Distance sampling: comparing walked transects and road transects for
rock ptarmigan densities and time series
Abstract
We compared times series for rock ptarmigan densities (20032019)
derived from walked transects and road transects in Mosfellsheiði and
Slétta in southwest and northeast Iceland, respectively. The walked
transects were laid out according to a random rule, but the road
transects were not. Data from both survey types were analysed according
to the distance sampling methodology. The purpose was to compare results
derived from the two survey types. Road transects are easier to conduct
than walked transects, but they break a fundamental rule of distance
sampling, namely random placement of transects. Our results show that
road transects are more than five times faster to conduct than walked
transects. There were no indications that ptarmigan avoided roads.
Population indices were similar for the two survey methods in both study
areas. Density estimates in Mosfellsheiði were identical for the two
survey methods but not in Slétta, where density estimates for road
transects were lower. The Slétta road transects were biased low because
parts of the road transect intersected habitat that ptarmigan do not
occupy. This bias can be remedied at least partially by considering
non-habitats by applying multipliers in the distance analysis.