Acoustic Indices #

Marina D. A. Scarpelli - PhD candidate at QUT
@ScarpelliNina

FCS

Two approaches: Bioacoustics vs Ecoacoustics #

  • Species-specific approach: consider the communication system exclusively between sender and receiver
    • Usually called bioacoustics

Communication1to1

  • Communication structure is more similar to a network (Sueur, 2014)
    • Ecoacoustics or soundscape approach
      • Soundscape: all the sounds in the landscape
      • Biophony + Geophony + Technophony
    • Enables the detection not only of species calls, but also social, behavioural and ecological cues

Communicationnetwork

Passive Acoustic Monitoring (PAM) #

  • Passive acoustic recorders facilitated the investigation of more comprehensive questions, enabling research on:
    • Soundscapes
    • Multiple species
    • Upscaling bioacoustics questions: temporal and spatially
  • However analytical tools still needed to be improved

sensor2

Approaches to analyse the data #

  • Manually listening/annotating
  • Species recognisers (automatic/semi-automatic)
  • Acoustic indices (AI): summarising information
    • More than 69 existing indices (Alcocer et al., 2022)

sugai-et-al-2019

Sugai LSM, Silva TSF, Ribeiro JW, Llusia D. 2019. Terrestrial Passive Acoustic Monitoring: Review and Perspectives. BioScience 69:5–11.

Acoustic indices #

  • Linked to the “Soundscape” or Ecoacoustics approach (Alcocer et al., 2022)
  • Mathematical function applied to waveform or spectrogram
    • Can be derived from traditional ecological indices (i.e.: diversity, richness, etc)
  • Summarise the acoustic information with a community perspective
    • Moving away from species-specific questions

Types of Indices #

Sueur et al., 2014 #

  • Intensity indices: measure sound intensity (in dB) with different frequency weighting and time averaging
  • They usually miss information on frequency and temporal patterns of soundscapes

towsey-et-al-2014

Types of Indices #

Sueur et al., 2014 #

  • Complexity indices: more species and individuals communicating will increase the complexity of the soundscape
  • Heterogeneity would be a proxy of animal acoustic activity

pieretti-et-al-2011

Types of Indices #

Sueur et al., 2014 #

  • Soundscape derived indices: estimates the relative contribution of biophony in relation to technophony and geophony
  • Splits the spectrum into frequency bands:
    • 0.2 to 2 kHz = technophony
    • 2 to 8kHz = biophony
    • geophony: occupies the entire spectrum

gage-and-axel-2014

Soundscape indices #

Splitting frequency bands #

Sometimes it works…

Technophony technophony-example

Soundscape indices #

Splitting frequency bands #

Sometimes it works…

Biophony biophony-example

Soundscape indices #

Splitting frequency bands #

Sometimes it works…

Geophony geophony-example

Soundscape indices #

Splitting frequency bands #

Sometimes it does not work…

Geophony rustling-example

Soundscape indices #

Splitting frequency bands #

Sometimes it does not work…

Biophony boobook-example

Soundscape indices #

Splitting frequency bands #

Sometimes it does not work…

Biophony birds-example

Acoustic indices disadvantages #

  • Sounds are multidimensional: frequency, time and amplitude
  • Environmental recordings have an additional constraint:
    • It is hard to determine the distance between sound source (e.g.: animal) to microphone
      • Amplitude metrics depend on distance because of attenuation effects
  • Other factors might influence values:
    • E.g. background noise, intensity and repetition of calls by one individual, overlaps between calls,etc
    • Therefore the relationship between one index/one ecological metric is still unclear

Acoustic indices measuring biodiversity #

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Summary of meta-analysis #

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  • AI had an overall moderate positive correlation with biodiversity
  • Best biodiversity metric correlated with AI was abundance of sounds
  • There was no single best AI

Summary of meta-analysis #

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  • Majority of studies investigated terrestrial habitats and birds are the main group studied;
  • Species richness and abundance of sounds are the most explored metric to relate acoustic indices values to biodiversity measures

Now, let’s do some practical exercises!

Go to the practical.