Worm munch matters
The lives that slither and scurry beneath the soil’s surface are vital for ecological health: now we can listen in to them- by Ella BrowningRead on Aeon.
Earthworms, which lack mouths, vocal cords, or ears, produce subtle sounds as they move and eat underground. These sounds resemble soft, Velcro-like scrapes and can be recorded using sensitive microphones. For example, a recording from Oxfordshire grassland captures the scraping noises of earthworms feeding on soil and decaying matter. Such sounds were previously overlooked but now offer a new way to study the hidden world beneath our feet. The article compares these sounds to familiar noises, like a laser or a buzz, to help readers visualize the unfamiliar. While earthworms themselves do not make intentional sounds, their movements and feeding behaviors create vibrations that can be detected and analyzed.
Ecoacoustics is a scientific field that studies natural soundscapes to understand ecosystem health. It combines elements of bioacoustics (the study of animal sounds) and soundscape ecology (the study of environmental sound patterns). Ecoacoustics examines three types of sounds: biophony (animal sounds like bird calls or insect chirps), geophony (natural non-biological sounds like wind or rain), and anthropophony (human-made sounds like traffic or machinery). A key principle is that healthier ecosystems tend to produce more diverse and complex soundscapes. For instance, a healthy tropical forest at dawn might feature a rich mix of bird songs and animal calls, while a degraded or polluted area may fall silent. The field was formally established in 2014, though its roots trace back to the 1960s.
Passive acoustic monitoring involves placing microphones or hydrophones in an environment to record sounds without disturbing the ecosystem. This method has been used to study hard-to-reach habitats like the deep ocean or the night sky, where traditional observation is difficult. For example, in the 1950s, the US Navy’s Sound Surveillance System (SOSUS) accidentally discovered whale calls while tracking submarines. Similarly, in the 1930s, Donald Griffin and George W Pierce used ultrasound technology to reveal how bats navigate using echolocation, a process where they emit high-pitched sounds and listen for echoes to map their surroundings. Passive acoustic monitoring allows scientists to collect long-term data on animal behavior and ecosystem health.
Soil is a complex ecosystem hosting diverse organisms like earthworms, ants, and beetles, all of which contribute to the underground soundscape. Early research suggests that healthier soils produce more varied and frequent sounds. For example, a study in UK grasslands found that rewilded areas (where nature is allowed to recover) had higher sound diversity and more activity, particularly after midnight when earthworms are most active. Traditional methods for assessing soil health, such as chemical or biological tests, are labor-intensive and limited in scope. Passive acoustic sensors, however, can be buried in the soil and left to record continuously, offering a scalable and non-invasive way to monitor soil health over time.
Earthworms play a crucial role in soil health by acting as ecosystem engineers. Their burrowing activities improve soil structure by creating channels that allow air and water to circulate. They also break down dead plant and animal matter, recycling nutrients back into the soil. Different earthworm species have distinct burrowing behaviors—some dig deep vertical tunnels, while others create shallow horizontal networks. Their presence and activity levels are often used as indicators of soil quality in agricultural settings. In the study mentioned, earthworms were the most abundant soil invertebrates, and their movements were linked to the recorded soil soundscape, suggesting their sounds contribute significantly to the underground acoustic environment.
To meet international biodiversity restoration goals, such as those set by the Convention on Biological Diversity (CBD), scientists need reliable methods to monitor soil health. Traditional techniques involve manual sampling and laboratory analysis, which are time-consuming and limited in scale. Passive acoustic sensors offer an alternative by providing continuous, long-term data on soil soundscapes. These sensors can be deployed across multiple locations simultaneously, capturing dynamic patterns like the post-midnight peaks in earthworm activity. While the field is still developing, early results indicate that ecoacoustic measurements can effectively assess soil health and the success of nature recovery efforts, such as rewilding projects.
Soil ecoacoustics is a relatively new field, and researchers are still refining methods to analyze underground soundscapes. One challenge is that existing ecoacoustic tools were originally designed for above-ground sounds, such as bird calls, which typically fall within the 2,000–15,000 Hz range. However, soil sounds often occur at lower frequencies, creating gaps in current analytical frameworks. Additionally, distinguishing between biotic sounds (made by animals) and anthropogenic sounds (human-made) can be difficult, as human activities like machinery may overlap with natural frequencies. Despite these hurdles, the potential of soil ecoacoustics to revolutionize soil monitoring is significant, offering a new lens to understand the hidden world beneath our feet.
