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


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  • Riparian areas in the Pacific Northwest provide important biotic and abiotic features, such as down wood, moist microsites, and abundant invertebrate prey that benefit aquatic and terrestrial amphibians. Reported high densities of amphibians from streams and riparian areas in the Pacific Northwest highlight their importance in riparian food webs. Amphibians provide an important trophic link between terrestrial and aquatic systems because they may exploit both terrestrial and aquatic prey and in turn they are prey for other vertebrates. In the Oregon Coast Range, riparian vegetation is often more diverse than upland vegetation and there is typically a considerable deciduous component. However, forestry regulations in Oregon require landowners to promote the growth of conifer over deciduous trees in riparian areas to benefit salmonids and other fishes. The goal of our research was to examine associations between the distribution and diets of aquatic and terrestrial amphibians and vegetation in second- and third-order Oregon Coast Range streams and riparian areas. Sites reflected overstory conditions in managed forests of the central Oregon Coast Range, ranging from primarily red alder (Alnus rubra) in the riparian zone to mostly Douglas- fir (Pseudotsuga menziesii). The two most common amphibians detected were the aquatic Pacific Giant Salamander larvae (Dicamptodon tenebrosus) and the terrestrial Western Redback Salamander (Plethodon vehiculum). Our first research objective was to identify habitat characteristics with an emphasis on riparian vegetation, that could be associated with the presence of these two species. We used logistic regression to examine the presence of these salamanders among sites and the information-theoretic approach using Akaike Information Criterion (AIC) methods to compare the strength of evidence of a set of candidate models formed from a priori hypotheses. The highest ranked model explained 95% of the variability of the presence of Dicamptodon tenebrosus larvae among sites and included variables representing percent cover of Douglas- fir, elevation, amount of wood cover, and lithology type. The odds of the presence of D. tenebrosus increased with the percent cover of Douglas- fir over the wetted width of the stream. Two competitive models (<2 AICc units) explained 54% of the variability in the presence of Plethodon vehiculum among sites and included variables for percent cover of western hemlock (Tsuga heterophylla), percent total overstory cover, elevation, and lithology type. The odds of observing P. vehiculum at a site increased with the percent cover of western hemlock in the riparian area. Our second research objective was to describe diet-habitat relationships, emphasizing riparian vegetation. We evaluated a candidate set of models using generalized linear least squares regression and AIC methods; we used a measure of relative stomach fullness, based on the amount of material in a stomach sample after accounting for the size of an individual, as the response variable. For both salamander species, models that included an variables representing canopy cover and stream or forest floor characteristics were among the highest ranked models. The stomach fullness of D. tenebrosus larvae was positively associated with the percent cover of western hemlock (Tsuga heterophylla) over the stream channel. P. vehiculum stomach fullness increased as the percent cover of red alder in the riparian zone increased. D. tenebrosus most frequently consumed benthic macroinvertebrates, including larval Diptera, Ephemeroptera, Plecoptera, and Trichoptera. Collembola numerically dominated P. vehiculum stomach samples; however Gastropoda, predominantly snails, and Isopoda were eaten in greater proportion than their relative availability. Altering vegetation may potentially impact amphibians in riparian areas by changing the thermal regime, microhabitats, or prey availability. Conifers moderate forest floor microclimate year round and deciduous trees provide high quality prey resources. Our results suggest that retaining both conifers and deciduous trees in the riparian zone of small streams in the Oregon Coast Range may benefit both D. tenebrosus and P. vehiculum.
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