Sampling duration and season recommendations for passive acoustic monitoring of bats after white-nose syndrome

Since 2006, white-nose syndrome (WNS) has caused drastic declines in populations of several hibernating bat species throughout eastern North America. Thus, there is a growing need to establish long-term monitoring programs to assess changes in bat populations over time. Information on the seasonal timing of species occurrence and the sampling effort required to acoustically detect individual bat species and obtain complete inventories is needed to design and implement effective monitoring programs. From April through October in 2018-2021, we passively sampled for bats using full-spectrum detectors at eight permanent locations at Fort Indiantown Gap National Guard Training Center, Pennsylvania. We examined seasonal activity patterns and estimated bat species richness among detector locations and seasons using species accumulation curves. We also estimated probability of detection (p) and occupancy (Ψ) using single-season occupancy models in PRESENCE software, and then determined the number of sampling nights needed to reliably infer absence of each species. We identified 351,771 bat passes of eight species or species groups in 5,856 detector-nights. Seasonal patterns of activity varied among species. On average, approximately 30 sampling nights were needed to detect 90% of the total species richness among locations and seasons. Relatively few nights (≤12 nights) were needed to detect most species during summer, however, many more nights were needed to detect acoustically rare species. Our results indicate that the acoustic sampling effort currently required to determine presence or probable absence of federally endangered Indiana myotis Myotis sodalis and northern long-eared myotis M. septentrionalis may also be adequate for tri31 colored bats Perimyotis subflavus, but a greater level of effort may be needed for little brown myotis M. lucifugus in some areas. Monitoring programs that incorporate efficient sampling methodologies will be critical for future conservation efforts as populations of several bat species continue to decline.