Differences in acoustic detectibility of bat species hamper Environmental Impact Assessment studies

Different bat species are known to differ in their detectability. Having available presence-absence data from 100 randomly stratified selected 1 km(2) squares in the north of the Netherlands, collected during autumn 2009 and spring 2010 following Environmental Impact Assessment protocols, we calculated probabilities of occupancy and detection for ten bat species. Not only did we investigate their presence in general but also of the three main functions a landscape has for a bat: roosting, commuting and foraging. The four most commonly detected species were Pipistrellus pipistrellus, Pipistrellus nathusii, Eptesicus serotinus and Myotis daubentonii. For all species, roosting was the function detected least while the function of foraging was detected most for most species. Probability of detection was highest for P. pipistrellus (0.79), followed by P. nathusii and E. serotinus. They are all relatively loud species, whose presence is hardly missed. For the other seven species, probability of detection was below 0.4 with the lowest value for Plecotus auritus (0.11). The latter species has a very soft echolocation call and is thus often not detected even when present. Our study is the first to use occupancy modelling for European bats. Our results show that the number of visits required to obtain a reliable approximation of occupancy differs widely: from two visits for both Pipistrellus species, to three for E. serotinus and M. daubentonii and even ten for P. auritus. Especially for the latter species, other survey methods may be better employed. This has implications for the design of surveys for Environmental Impact Assessments.

Automated summary

Different bat species have different detectability, with Pipistrellus pipistrellus, Pipistrellus nathusii, Eptesicus serotinus, and Myotis daubentonii being the most commonly detected species. Foraging is the most easily detected activity, while roosting is the least easily detected.