Detection probabilities and sampling rates for Anisoptera exuviae along river banks: influences of bank vegetation type, prior precipitation, and exuviae size

Abstract

Exuviae collections have considerable value in population studies of Odonata, but methods for standardizing collections or estimating densities and detection probabilities have been little studied. I measured sampling rates for Anisoptera exuviae and used a maximum likelihood, four-pass, depletion population estimator to standardize collections and to estimate exuvial densities and detection probabilities along 10 riverbank stations in Wisconsin. First-pass sampling rates averaged slower than the overall average for experienced collectors (0.53 m min^–1 compared to 0.90 m min^–1) because more exuviae were present on the first pass, increasing picking and handling time. Neither bank vegetation type (grassy versus forested) nor amount of prior precipitation affected sampling rate. Exuviae detection probabilities for a single pass ranged from 0.49 to 0.75, and averaged 0.64. The mean cumulative probability of detection increased to 0.87 after two passes, 0.95 after three passes, and 0.98 after four passes. A strong negative relationship existed between detectability and the amount of prior precipitation. Bank vegetation type did not affect detection probability. Smaller exuviae had an 8% lower probability of detection than larger exuviae. If four sampling passes are cost-prohibitive for some exuviae studies, making just two passes may provide an adequate estimate of sampling efficiency. The assumption that exhaustive collecting efforts will find all or most of the exuviae along vegetated natural banks is unfounded.

Keywords: Odonata, dragonfly, Anisoptera, exuviae, detection probability

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