|Gruber, B; Reineking, B; Calabrese, JM; Kranz, A; Polednikova, K; Polednik, L; Klenke, R; Valentin, A; Henle, K: A new method for estimating visitation rates of cryptic animals via repeated surveys of indirect signs, Journal of Applied Ecology, 45(2), 728–735 (2008), doi:10.1111/j.1365-2664.2007.01406.x|
|Key words: Eurasian otter, Lutra lutra, Mammalia: monitoring, maximum-likelihood estimate, track counts, visitation probability, visitation rate|
1. Visitation rate is a measure of the frequency with which animals visit specific locations of interest. This information is crucial for various problems in the conservation and management of animal species, e.g. to determine the risk wildlife poses to human managed resources or the predation risks of nests. An important assessment tool, especially for cryptic animals, is to count indirect signs of presence, such as tracks or faeces. 2. Here we propose a maximum likelihood-based method that uses information on the age of tracks or signs, and we show that existing visitation rate/probability estimators are special cases of this more general approach. 3. Using simulated data, we compared the performance of the new visitation rate estimator to three other estimators, including the most commonly used estimator. These estimators make use of either fresh, aged or total signs, whereas our approach uses information simultaneously on both fresh and aged signs. The new estimator is, on average, in excess of three times more accurate than the next best estimator. Moreover, the new approach is very flexible and can be applied for sampling regimens with irregular time intervals between sampling. 4. We demonstrate the application of the method to field data by estimating the visitation rate of Eurasian otters (Lutra lutra) to a commercial fish pond. To facilitate the use of this method, we provide an easy-to-use Excel workbook and give recommendations on the most efficient sampling regimens. 5. Synthesis and applications. Visitation rate is an important quantity that can be estimated by repetitive sampling of indirect signs. We demonstrate the advantage of incorporating information explicitly on the age of signs over existing approaches. The new estimator can be applied to any species for which it is possible to discriminate between aged and new signs or tracks, and should be widely applicable in ecology and conservation biology.