Vegetation patterns and abundances of amphibians and small mammals along small streams in a Northwestern California watershed
Welsh, H.H., J.r
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Our goal was to describe and evaluate patterns of association between stream size and abundances of amphibians and small mammals in a watershed in northwestern California, USA. We sampled populations at 42 stream sites and eight upland sites within a 100-km2 watershed in 1995 and 1996. Stream reaches sampled ranged from poorly defined channels that rarely flowed to 10-m-wide channels with perennial flow. The majority of reaches flowed only intermittently. Aquatic vertebrates were sampled by conducting area-constrained surveys, and terrestrial vertebrates were sampled along three 45-m-long transects using cover boards, drift fence/pitfall trap arrays, and two types of live trap. Vegetation characteristics (including basal area of live trees by species, ground cover, shrub cover and numbers of logs) were strongly associated with measures of stream size, especially channel width. Compared to upland sites, mean numbers of plant species in the herbaceous layer were significantly greater along streams with active channel widths as small as 0.9-1.3 m. Larval Pacific giant salamanders (Dicamptodon tenebrosus) were found only in stream reaches with continuous flow or in channels >=2.4-m wide, and larval tailed frogs (Ascaphus truei) were found only at sites with continuous or nearly continuous flow. Allen's chipmunks (Tamias senex) and deer mice (Peromyscus maniculatus) occurred at nearly every site sampled but were more abundant at reaches along larger streams than at reaches along smaller streams or at upland sites. None of the vertebrate species evaluated was significantly associated with intermittent streams having channels less than about 2-m wide and drainage areas less than about 10 ha. Our results provide additional information on the ecological role of small, intermittent streams.