Balestrieri Alessandro, Remonti Luigi, Prigioni Claudio
Department of Animal Biology, Pavia University, Botta Square 9, 27100, Pavia, Italy.
Naturwissenschaften. 2011 Jan;98(1):23-31. doi: 10.1007/s00114-010-0737-0. Epub 2010 Nov 16.
False negative detections may bias the surveys for rare species and reduce the reliability of models based on the proportion of occupied patches. We assessed the detectability of the Eurasian otter Lutra lutra through the standard survey method by analysing the detection history of 28 sampling stretches surveyed monthly between March 2001 and January 2003. Each survey negative for otter spraints was considered as a false negative if the otter had been recorded in the previous and/or following month (respectively, cFN and FN). Otter marking intensity (MI) (MI=N° of spraints per kilometre) was calculated and assumed to represent an index of its relative abundance. Spraints were found in 81.7% of all surveys. Yearly MI ranged from 1.02 to 101.4 spraints per kilometre. In 2002, mean MI was significantly lower than in the previous year, while no clear seasonal trend could be outlined. The minimum number of surveys required to establish the occurrence of the otter, as estimated by a probability model, was 2.6 and was inversely related to MI. For a sub-sample of 18 sampling stretches, the relation between the frequency of both cFN and FN and five variables of potential interest for otters was tested by means of stepwise linear multiple regressions, yielding two highly significant models, which both included only MI as the explanatory variable. The frequency of both FN and cFN was correlated to MI and the resulting equations used to assess the percentage of surveys positive for otters in both years. After the correction for non-detections, otter site occupancy did not vary between the 2 years, except for one river when applying the more conservative estimate of false negatives (cFN). Multiple visits and the assessing of MI should become standard components of otter surveys. This approach has broad applicability and may be applied to assess the large-scale distribution of other rare or elusive mammalian carnivores.
假阴性检测可能会使珍稀物种的调查产生偏差，并降低基于占据斑块比例的模型的可靠性。我们通过标准调查方法，分析了2001年3月至2003年1月期间每月调查的28个采样河段的检测历史，评估了欧亚水獭（Lutra lutra）的可检测性。如果在前一个月和/或接下来的月份（分别为连续性假阴性和假阴性）记录到水獭，那么每次未检测到水獭粪便的调查都被视为假阴性。计算了水獭标记强度（MI）（每公里粪便数量），并假定其代表相对丰度指数。在所有调查中，81.7%发现了水獭粪便。每年的MI范围为每公里1.02至101.4个粪便。2002年，平均MI显著低于上一年，同时未发现明显的季节性趋势。根据概率模型估计，确定水獭出现所需的最少调查次数为2.6次，且与MI呈负相关。对于18个采样河段的子样本，通过逐步线性多元回归测试了连续性假阴性和假阴性的频率与水獭潜在感兴趣的五个变量之间的关系，得出了两个高度显著的模型，这两个模型都仅将MI作为解释变量。假阴性和连续性假阴性的频率均与MI相关，所得方程用于评估这两年中水獭阳性调查的百分比。在对未检测情况进行校正后，除了一条河流在应用更保守的假阴性估计（连续性假阴性）时，两年间水獭的栖息地占有率没有变化。多次访问和MI评估应成为水獭调查的标准组成部分。这种方法具有广泛的适用性，可用于评估其他珍稀或难以捉摸的哺乳动物食肉动物的大规模分布。
