Linking the forest soundscape with structural and ecological attributes: a bioacoustic approach

A soundscape is a useful indicator of biological activities in any ecosystem. This study evaluated the potential of five acoustic indices - Acoustic Complexity Index (ACI), Acoustic Diversity Index (ADI), Acoustic Evenness Index (AEI), Normalised Difference Soundscape Index (NDSI), and Bioacoustic Index (BI) in depicting the soundscape of a tropical forest in South Asia. We analysed 8,400 sound samples (55 s each; 10 samples per hour at each location; totalling 7,700 min) from 35 locations in the study area across four diel periods - morning, midday, evening, and night. Among the acoustic indices, ACI, ADI, AEI, and BI showed significant diel variations, while NDSI remained stable, indicating consistent biophonic and anthropogenic noise throughout the day. ACI, AEI, and BI showed significant correlation with bird species richness during peak activity period (morning) and across 24 h. A spectrogram analysis with BirdNet and expert validation identified the presence of 47 distinct acoustic signatures of birds. Additionally, generalised additive models (GAMs) revealed that forest structural and ecological attributes such as tree height, density, canopy openness, elevation, and disturbance indices were associated with ADI, AEI, and NDSI. Overall, the study suggests that the structural and ecological attributes of the explored forest ecosystem are strongly correlated with soundscape acoustic indices.