Survey coverage, recording duration and community composition affect observed species richness in passive acoustic surveys

Abstract

Abstract Bioacoustic assessments of species richness are rapidly becoming attainable, but uncertainty regarding the optimal acoustic survey design remains. Selecting the duration of recording and the number of recording units are critical decisions, and we used both simulated and empirical data to quantify the trade‐offs those choices present. We evaluated the performance of 30 hypothetical acoustic survey designs (e.g. continuous recording, every other 5 min, etc.). Simulated bird species' (n ≤ 60) abundance across the study area, probability of daily availability and time‐dependent probability of vocal activity varied randomly within ranges of realistic values. Field data, collected in central New York, USA (747 hr) and in the northern Sierra Nevada, USA (1,090 hr), was analysed with a novel machine‐learning algorithm, BirdNET. All three datasets were subsampled at 5‐min intervals, observed species richness was compared across survey designs, and detection probability was calculated for each species. Observed species richness increased with survey coverage (number of recording units) and with recording duration in all three datasets. The impact of differences in survey coverage decreased as recording duration decreased. Species' detection probabilities were negatively affected by reducing the number of days of recording and by reducing the daily recording duration. The more rare species a community had, the more species richness was underestimated as survey coverage decreased. Rarefaction curves indicated that increasing recording time has diminishing marginal utility but that the asymptote varies among communities. The cost per species observed decreased with increasing recording duration. Discontinuous and reduced‐coverage sampling may still yield fairly accurate assessments of biodiversity but reducing recording duration or coverage will result in different species remaining undetected. Whether the performance of a study design is ‘good’ or ‘bad’ depends on researchers' constraints and scientific questions to be answered. More hardware and longer recording durations are not always better, but we caution researchers against doing the bare minimum required for their present needs without pressing financial reasons to do so.