Distance sampling: Comparing walked transects and road transects for rock ptarmigan densities and population trends

We compared population trends for rock ptarmigan (Lagopus muta) densities
(2003‒2019) derived from walked transects and driven road transects in
Mosfellsheiði and Slétta in southwest and northeast Iceland, respectively.
The walked transects were laid out according to a random rule.
Convenience-based road transects could give biased population density
estimates if roads affect the distribution of ptarmigan. We used distance
sampling to compare density estimates provided by the two survey types.
Our results showed that road transects were more than five times faster to
conduct than walked transects. Estimated ptarmigan density changed in
synchrony for the two survey methods in both study areas. Mean density
estimates in Mosfellsheiði were similar for the two survey methods (walked
transects 1.6 males × km-2, 95% CI 1.4‒1.8; road transects 1.7 males ×
km-2, 95% CI 1.4‒2.0), but not in Slétta, where density estimates for road
transects were significantly lower (walked transects 5.2 males × km-2, 95%
CI 4.7‒5.7; road transects 3.2 males × km-2, 95% CI 2.8‒3.7). Density
estimates from the Slétta road transects were biased low because parts of
the road intersected areas that ptarmigan did not occupy. This bias was
remedied, at least partially, by considering the area of non-habitats
within the surveyed belt by applying multipliers in the distance analysis.
Collectively, our results demonstrate that road-based surveys and distance
sampling can provide an economical means for estimating density and annual
population trends for open country grouse (Tetraonini) like ptarmigan.
Still, density estimates can be biased without proper consideration for
survey design.

本研究对比了2003-2019年冰岛西南部莫斯费尔斯海迪（Mosfellsheiði）与东北部斯莱塔（Slétta）区域内，由步行样线（walked transects）和驾车道路样线（driven road transects）得到的岩雷鸟（rock ptarmigan，*Lagopus muta*）种群密度变化趋势。其中步行样线按照随机原则布设。若道路影响岩雷鸟的分布，基于便利性布设的道路样线可能会导致种群密度估计出现偏差。本研究采用距离抽样（distance sampling）方法，对比两种调查方式得到的密度估计结果。

研究结果显示，道路样线调查的开展速度较步行样线快五倍以上。两个研究区域中，两种调查方法得到的岩雷鸟密度估计值均呈现同步变化趋势。莫斯费尔斯海迪区域的两种调查方法得到的平均密度估计值较为接近（步行样线：1.6只雄性·km⁻²，95%置信区间（95% CI）1.4~1.8；道路样线：1.7只雄性·km⁻²，95%置信区间1.4~2.0），但斯莱塔区域并非如此：道路样线得到的密度估计值显著偏低（步行样线：5.2只雄性·km⁻²，95%置信区间4.7~5.7；道路样线：3.2只雄性·km⁻²，95%置信区间2.8~3.7）。

斯莱塔区域道路样线的密度估计值偏低，是因为部分道路穿过了岩雷鸟未栖息的区域。通过在距离抽样分析中引入修正系数，将调查样带内非栖息地面积纳入考量，该偏差至少得到了部分修正。综合来看，本研究结果表明，基于道路的调查法结合距离抽样法，可为岩雷鸟这类开阔地带松鸡（Tetraonini）的种群密度及年度种群趋势估算提供一种经济高效的手段。但如果未对调查设计进行合理考量，密度估计值仍可能存在偏差。