ud (Dicamptodon tenebrosus) and the terrestrial Western Redback Salamander (Plethodon\ud vehiculum).\ud Our first research objective was to identify habitat characteristics with an\ud emphasis on riparian vegetation, that could be associated with the presence of these two\ud species. We used logistic regression to examine the presence of these salamanders\ud among sites and the information-theoretic approach using Akaike Information Criterion\ud (AIC) methods to compare the strength of evidence of a set of candidate models formed\ud from a priori hypotheses. The highest ranked model explained 95% of the variability of\ud the presence of Dicamptodon tenebrosus larvae among sites and included variables\ud representing percent cover of Douglas- fir, elevation, amount of wood cover, and\ud lithology type. The odds of the presence of D. tenebrosus increased with the percent\ud cover of Douglas- fir over the wetted width of the stream. Two competitive models (