Abstract
Soundscape analysis is a potentially powerful tool in ecosystem monitoring. Ecoacoustic metrics, including the Acoustic Complexity Index (ACI) and Acoustic Entropy (H), were originally developed for terrestrial ecosystems and are now increasingly being applied to investigate the biodiversity, habitat complexity and health of marine systems, with mixed results. To elucidate the efficacy of applying these metrics to marine soundscapes, their sensitivity to variations in call rate and call type were evaluated using a combination of field data and synthetic recordings. In soundscapes dominated by impulsive broadband snapping shrimp sounds, ACI increased non-linearly with increased snapping rate (∼100–3500 snaps/min), with a percent range of variation (∼40–50%) that exceeds that reported in most studies. H, however, decreased only slightly (∼0.04 units) in response to these same snap rate changes. The response of these metrics to changes in the rate of broadband snapping was not strongly influenced by the spectral resolution of the analysis. For soundscapes dominated by harmonic fish calls, increased rates of calling (∼5–120 calls/min) led to decreased ACI (∼20–40% range of variation) when coarse spectral resolutions (Δf = 94 or 47 Hz) were used in the analysis, but ACI increased (∼20% range of variation) when a finer resolution (Δf = 23 Hz) was employed. Regardless of spectral resolution used in the analysis, H decreased (∼0.20 units) in response to increased rates of harmonic calling. These results show that ACI and H can be modulated strongly by variations in the activity of a single sound-producing species, with additional sensitivity to call type and the resolution of the analysis. Variations in ACI and H, therefore, cannot be assumed to track call diversity, and the utility of these metrics as ecological indicators in marine environments may be limited.
Highlights
The assessment of biodiversity is critical in addressing questions of ecosystem health and resilience
The richness of sound types was correlated with Acoustic Complexity Index (ACI) in temperate ponds in France (Desjonquères et al, 2015). While these results suggest that ACI may respond positively to an increase in acoustic diversity, these patterns are not universal (e.g., Kaplan et al, 2015; Butler et al 2016) and other work has suggested that ACI can be influenced strongly by the rate of sound production associated with a single, or small number of species
For soundscapes dominated by impulsive broadband snapping, ACI initially increased with increased snapping, but the response saturated at high snap rates (Figures 2a and 8a)
Summary
The assessment of biodiversity is critical in addressing questions of ecosystem health and resilience. As many species emit sounds while moving, foraging and interacting with one another, passive acoustic monitoring provides an alternative method to constrain the diversity and behavior of animals (Rountree et al, 2006; Farina, 2014; Farina and Gage, 2017) This approach is relatively low-cost, minimally-disruptive, and can be deployed semi-continuously over long time periods. We review two soundscape indices frequently used to assess biodiversity in marine habitats, the Acoustic Complexity Index (ACI) and Acoustic Entropy (H), with the aim of assessing their effectiveness and potential shortcomings Both field and synthetic soundscape recordings are used to investigate the sensitivity of ACI and H to changes in the rate and composition of biological sound production. Based on our review of the marine soundscape literature and the results of this analysis, we conclude with recommendations for the use of these acoustic metrics to assess marine ecosystems
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