4.3. Considerations about the methodology
Although several automated indexes have been proposed for soundscape
and biodiversity analyses, the application of a single index can
hardly account for all the biological components (Sueur et al., 2014).
Therefore, the simultaneous use of acoustic indices could provide additional
insights on biophony reactions to mining noise, and may help to
improve understanding of species reactions. Nevertheless, the separation
of the two different soundscape components (technophony and
biophony) still represents a difficult task because they overlap, particularly
in the lower frequency bands. Consequently, when automatic processing
procedures are applied in noise-polluted environments,
technophony might be read as biophony and vice versa. In this study,
one algorithmwas selected for the processing phase, the Acoustic Complexity
Index (ACI), because it has been previously proven to be effective
for filtering out constant sounds (Pieretti et al., 2011) such as
trucks' transits or background buzz frommining activity,while enhancing
the variability of biological sonic productions. Therefore, ACI is
regarded here as a proxy of biophonies that are compared across two
different soundscapes, noisy and natural.
Future research might consider the application of several indices in
recordings characterized by different kinds of noise, in order to analyze
their pros and cons togetherwith the emergent properties of their combined
use. Indeed, studies describing the use of acoustic indices to investigate
animal communities in noise-polluted environments are needed.
Additionally, it is important to note that soundscape measurements are,
currently, not able to provide precise and detailed information at a
species-specific level. There is still the need for a comparison with classical
fieldwork data, e.g. species aural census, in order to interpret the
automatic procedure results correctly. When dealing with noisepolluted
habitats, analytic compromises must be defined, such as to
split the analyses into specific frequency bands (for example here
0–1.5 kHz and 1.5–22 kHz). Nevertheless, the exploration of acoustic
communities and soundscapes offers an efficient way to analyze largescale
phenomena (Sueur et al., 2014). The assessment of acoustic temporal
and spectral changes can provide a general overview of circadian