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Eavesdropping on ecosystems: Using soundscape ecology to monitor biodiversity

Take a step outside and just for a moment, close your eyes, and listen. You may hear the melodic chatter of birds, the thrum of insects, and the dry croak of a frog. The growing field of soundscape ecology investigates nature’s sound signatures, and the arrangement of vocalising organisms in their environment. This eavesdropping tool is allowing scientists and conservationists to explore the composition and the health of an ecosystem.

As is the case with shelter, food, and mates, sound is a limited and partitioned resource for which organisms must compete. These interactions between animals have driven powerful evolutionary processes, which have resulted in organisms occupying their own space within the frequency spectrum. This allows them to vocalise clearly above the cacophony of sounds in an ecosystem, in order to communicate, court mates, defend territory, and warn when danger is approaching. This concept is encompassed in the acoustic niche hypothesis, which likens the acoustic niche of a species to a specific radio bandwidth that enables clear sound transmission.

The acoustic niche hypothesis describes how sound is divided between organisms - just like physical space. Image courtesy of Zuzana Burivalova

Traditionally, sound recordings have been used to monitor individual bird species, which caw and cackle at a certain frequency within the sound spectrum. This reductionist approach has more recently been expanded into an all-encompassing view of the ecosystem and its vocalising organisms, in which the full acoustic environment is recorded and analysed. This provides a tool to monitor an ecosystem’s health, and therefore track the impact of environmental changes. Whilst satellite images can be lacking detail, and field surveys are time consuming and labour intensive, soundscape ecology can be a low-cost and non-invasive alternative.

A spectrogram illustrates the complexity of sound using colours. Image courtesy of Zuzana Burivalova

The boundless richness of tropical forests provides a key example of where bioacoustic monitoring plays an integral role in understanding biodiversity and conservation. Despite only occupying less than 2% of the earth’s land masses, tropical forests house an estimated 50% of all life. An understanding of bioacoustics allows for the rapid measurement of hundreds of species, where measures of sound such as soundscape saturation provides an insight into the species which dwell within the habitat. Scientists can then use this data to identify the most diverse parts of the forest, and the effectiveness of local conservation strategies. This provides vital information on the most at risk areas, and how to optimise conservation efforts, so that we can balance the needs of the ecosystem with anthropogenic use of the land.

Zuzana Burivalova, a tropical forest ecologist from Princeton University, has ventured into the depths of the jungle in Borneo, to record the orchestral arrangements of the vocalising creatures. Even if the vocalising organisms within an ecosystem are swooping through the forest canopy or rummaging amongst the leaf litter, the sound recorders provide a 360 degree view, about 50m into every direction. This casts a view into even the most obscure corners, or towering high habitats, which would usually be hard to access. The resulting soundscape is awash with noise from all forms of life, no matter how small or hidden.

As the use of sound provides a measure of the overall state of the forest, Zuzana was able to monitor the impacts of different land uses and conservation efforts on biodiversity. The affected soundscape from forest disturbances indicated that even benign uses of the forest, such as selective logging, resulted in a more homogenous forest with lower species richness.

The recorders enable long term monitoring, with minimal labour intensive work and a small price tag. This means large databases of sound recordings spanning many years can be built up, and used to monitor long term changes in an ecosystem.

Stepping from the terrestrial habitat and diving into the oceans, a similar scape of distinctive diversity can be found in coral reefs. These habitats house some of the greatest biodiversity on the planet, with one quarter to a third of all marine life residing in these underwater jungles. In this submerged world, animals use sound to entice mates, safeguard their homes, and coordinate spawning events between males and females. As with life on land, the complexity and saturation of sound production can be used as a signal for the variability and diversity of life within the reef, meaning scientists can also keep an ear on the coral reef, to identify changes in animal abundance and distribution. They can also monitor impacts of anthropogenic interactions, including boat traffic, chemical spills, or fishery activities, as well as the effectiveness of conservation strategies.

The low-cost and non-invasive sound monitoring methods play an important role in monitoring underwater ecosystems as coral reefs are faced with a warming climate and increased acidity of the oceans. This may result in coral bleaching, in which the symbiotic algae that provide food for the coral evacuate their habitat, and with that, trigger events that can cascade into biodiversity reduction in coral reefs. If the biodiversity were to decrease, the frequencies occupied would correspondingly plummet, resulting in an incoherent and patchy soundscape. Underwater noise is thought to play a role in habitat selection for juvenile reef dwelling animals, thus sound is essential in replenishing and maintaining healthy coral reefs. Therefore, disruptions to this process could result in a vicious cycle if the sounds produced by a coral reef become less variable and saturated, and hence less attractive to these organisms.

The good news is that once we understand the underwater sound fingerprints, the role they play, and what different saturations and complexities of frequencies convey to marine organisms, it may be possible to broadcast these sounds and attract the larvae that are vital to replenish and restore the bleached coral reef.

Acoustic monitoring is becoming an increasingly important tool in prioritising habitats to receive conservation efforts, balancing human needs with conservation, and preserving the biodiversity and ecosystem services that shape Earth. With increasing anthropogenic pressures, understanding the impact of our actions and how these can be minimised is fundamental to our sustainable existence on this exceptional planet. In the words of Bernie Krause, a picture is worth a thousand words, but with the unparalleled insights that acoustic ecology is providing, perhaps a soundscape is worth one thousand pictures.