Estimating factors influencing the detection probability of semiaquatic freshwater snails using quadrat survey methods
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 semiaquatic snail, as a case study to test detection and accuracy issues surrounding quadrat searches. Quadrats (9 × 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-min visual searches for shells. The probability of detecting a shell when present was 67.4 ± 3.0%, but it decreased with the increasing litter depth and fewer number of shells present. The mean (± SE) observer accuracy was 25.5 ± 4.3%. Accuracy was positively correlated to the number of shells in the quadrat and negatively correlated to the number of times a quadrat was searched. The results indicate quadrat surveys likely underrepresent true abundance, but accurately determine the presence or absence. Understanding detection and accuracy of elusive, rare, or imperiled species improves density estimates and aids in monitoring and conservation efforts.
KeywordsWetland Spring Endangered species Survey Invertebrates Conservation evaluation
Funding for this research was provided by the U.S. Geological Survey (Cooperative Agreement No. G13AC00051). The authors thank F. Anaya, L. Clark, A. Godar, B. Johnson, K. Leuenberger, K. Metzger, J. Sanchez, F. Truetken, and B. Wadlington for their participation in this experiment. This manuscript benefited from the comments and suggestions provided by S. Fritts and M. Barnes. Phantom springsnail shells were provided by C. Funkhouser. The authors also thank Cooperating agencies for the Texas Cooperative Fish and Wildlife Research Unit and University of Hawaii system, Hawaii Department of Land and Natural Resources, the U.S. Geological Survey, Texas Tech University, Texas Parks and Wildlife, the Wildlife Management Institute, and the U.S. Fish and Wildlife Service. The use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
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