The impact of vehicular noise on acoustic indices within simulated bird assemblage soundscapes

Passive acoustic monitoring (PAM) is a sampling technique that has gained increasing popularity in the field of wildlife monitoring and research since it allows for non-invasive and cost-effective collection of acoustic information. Retrieving biological information from PAM recordings can often involve time-consuming sound annotation methodologies, but the advent of acoustic indices can help expedite this process. While correlations between acoustic indices and species richness have been observed in a variety of ecological contexts, these relationships faulter in environments with increased vehicular noise. Here, we assessed the direct impact of vehicular noise on nine acoustic indices through controlled manipulation of vehicular noise within computer-generated bird assemblage soundscapes. Our results demonstrate that recording distance from roadsides and number of passing cars per minute have notable and persistent impacts on acoustic index values, but the magnitude of the effect varies across indices. Four acoustic indices demonstrated greater resilience to vehicular noise interference and may therefore be better suited for developed areas: Bioacoustic Index, Acoustic Complexity Index, Acoustic Diversity Index, and Acoustic Evenness Index. By contributing to the collective understanding of acoustic index behaviours under anthropogenic noise pollution, we hope to better inform their ecological application in human-developed contexts.