Optimising bat bioacoustic surveys in human-modified neotropical landscapes

During the last decades, the use of bioacoustics as a non-invasive and
cost-effective sampling method has greatly increased worldwide. For bats,
acoustic surveys have long been known to complement traditional
mist-netting, however, appropriate protocol guidelines are still lacking
for tropical regions. Establishing the minimum sampling effort needed to
detect ecological changes in bat assemblages (e.g., activity, composition
and richness) is crucial in view of workload and project cost constraints,
and because detecting such changes must be reliable enough to support
effective conservation management. Using one of the most comprehensive
tropical bat acoustic datasets, collected in the Amazon, we assessed the
minimum survey effort required to accurately assess (1) the completeness
of assemblage inventories and (2) habitat selection in fragmented forest
landscapes for aerial insectivorous bats. We evaluated a combination of 20
different temporal sampling schemes, which differed regarding: (i) number
of hours per night, (ii) number of nights per site and (iii) sampling only
during the wet or dry season, or both. This was assessed under two
different landscape scenarios: (a) in primary forest fragments embedded in
a matrix of secondary forest, and (b) in the same forest fragments, but
after they had been re-isolated through clearing of the secondary forest.
We found that the sampling effort required to achieve 90% inventory
completeness varied considerably depending on the research aim and the
landscape scenario evaluated, averaging ~80 and 10 nights before and after
fragment re-isolation, respectively. Recording for more than four hours
per night did not result in a substantial reduction in the required number
of sampling nights. Regarding the effects of habitat selection, except for
assemblage composition, bat responses in terms of richness, diversity, and
activity were similar across all sampling schemes after fragment
re-isolation. However, before re-isolation, a minimum of four to six
sampling hours per night after dusk and three to five nights of sampling
per site were needed to detect significant effects that could otherwise go
unnoticed. Based on our results, we propose guidelines that will aid to
optimize sampling protocols for bat acoustic surveys in the Neotropics.