A new paper, recently published in the Journal of Applied Ecology, provides a method for acoustic monitoring that
calculates the area listened to, by an array of microphones. C·I·B senior researcher, John Measey, and colleagues from the
University of Cape Town and the University of St. Andrews, Scotland, used microphones to monitor the Cape peninsula moss frog
(Arthroleptella lightfooti) in Table Mountain National Park, South Africa.
A Cape peninsula moss frog (Arthroleptella lightfooti) sitting on one of the microphones used in the
study. Note the very small size of the frog (~20 mm) which makes it difficult to monitor moss frog populations in any way other
than through acoustic monitoring (Photo Credit: Res Altwegg).
The revolution in digital media has enabled conservation organisations wishing to monitor vocalising species to record sounds
from the environment and use automated voice recognition computer software to pick out particular species from their calls. Acoustic
monitoring is suitable for monitoring the abundance of a species over time as it has minimal impact on the species being monitored. Many
managers are now turning to digital acoustic media to fulfil their need to see how the abundance of wild animals changes over time, for
example, with changes in climate.
However, there are some technical difficulties in analysing acoustic data collected over large areas of space. First, the
area a microphone listens to is not easily defined. The slightest wind or even a change in humidity can change the distance over which sound
propagates, with the result that microphones listen to different areas every time they record.
In their study, Measey et al showed that the area over which their microphone array could detect frog calls nearly doubled
(from 400 to 800 m2) during the winter breeding season. However, because the technique accounted for this change in the size of
the sampling area, they could effectively monitor the calling density of this threatened species.
“Microphone arrays are a very exciting advance because we can now monitor our environment without damaging the species
we aim to monitor,” Measey said, “but without considering where they are listening, managers risk making false assumptions
about the data they collect.”
An acoustic array at the study site in Table Mountain National Park. (Photo Credit: John Measey).