Abstract
Ultrasonic bat detectors are useful for research and monitoring purposes to assess occupancy and relative activity of bat communities. Environmental “clutter” such as tree boles and foliage can affect the recording quality and identification of bat echolocation calls collected using ultrasonic detectors. It can also affect the transmission of calls and recognition by bats when using acoustic lure devices to attract bats to mist-nets. Bat detectors are often placed in forests, yet automatic identification programs are trained on call libraries using echolocation passes recorded largely from open spaces. Research indicates that using clutter-recorded calls can increase classification accuracy for some bat species and decrease accuracy for others, but a detailed understanding of how clutter impacts the recording and identification of echolocation calls remains elusive. To clarify this, we experimentally investigated how two measures of clutter (i.e., total basal area and number of stems of simulated woody growth, as well as recording angle) affected the recording and classification of a synthesized echolocation signal under controlled conditions in an anechoic chamber. Recording angle (i.e., receiver position relative to emitter) significantly influenced the probability of correct classification and differed significantly for many of the call parameters measured. The probability of recording echo pulses was also a function of clutter but only for the detector angle at 0° from the emitter that could receive deflected pulses. Overall, the two clutter metrics were overshadowed by proximity and angle of the receiver to the sound source but some deviations from the synthesized call in terms of maximum, minimum, and mean frequency parameters were observed. Results from our work may aid efforts to better understand underlying environmental conditions that produce false-positive and -negative identifications for bat species of interest and how this could be used to adjust survey accuracy estimates. Our results also help pave the way for future research into the development of acoustic lure technology by exploring the effects of environmental clutter on ultrasound transmission.
Highlights
Acoustic recording of bats is a widely used monitoring tool for assessing occupancy and relative activity of bat communities
Our results were aligned with our prediction and previous studies that detector angle would play an important role in echolocation call quality received [24,25], our findings that stem density and basal area did not influence outcomes as much were contrary to our expectations based on previously reported field studies [6,11,20,39]
Recording quality and lure effective range is probably going to be relatively poor in a cluttered environment as compared to recording or broadcasting in open areas, researchers often are constrained to survey in cluttered habitat for clutter-adapted foraging species
Summary
Acoustic recording of bats is a widely used monitoring tool for assessing occupancy and relative activity of bat communities. Clutter can absorb or reflect ultrasonic signals, alter recorded parameters, create echoes, or block all or a portion of the signal [9,10,11,12,13] These factors could result in poor acoustic survey results with an unacceptable level of false positives or false negatives that may bias survey findings and misrepresent bat abundance and their habitat associations [14]. The ability to account for clutter in occupancy and relative activity modeling and acoustic lure deployment studies is critical in the post-WNS landscape where populations of impacted bat species are at unprecedented lows to the level that traditional mist-net capture techniques perform poorly [7]
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