Applications of Ecoacoustics in Soil Ecosystems

Since the Industrial Revolution, human activities have increasingly degraded natural environments. In response to this escalating crisis, the United Nations declared the period from 2021 to 2030 the "Decade on Ecosystem Restoration", urging nations worldwide to protect and restore ecosystems in order to halt ecological decline and safeguard the integrity of natural systems.

Among terrestrial ecosystems, soil forms the fundamental foundation for most life on land. It is estimated to host roughly 59% of the world's species, making it one of the most biodiverse habitats on Earth. Despite this immense biological richness, soil ecosystems have been severely degraded by human activity. Intensive agriculture, industrial pollution, and other unsustainable practices have accelerated soil deterioration on a global scale. Protecting soil is therefore a critical priority. Doing so helps preserve terrestrial biological communities and also ensures the long-term stability of global food production systems.

Monitoring biodiversity is a key component of ecosystem conservation. Reliable monitoring allows conservation practitioners to interpret ecological data, evaluate whether restoration efforts are successful, and adjust management strategies when necessary. Because of this, researchers have increasingly emphasized the need to develop new biodiversity monitoring tools while also improving existing ecological monitoring methods.

A study published in 2024 explored the use of ecoacoustic techniques to monitor soil biodiversity by recording the sounds produced by soil-dwelling invertebrates. By analyzing these acoustic signals, researchers assessed the diversity of invertebrate species within the soil and evaluated how reliable ecoacoustic approaches may be for this purpose.

Introduction to Ecoacoustics

Ecoacoustics is a rapidly developing field within ecology that focuses on analyzing environmental soundscapes. The approach is increasingly being used to monitor biodiversity. By detecting and analyzing biological sounds produced by different species, ecoacoustic methods allow researchers to infer the presence, abundance, distribution, and composition of organisms within ecosystems. In some cases, acoustic signals can even reveal aspects of animal behavior.

Ecoacoustic monitoring also offers several practical advantages. It can be efficient, relatively inexpensive, and minimally invasive compared with many traditional ecological survey methods.

At present, ecoacoustic techniques are widely used to monitor elusive or protected species. These include birds, bats, whales, fish, and various invertebrates. In soil environments, ecoacoustics has already been applied successfully to detect acoustic signals produced by medium-sized and large soil organisms. Research conducted in 2023 further demonstrated that ecoacoustic methods could effectively monitor soil biological communities within temperate forests in the United Kingdom. Nevertheless, additional studies are needed to evaluate how reliable soil ecoacoustics is across different climates and ecosystems around the world.

Research Design

Study site（圖片來源：Robinson, J. M et al.(2024). ，採用 CC BY-NC 4.0 授權。 ）

The 2024 study was conducted in grassy woodland ecosystems at Mount Bold in South Australia. The study area covers approximately 5,500 hectares. Due to agricultural expansion, climate change, and urbanization, grassy woodland ecosystems in Australia have declined dramatically, with only about 10–15% of their original extent remaining. As a result, numerous restoration programs are currently underway.

The grassy woodland ecosystem at Mount Bold differs substantially from the temperate forests previously studied in the United Kingdom. The region has a Mediterranean climate, characterized by hot, dry summers and cool winters. Differences in ecosystem structure, species composition, and climate conditions provide an opportunity to test whether ecoacoustic monitoring remains effective under different environmental conditions.

Historically, the Mount Bold landscape experienced more than a century of livestock grazing, which continued until 2003. Vegetation restoration projects were later implemented between 2005 and 2009. This history allowed researchers to construct a restoration timeline that included three types of sites:

1. Areas where original vegetation remained intact

2. Areas where vegetation had been cleared

3. Areas where vegetation restoration had been carried out

For each of these three categories, researchers selected two sampling locations. Acoustic recordings were collected over five days using two approaches: recordings made directly in the field and recordings made inside a soundproof chamber.

After acoustic recordings were completed in the soundproof chamber, soil samples were examined to identify and count the invertebrates present. Once identification and counting were finished, both the soil and the organisms were returned to their original locations. The resulting invertebrate counts were then compared with the acoustic recordings.

Results

The results revealed that sites where vegetation had been cleared contained significantly fewer invertebrates and lower species diversity than either restored sites or areas with intact vegetation. Ecoacoustic measurements reflected the same pattern.

In other words, higher acoustic complexity and diversity within soil recordings corresponded to greater numbers of invertebrates and higher species richness. This indicates that soil soundscapes can serve as a meaningful indicator of soil biodiversity.

The findings also suggest that the restoration efforts carried out at Mount Bold successfully helped recover soil biological communities. These conclusions are consistent with earlier research conducted in 2018 using molecular methods to monitor soil biodiversity.

Differences in invertebrate abundance among the three groups, with ants—whose sounds could not be detected by the acoustic equipment—shown separately（圖片來源：Robinson, J. M et al.(2024). ，採用 CC BY-NC 4.0 授權。 ）

The study further showed that each stage of ecosystem restoration supported a distinct invertebrate community composition. Although some overlap occurred between groups, the differences were clear. The restored sites were most similar to the intact native vegetation sites, whereas both differed substantially from the cleared areas.

An interesting observation concerned earthworms. In many restoration sites elsewhere, earthworm populations increase dramatically. However, at Mount Bold earthworms were relatively rare. Instead, ants represented the most abundant group of soil invertebrates. This factor influenced the reliability of ecoacoustic measurements.

Because ants are extremely small, current recording equipment has difficulty detecting the sounds they produce. As a result, other monitoring methods may sometimes be more suitable in such environments. Alternatively, improvements in acoustic equipment, particularly greater microphone sensitivity, could allow future studies to detect these subtle sounds more effectively.

Conclusion

This study demonstrates that soil ecoacoustic monitoring holds considerable potential as a non-invasive method for assessing soil biodiversity. In the grassy woodlands of Mount Bold, both intact vegetation sites and restored sites exhibited greater acoustic complexity and diversity than cleared areas. These acoustic indicators were strongly correlated with the abundance and diversity of soil invertebrates.

Such results confirm that acoustic metrics can reflect the biological condition of soil ecosystems.

Nevertheless, further improvements are still possible. In addition to enhancing recording equipment, future studies should consider additional factors such as day–night cycles and variations in soil depth, both of which may influence soil soundscapes.

As research in soil ecoacoustics continues to develop, this technique may become an important tool for understanding biodiversity across different ecosystems. It may also provide valuable information for environmental policy and ecosystem management.

Author: Bai Leng

Reference:

Robinson, J. M., Taylor, A., Fickling, N., Sun, X., Breed, M. F. (2024). Sounds of the underground reflect soil biodiversity dynamics across a grassy woodland restoration chronosequence. Journal of Applied Ecology.