Do acoustic indices reflect the characteristics of bird communities in the savannas of Central Brazil?

Abstract

Habitat loss and fragmentation represent two of the most significant threats to biodiversity. In some regions, like the Brazilian Cerrado, the deforestation rate can reach nearly 1 million hectares per year. Ecoacoustics and acoustic indices can be used to promote rapid assessments in threatened regions. We evaluated how two particular indices (the acoustic diversity index-ADI-and normalized difference soundscape index-NDSI) reflect bird species richness and composition in a protected area near Brasilia city. We hypothesized that ADI should reflect the characteristics of birds in the cerrado and in the gallery forest, i.e., with higher values in gallery forest than in the cerrado. Based on habitat structure, we also hypothesised that NDSI should be lower in less complex habitat, and lower in areas close to urbanized areas. We assessed 30 locations by installing automatic recorders to generate 15 min wave files (48 kHz, 16 bits, stereo). The manual hearing of the files revealed the presence of 107 bird species (74 in gallery forest and 47 in cerrado). Our results showed that ADI was significantly associated to species richness, being higher in gallery forest than in the cerrado. We found that NDSI values were lower in areas close to highways, an important source of impact for bird diversity. We argue that acoustics indices are a valid approach for rapid biodiversity assessment, however basic knowledge on species occurrences is essential to interpret the values provided by these indices.

Comparison between lek counts and bioacoustic recording for monitoring Western Capercaillie (Tetrao urogallus L.)

“…Similar analytical methods to those used here have been undertaken in previous studies on avian acoustics (e.g. Machado et al 2017). A semi-automated process can substantially reduce the time required for analysis, but introduces the potential for machine errors in call recognition, with both false negative and false positive classifications likely to result.…”

Impact of stand variables on characteristics of avian soundscape in common oak (Quercus robur L.) forests

“…Differences in ADI and NDSI meanings correspond to differences in the species composition and structure of bird communities between the gallery and cerrado forest in Central Brazil. (Machado et al, 2017). The results of studies conducted at sites of varying degrees of degradation (from forests to agricultural land) in southeastern England and northwestern Ecuador showed that the greatest correlation (up to 65%) between acoustic indices and the number of bird species was observed in a temperate climate (Eldridge et al, 2018).…”

A roadmap for survey designs in terrestrial acoustic monitoring

“…; Machado et al. ). However, this method has two main drawbacks: (i) rotation procedures precludes simultaneous recording across sampling sites, potentially introducing bias from seasonal or weather changes, which must be accounted for; (ii) the number of monitoring days before rotating will influence species detectability, especially for rare species.…”