Factors related to amphibian occurrence and abundance in headwater streams draining second-growth Douglas-fir forests in southwestern Washington

Abstract

Forested headwaters of the US Pacific Northwest are an important habitat resource for a varied amphibian fauna. Factors related to occupancy and relative abundance for many of these species are poorly known, adding uncertainty to conservation decisions in managed forestlands. We sampled occurrence and abundance of amphibians in 40 perennial headwater streams traversing 50–65-year-old second-growth forests in the coast range of southwestern Washington. Streams were divided among basalt ( n=18) and marine sediment ( n=22) lithologies. Our samples resulted in collections of 1141 amphibians of six taxa — three stream-breeders and three woodland salamanders. Stream-breeding taxa included larval and neotenic giant salamanders ( Dicamptodon spp.), Columbia torrent salamanders ( Rhyacotriton kezeri) and larval tailed frogs ( Ascaphus truei). Pacific giant salamanders ( D. tenebrosus) and/or Cope’s giant salamanders ( D. copei) occupied 95% of sampled streams, accounting for 57% of total amphibians collected. Streams traversing basalt lithology had almost twice the giant salamander abundance of marine sediment streams. Adjusting for lithology, giant salamanders increased in abundance with increasing pool frequencies in combination with increasing large woody debris (LWD) accumulations in adjacent riparian areas, and decreased with increasing accumulations of large (>60 cm diameter) woody debris in the channel. Torrent salamanders occupied 53% of sampled streams. The likelihood of habitat occupancy by torrent salamanders increased as channel gradient increased and basin area decreased. When adjusted for basin area, torrent salamander abundance increased as the proportion of the active channel with flowing water decreased, and at more northerly aspects. Larval tailed frogs larva were found in 13% of sampled streams, exclusively occupying basalt streams at elevations >300 m. At least one of the three species of woodland salamanders ( Plethodon spp.) occupied habitats adjacent to 93% of sampled streams. Western red-backed salamanders ( P. vehiculum) were most ubiquitous, occupying habitats adjacent to 85% of sampled streams. Dunn’s salamanders ( P. dunni) occupied habitats adjacent to 58% of sampled streams, likelihood of occurrence increasing with increasing gradient of the steepest sideslope. Van Dyke’s salamanders ( P. vandykei) occupied habitats adjacent to three streams, all of which traversed basalt lithologies on north-facing slopes. Our results suggest that habitat quality for headwater amphibians in this region is strongly influenced by landform characteristics, including basin lithology. These associations provide managers an opportunity to improve headwater amphibian conservation strategies by prioritizing stream segments with respect to their likely amphibian fauna.