Comparative Effectiveness of Three Techniques for Salamander and Gastropod Land Surveys
McDade, Kirsten A., Maguire, Chris C., The American Midland Naturalist
We compared the effectiveness and efficiency of three terrestrial salamander and gastropod trapping techniques: pitfall traps, ground searches and cover boards. The study was conducted on 18 stands with three management histories in the Umpqua National Forest, southern Oregon Cascades. A total of 648 pitfall traps were open for 28 consecutive days in fall 1999. Two hundred twelve amphibians (eight species) and 202 gastropods (six species) were captured. Also in fall 1999, 36 h of ground searches covering 3600 m^sup 2^ resulted in the detection of 19 amphibians (two species) and 130 gastropods (six species). Four cover boards (100 × 100 cm) in stacks of two were placed in each stand and checked four times in fall 1999 and once in spring 2000 after snow melt. Cover boards concealed no amphibians and only two gastropods (one species). Pitfall traps were more efficient at capturing amphibians than ground searches (0.41 vs. 0.25 captures per hour of effort), but less efficient at capturing gastropods than ground searches (0.39 vs. 1.73 captures per hour of effort). Cover boards as used were ineffective at capturing either amphibians or gastropods. Climatic conditions of the southern Oregon Cascades likely influenced the results.
In the Pacific Northwest, two methods commonly are used to sample terrestrial salamanders: pitfall trapping and ground searches. Cover boards, a third sampling method, have been used sparingly in the Northwest, despite their success in the eastern United States (DeGraaf and Yamasaki, 1992; Harpole and Haas, 1999; Monti et al., 2000; Hyde and Simons, 2001; Jaeger et al., 2001).
There are many pitfall trap variations, but generally a pitfall consists of a deep depression in the ground that entraps animals that fall into it and restricts their escape. Pitfall traps have been used to estimate the seasonal activity, reproductive status and abundance of species (e.g., Campbell and Christman, 1982; Corn and Bury, 1990). There is evidence, however, that estimates of salamander abundance derived from pitfall trapping are biased (Corn and Bury, 1991; Heyer et al., 1994, p. 75-141). Sub-surface activity, arboreal junkets (see Nussbaum et al., 1983, p. 11-25) and sedentary behavior all decrease the probability of capture. Consequently, trappability differs widely among species (Campbell and Christman, 1982; Bury and Corn, 1987; Corn and Bury, 1990).
Ground searches entail actively probing for salamanders on the forest floor within a defined area (area-constrained search), over a defined time period (time-constrained search) or a combination of both (time- and area-constrained search) (Welsh, 1987; Corn and Bury, 1990; Heyer et al., 1994, p. 75-109). Searches are performed during the night or day and vary in intensity from "low" to "high" depending on the amount of forest floor disruption (Heyer et al., 1994, p. 84-92). Ground searches provide information on salamander presence/absence and microhabitat use and they commonly are used for inventory purposes, e.g., as directed in the Survey and Management Provision of the Northwest Forest Plan (Corn and Bury, 1990; Oison, 1999). Similar to pitfall trapping, however, ground searches are inappropriate for abundance estimation because of possible search bias (Corn and Bury, 1990). Not only are salamanders difficult to detect in structurally complex environments, but detection efficiencies also may differ among investigators.
Cover boards are wooden boards of various dimensions that are placed on the forest floor to simulate natural down wood. Several studies have effectively used cover boards to capture salamanders (DeGraaf and Yamasaki, 1992; Davis, 1998; Harpole and Haas, 1999; Monti et al., 2000; Hyde and Simons, 2001; Jaeger et al., 2001). Because boards can be checked repeatedly without forest floor disruption and they require less effort to search than many natural forest floor objects, they have been proposed for long-term monitoring of terrestrial amphibian populations (Jung et al. …