Passive Acoustic Monitoring to Assess Biodiversity Recovery in Tropical Rainforest Restoration Projects

Abstract

Monitoring methods must be reliable, appropriate, and able to measure vegetation recovery, as well as the return of animal communities and ecological functions, in tropical rainforest restoration projects. This study compares the effectiveness of passive acoustic monitoring for assessing biodiversity recovery in restored rainforest plots, degraded control areas, and mature reference forest sites. Automated acoustic recorders were set up at fixed sampling points and set to record soundscapes at dawn, evening, and night, in both wet and dry seasons. The study was conducted on the acoustically active faunal groups, such as birds, frogs, insects, and selected mammals, because they are useful for reflecting the habitat quality, microclimatic stability, and ecological recovery. The acoustic data were processed using noise filtering, spectrogram inspection, species-level call identification, and the computation of the acoustic indices used: Acoustic Complexity Index, Acoustic Diversity Index, Bioacoustics Index, and Acoustic Entropy. There was a clear difference in results when considering habitat categories. Low species call richness and weak calling activity were observed at degraded control sites, in addition to low acoustic complexity. In restored rainforest areas, there was an increase in acoustic diversity, calling activity on the dawn chorus and at night, and species call richness, which suggests faunal recolonization was occurring. Mature and stable forest soundscapes were found in the reference forests with the highest acoustic values. Positive associations were detected between canopy cover, vegetation density, and acoustic diversity, with statistical analysis revealing significant differences between the habitat types. Results indicate that passive acoustic monitoring is a                   non-invasive, continuous, and practical tool for assessing the recovery of biodiversity in rainforest restoration initiatives. It can be used to monitor long-term conservation planning by identifying animal return, habitat improvement, and ecosystem functional recovery.