Amphibian assemblages in zero-order basins in the Oregon Coast Range

Abstract

Zero-order basins, extending from ridgelines to the initiation of first-order streams, were sampled in the Coast Range of Oregon to (i) characterize spatial distribution patterns of amphibian species and assemblages along longitudinal and lateral gradients, and relative to three geomorphic surfaces (valleys, headmost areas, and slopes); and (ii) develop empirical species–habitat models. Unmanaged zero-order basins were hotspots for amphibian diversity, with significant differences across geomorphic gradients. Captures of riparian-associated amphibians were higher in valley areas, usually within 2 m of basin center. Upland-associated amphibians were captured two times farther from basin centers than riparian-associated species, but highest densities occurred only 2–5 m from basin center. The most useful empirical models related captures of individual amphibian species to geomorphic, disturbance, moisture, and overstory variables. Ordination and indicator species analysis characterized geomorphic and other environmental gradients in amphibian assemblages and suggested spatial compression of fluvial habitats and riparian-associated species in zero-order basins, in comparison with downstream areas. Our findings have implications for headwater areas managed to hedge risk to and uncertainty in amphibian persistence, namely in the delineation of zones with species management priority, and in the maintenance of natural fluvial and hillslope disturbance regimes, along with the microhabitat features created by these regimes.