Are acoustic indices useful for monitoring urban biodiversity?

Monitoring changes in biodiversity resulting from urban sprawl is an important topic for conservation. Automated biodiversity monitoring methods can quickly collect and process large datasets at minimal cost facilitating effective biodiversity monitoring. Despite being promising, the efficacy of acoustic monitoring in urban areas has not been established. Herein, our aim was to assess the effectiveness of acoustic indices in monitoring urban biodiversity. Thus, we acoustically sampled birds from a large neotropical city (Brasília, Brazil) across a gradient of urbanization. Human identification of recorded species revealed a clear reduction in bird richness in areas of greater urbanization. However, none of the six automated acoustic indices we tested [Normalized Difference Soundscape Index (NDSI), Acoustic entropy (H), Bioacoustic Index (BI), Acoustic Evenness (AEI), Acoustic Diversity Index (ADI), and Acoustic Complexity Index (ACI)], correlate with the degree of urbanization, indicating that these indices may not be a suitable tool for monitoring biodiversity in urban environments. In urban areas, it seems more appropriate to use traditional metrics, that make it possible to accurately identify the heard species. We recommend the need to ground truth the indices and explore alternative acoustic signal’s ability to monitor biodiversity in the complex soundscape of urban environments.