Monitoring soil fauna with ecoacoustics

Abstract

Ecoacoustics-or acoustic ecology-aids in monitoring elusive and protected species in several ecological contexts. For example, passive acoustic monitoring (PAM), which involves autonomous acoustic sensors, is widely used to detect various taxonomic groups in terrestrial and aquatic ecosystems, from birds and bats to fish and cetaceans. Here, we illustrate the potential of ecoacoustics to monitor soil biodiversity (specifically fauna)-a crucial endeavour given that 59% of species live in soil yet 75% of soils are affected by degradation. We describe the sources of sound in the soil (e.g. biological, geological and anthropogenic) and the ability of acoustic technology to detect and differentiate between these sounds, highlighting opportunities and current gaps in knowledge. We also propose a roadmap for the future development of optimized hardware, analytical pipelines and experimental approaches. Soil ecoacoustics is an emerging field with considerable potential to improve soil biodiversity monitoring and 'soil health' diagnostics. Indeed, early studies suggest soil ecoacoustics can be successfully applied in various ecosystems (e.g. grasslands, temperate, tropical and arid forests) and land uses (e.g. agriculture, viticulture, natural and restored ecosystems). Given the low cost, minimal intrusiveness, and effectiveness in supporting soil biodiversity assessments and biosecurity risks, we advocate for the advancement of soil ecoacoustics for future land management applications.

Keywords: bioacoustics; biomonitoring; ecoacoustics; soil biodiversity; soil health.

Figure 1.

A basic soil ecoacoustics field recording set-up. This comprises an audio recorder with an impedance filter and pre-amp (e.g. +20 dB), a piezoelectric (contact) microphone, which typically contains a piezoelectric ceramic, such as lead zirconate titanate (PZT) or a polymer such as polyvinylidene fluoride (PVDF), a metal probe, ideally aluminium for its sound propagation properties, and some headphones. The mechanical vibrations caused by invertebrates are converted to an electrical signal and the data are stored on an secure digital (SD) card, ready for downstream analysis.

Figure 2.

Sources of anthrophony/technophony that could potentially impact soil biota and ecological functions, with a graphical example of a research question relating to the duration of anthrophony on the activity or health of soil invertebrates and possible negative, neutral or positive outcomes.

Figure 3.

A myriad of methodological possibilities: (a) testing different sampling devices in the field, (b) running controlled mesocosm trials, (c) testing the acoustic signals of different taxa and studying their associated morphological and behavioural traits, and (d) creating classifiers using cluster-based machine learning techniques—based on acoustic parameters such as frequency, amplitude, inter-pulse interval and duration.

Figure 4.

Detection plate innovation is currently being developed for surface-moving invertebrates. This could be combined with an Internet of Things (IoT) module to provide remote, real-time data transmission (see the IoT section for more information).