Efficacy of visual encounter surveys for coastal tailed frog detection

Abstract

Coastal tailed frogs Ascaphus truei inhabit montane streams and forested habitats in the Coast and Cascade Mountains from northern California, USA, to the Skeena River watershed in northwestern British Columbia (BC), Canada. Terrestrial adults and juveniles of this cryptic biphasic species are difficult to survey as they are small, do not vocalize, and may be associated with woody ground structures or subsurface refugia at considerable distances from natal streams. We performed a comparative analysis of the detection rate of post-metamorphic coastal tailed frogs and ecological factors hypothesized to influence detection when conducting visual encounter and pitfall trap surveys. We conducted concurrent surveys in northwestern BC at six sites over similar time periods using both techniques. The average detection rate of visual encounter surveys (  = 0.249, SD = 0.702) was greater than that of pitfall sampling ( = 0.138, SD = 0.773) when cool temperatures and high humidity favor above-ground movement during the daytime. Light-touch ground searches of refuge habitats likely enhanced detection during visual surveys. Although the average detection rate was less, pitfall traps provided 24-hour sampling and were less affected by the experience of the surveyor and the occurrence of ground cover. In general, variation in seasonal behavior influenced detection regardless of method. The relatively higher cost and fixed nature of pitfall traps should be weighed against the ability to apply more cost-effective visual encounter surveys to a greater number of sites.