The Herpetological Journal

Authors: Walker, Ryan C.J.

Abstract: Understanding the range of threatened species is important for developing sound conservation initiatives. However, different survey methods can yield varying results when applied to cryptic vertebrates. Here, I established detection probabilities and compare detection rates of time-dependent searches against line-transect sampling for a rare, small and cryptic spider tortoise (Pyxis arachnoides) in the coastal dry forests of southwest Madagascar. The detection probability was 1.00 for field surveys undertaken during periods of highest tortoise activity. Significant differences in mean detection rates of 4.15 and 2.29 tortoises per man hour were recorded for time-constrained searching and line-transect sampling, respectively. Only time-constrained searches detected tortoises at all survey sites. There was no size-dependent variation in tortoise detection for either method. A GIS-based spatial model revealed that 12.54% of the range detected through timed searching would have been missed if transect sampling alone was applied. Higher detection rates for the timed search method are probably a result of surveyors applying greater effort to the species preferred microhabitat. Dependent on the desired output of the study, time-dependent searches or a combination of time-dependent searching and linear transect sampling is suggested.

Keywords: PYXIS ARACHNOIDES, LINE-TRANSECT SAMPLING, CRYPTIC SPECIES, ARID FOREST ENVIRONMENTS, DISTRIBUTION MAPPING, TIME-CONSTRAINED SEARCH