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Roesler EL, & TB Grabowski. 2018. Estimating factors influencing the detection probability of semi-aquatic freshwater snails using quadrat survey methods. Hydrobiologia 808:153-161. doi: 10.1007/s10750-017-3415-9


Developing effective monitoring methods for elusive, rare, or patchily distributed species requires extra considerations, such as imperfect detection. Although detection is frequently modeled, the opportunity to assess it empirically is rare, particularly for imperiled species. We used Pecos assiminea (Assiminea pecos), an endangered semi-aquatic snail, as a case study to test detection and accuracy issues surrounding quadrat searches. Quadrats (9 x 20 cm; n = 12) were placed in suitable Pecos assiminea habitat and randomly assigned a treatment, defined as the number of empty snail shells (0, 3, 6, or 9). Ten observers rotated through each quadrat, conducting 5-minute visual searches for shells. The probability of detecting a shell when present was 67.4% ± 3.0, but decreased with increasing litter depth and fewer shells present. The mean (± SE) observer accuracy was 25.5% ± 4.3%. Accuracy was positively correlated to number of shells in the quadrat and negatively correlated to number of times a quadrat was searched. The results indicate quadrat surveys likely underrepresent true abundance, but accurately determine presence or absence. Understanding detection and accuracy of elusive, rare, or imperiled species improves density estimates and aids in monitoring and conservation efforts.


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February 2018

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  1. U.S. Geological Survey