The natural sounds of wild nature


It is a truism that walking a wild place is a sensual experience, whether it is your first time there, or if you are back again for that restorative effect you get in the soft fascination that naturalness and wildness offers (1). You make use of all your senses to sample the ecological schema around you, your eyes taking in the scene, its colours and forms, your attention caught by any motion, your sense of smell often complementing the visual cues you have captured, and your hearing adding another layer to the overall picture of what you have absorbed. There may also be sensation from touch, wind or rain, or your motion. I have written before about these affective experiences, of how a hunter gatherer would have processed a mixed array of stimuli like this that together provided the simple cues that fitted with a schema for survival (2). I saw this as being akin to the idiom of living by your wits, a cultural manifestation of the challenge and opportunity faced by early humans because of the physiological dependence in securing the food and shelter provided in those biophysical settings, and which required frequent moves through the landscape. This, as Rob Pheasant would say, was coupled with constantly living in a heightened state of arousal from fear of predation and other inanimate hazards (such as barriers or impediments to movement presented by impassable rivers, ravines, cliffs, and swamps, as well as natural hazards such as from fire) and the stress of prolonged periods of directed attention in locating familiar places or finding new ones while constantly processing the sign-stimuli around them. Thus they would have been managing risk on the basis of sign stimuli, and would have chosen locations for repose in the landscape that offered clear sight lines as well as refuge.

For Rob, it was the importance of the soundscape. Thus while the need to find fresh water would frequently have been met by visual stimuli, such as a glassy surface, they would also have provided auditory cues that signalled their presence, such as rivers, streams, waterfalls and cascades. Other indicative elements of the soundscape would have been the characteristic sounds of potential quarry animals, or their movement heard but unseen in complex vegetated landscapes. When the following were also added to the soundscape mix - bird song, territorial, courtship and mating calls; along with the forewarning to take shelter from advancing thunder storms; and the need for security on hearing the cooperative calling between predators as they search out new prey - it then becomes apparent how essential audition was to landscape characterization. Conservation biologist Michael Soule in considering some years ago the unanswered questions about biophilia, observed that the senses of hearing and vision were more recent developments in evolution,There is a growing literature that demonstrates the damage that can be done when anthropogenic noise rises above natural sounds, effectively masking those sounds. This is disruptive of wildlife communication and behaviour and were more closely associated with language (3). As he noted, many avian and mammalian alarm calls and other auditory and visual signals of animals were instantly recognized, even employed, by humans. However, there were many interesting questions that awaited study, such as were natural sounds (of animals) more relaxing than sounds of machinery? Were the songs and calls of seabirds less relaxing than savanna or forest bird sounds? He wondered how the complexity per se of sound and vision affected us? For example, were signals of higher complexity more appealing? If so, were such preferences universal, innate, and limited to humans?

Natural soundscapes are a biological process that should be restored and conserved

I discovered some years ago that the National Park Service (NPS) in America had a Natural Sounds Program (4). The NPS regard natural soundscapes as a Natural Resource, a biological process that should be restored and conserved, just like the native plants, animals, and communities (5). Thus in the section on Natural Resource Management in the NPS Management Policies, the natural soundscapes in the parks are regarded as a natural system that should be protected from unacceptable impacts, and where efforts should be made for restoration to a natural condition if it has been degraded by unnatural sounds (noise)(6). Interestingly, it sees the natural soundscape resources encompassing all the natural sounds that occur in parks, including the physical capacity for transmitting those natural sounds through air, water, or solid materials, and the sounds produced by physical processes, such as wind in the trees, claps of thunder, or falling water.

The Natural Sounds Program acknowledges that natural sounds are a vital part of the experience of the Parks – “They sharpen your senses and connect you to the park environment” (7). As you would expect, the natural sounds in American parks expose the depauperate state of our wild nature, but it is acknowledged that even sounds more commonly heard, such as bird songs, wind through grasses and trees, frog croaks, and chirping insects, are a part of the park experience. It notes that research shows that hearing these natural sounds brings health benefits for humans, such as lowered stress, improved mood, cognition and social well-being. If you can’t get to an American park to experience the soundscape yourself, there is an extensive library of sounds for Rocky Mountain National Park and Yellowstone National Park to let you know what you are missing (8,9). The sound of a grizzly bear in Yellowstone using its strong jaws to break through the bones in a dead bison is a stark reminder of the advice to back away slowly and keep your distance (10). The pair of wolves howling in the Lamar Valley in Yellowstone, and where I first saw them, reminds me that wolves don’t have the infuriating tendency of barking like domestic dogs (12). Hear the soundscape recorded at Yellowstone Lake where the sound of several bull elks (deer) bugling during the autumn rut, an impromptu brass ensemble that if you listen carefully is accompanied by the sound of drops of water (13)

All interesting stuff, but what is misplaced in all these musings over ecological schema is that it only looks at it, as did Soule, from an anthropocentric viewpoint, how humans react to stimuli, but not how other animals did. I briefly tried looking through the eyes of animals when writing about the implications of habitat fragmentation, how daunting it must be to small mammals to navigate the gaps we create when they usually depend on continuous woodland as cover to move safely (14). Of course, animals don’t just depend on visual cues, they must also react to auditory cues as they too pick out information about the ecosystems they inhabit. The Natural Sounds Program acknowledges this, noting that activities such as finding desirable habitat and mates, avoiding predators, protecting young, and establishing territories are all dependent on the acoustical environment (15). Thus a fox depends on its ability to hear the sound of rodents under snow when finding food (7). Birds, frogs, or katydids (bush crickets) use sounds to define territories or aid in attracting mates, bats use sounds to locate prey or navigate, and sounds received by mice or deer allow them to detect and avoid predators or other dangers (6). In order to continue with these activities, animals are being forced to adapt to increasing noise levels. However, given the intrinsic value of sounds to wild nature, the NPS commits to identifying what levels and types of unnatural sound constitute acceptable impacts on park natural soundscapes (4). Since parks were created in part for people's enjoyment, the NPS accepts a certain degree of noise is permissible (16) and that the frequencies, magnitudes, and durations of acceptable levels of unnatural sound will vary throughout a park, being greater where there are visitor facilities and along transportation corridors (6).

Noise effects wildlife when anthropogenic sounds rise above natural levels, a growing number of studies indicating that animals, like humans, are stressed by noisy environments (17). As background sound increases, the area in which animals can hear important sounds is reduced, masking natural sounds that would otherwise be heard, and thus interfering with wildlife communication and behaviour (18). A recent study showed that anthropogenic noise doubled background sound levels in 63% of U.S. protected areas, masking more than half of the natural sounds that would otherwise be heard (19). The endangered Sonoran pronghorn avoids noisy areas frequented by military jets; female frogs exposed to traffic noise have more difficulty locating the male's signal; gleaning bats avoid hunting in areas with road noise (18). When these effects are combined with other stressors such as winter weather, disease, and food shortages, sound impacts can have important implications for the health and vitality of wildlife populations within a park (20). As you might expect, wilderness areas experience the lowest noise exceedance, as they are often remote sites with low background sound levels, and there is a ban on motorized transport or machinery (19). However, even then the low background sound levels enhance the audibility of distant sound sources so that minimizing the intrusion of anthropogenic noise in wilderness requires noise management at a much larger scale.

Can acoustic measurements differentiate contours of wildness?

The differential between the noise pollution in wilderness and the National Parks is explicable, and suggests there may be a reciprocal gradient between unnatural (noise) and natural sounds, but it makes you wonder whether there is something more intrinsic about sounds in wild nature, that there is a correlation, a rising gradient between wildness and natural sounds. Would you expect there to be an acoustical environment richer in natural sounds in the biophysical reality of self-willed land compared to the simplified ecology in cultural land use? Does wilder land have more bird and insect sounds; greater wind sound reflected in the greater complexity of vegetation as its sounding board; are there more of the natural sounds that are outside of the hearing range of humans; is our hearing acute enough to be able to detect differences in sound loudness when some natural sounds are of very low volume?

This is what Rob wants to tackle next, whether acoustic measurements can differentiate wildness contours. It is perhaps fortunate then, that we came cross the work of Bernie Krause who has been involved in capturing wild soundscapes all over the Americas (21). Like me, Krause made the connection between hunter gatherers and their dependency on soundscapes for survival. After he had recorded humpback whales in Hawaii, and killer whales along the coasts of British Columbia and Alaska, he began to wonder just how those vocalizations fitted into the entire bio-spectrum of audio events, as they certainly weren't the only sounds present. In listening to the acoustical ambience – the normal sounds of a location - on land and in the water night after night, the thought of an extended hypothesis occurred to him that there was another way of hearing these sounds — ways in which the creatures themselves might be hearing them. His premise was that every location has a unique acoustical bio-spectrum that provides information on the dynamics of the ecosystem.

By sampling the bio-acoustics of a specific area over periods of time and under different conditions, Krause wondered whether there might be ways to predict how certain human induced, biological or geological audio changes might affect the ecosystem as a whole. Conversely, when normal audio patterns change, how they might be telegraphing other significant alterations in the system. This was Krauses acoustical habitat ambient theory. In a trip in 1982 to the Amazon Basin, he recorded fifteen minute samples every hour in one location over a period of many weeks. What he discovered was that given the same weather, time of day and season, there appeared to be a measurably stable acoustical bio-spectrum quite unique to that particular place, a sound key. What was so unusual about the spectrogram evaluation of the recordings - a visual representation of the spectrum of frequencies of sound as they varied with time - was that they gave very strong evidence that when one vocal creature ceased to reproduce sounds, it would soon be replaced by yet another in the same part of the audio spectrum, thus appearing to keep intact the special ambient sound key of that habitat. The questions he asked then was in what ways do animals utilize these bio-acoustical spectra as beacons? Are there regional and local bio-acoustic habitat grids through which they travel to help them orient to one place or another? What happens when there is an acoustical imbalance such as human-induced masking noise or a biological change in a given habitat?

Bio-acoustic consistency, a unique vocal fingerprint

Later, Krause suggested that every creature had an aural niche or its own particular voice and specific place in a habitat based on the relative frequency, amplitude, timbre and duration of the sound it produces - each creatures voice performs as an integral part of an animal orchestra (22). Over a number of years, he would return to the same sites only to find, when the recordings were analysed, that each place showed bio-acoustic consistency – there would be a unique vocal fingerprint, an aural zone so unique and important to creature life in a given location. It would seem, he thought, that there was a clear acoustical message being sent as to the biological health of these locations, so that disturbance to this soundscape would lead to the inability of creatures to successfully communicate or otherwise employ their auditory senses. Krause gave an example of when he recorded in new stands of trees planted in the Olympic peninsula by lumber companies, he found a profound lack of bio-diversity evidenced first by the obvious monoculture of rows of fast-growing pines and very little supporting vegetation growing on the forest floor, but more so by the overwhelming silence. When compared with recordings of a nearby healthy old-growth forests, the measurable differences were he wrote astounding. Well, I would be astounded if they weren’t different, so what we are looking for is some subtlety in differentiation.

In a paper from 1999, Krause demonstrated a human-induced interruption in the evening soundscape of Western spadefoot toads, as a result of a low-flying military jet over the Mono Lake basin, California, and which resulted in peril for the toads (23). A series of spectrograms illustrated his point (Figs 3-6) the first showed the synchronicity of vocalization that occurred among many toads at the same time so that no predator can detect the origin of sound emanating from any one organism. The next showed the effect of the flyover as it was occurring in the drop off of the numbers of creatures vocalizing, as well as breaks in synchronicity. The third showed recordings made 20 minutes after the flyover, and which demonstrated a continued inability on the part of the toads to resume synchronicity. It was during this period that two coyotes and a great horned owl were observed in the available light drawn to the edge of the pond apparently to feed on the few remaining vocal amphibians. It was then that all vocalizations ceased until 45 minutes from the end of the aircraft fly-by when coherent vocalizations resumed, and no more predators were observed in the vicinity.

Krause had begun to use the term biophony for all of the sounds generated by organisms in a given habitat at one time and in one place. He would use that term and another he had coined in an educational guide produced for the National Park Service on listening to and recording wildscapes in the National Parks (24). He noted that habitats generate two types of non-human sound, biophony and geophony, the latter being all the non-creature sounds like streams, weather, and effects of wind in trees and grasses. Thus biophony and geophony together would be the natural soundscape. At the same time, Krause and colleague Stuart Gage where making sound recordings of the biophonies and geophonies at selected times during each of the four seasons at four different sites in Sequoia National Park, with the aim of testing whether biophony was an Indicator of habitat fitness and dynamics (25). This work was eventually reanalysed and published in the journal Landscape Ecology where Krause used another term – anthrophony, the sound produced by human-generated mechanical sounds, such as aircraft, automobiles, generators, snowmobiles, jet-skis, radios, television sets, boom boxes, bells, wind turbines or automobile sound systems (26).

A unifying theory of soundscape ecology

Two other papers would come out from Krause and others at the same time, one that presented a unifying theory of soundscape ecology, based on the causes and consequences of biological (biophony), geophysical (geophony), and human-produced (anthrophony) sounds, giving examples of soundscape maps of birds in Tuscany that showed that soundtopes (three-dimensional map of acoustic complexity) constructed from acoustic arrays could be used to quantify the spatial dynamics of soundscapes; a summary of the Sequoia National Park study that attempted to quantify the effects of geophony on biophonic patterns, and which showed that animals that communicate in each habitat do so at different frequencies to avoid overlap; and near-continuous sounds in a variety of landscapes in northwestern Tippecanoe County, Indiana, that showed that the temporal patterns of soundscapes exhibited strong dawn and dusk chorus peaks that diminished with increasing human disturbance in the landscape. (27). The other paper was an introduction and overview of soundscape ecology, providing a lexicon for it, an integrative framework of soundscape dynamics, how to measure soundscapes acoustically, temporally and spatially, and soundscape conservation (28).

Rather than plough through those information-dense readings, you could instead watch Krause’s TED talk that is about 15 mins long, and is a good introduction to his ideas about soundscape ecology/acoustic ecology, its components in geophony, biophony and anthrophony, and how this can be used to evaluate the health of a habitat across the entire spectrum of life (29). The story of the toads and the jet overfly is retold, and I am very surprised that a sea anemone can make such a noise. Krause gives another example of the effects of human disturbance on biophony in the “selective logging” of Lincoln Meadow in the Sierra Nevada Mountains. It was an approach of taking out a tree here and there rather than clear-cutting a whole area, the logging company convincing local residents that there would be absolutely no environmental impact. He had been recording in Lincoln Meadow for many years and, with permission, he went to record both before and after the operation. Before the logging, the sound signature of a stream was represented across the bottom third of the spectrogram, while birds in the meadow were represented in the signature across the top. He returned a year later after the selective logging, and while the stream was still represented in the bottom third, bird sound in the top third was almost completely missing. Krause has returned to Lincoln Meadow 15 times in the last 25 years, and the density and diversity of that biophony had not yet returned to anything like it was before the operation. Worse still, a picture of Lincoln Meadow taken after showed very little visual difference, which would confirm the logging company's contention that there was no environmental impact. However, Krause said that our ears told us a very different story.

As Krause says, we typically try to understand the world from what we see, but he thinks a much fuller understanding can be got from what we hear. I would say that the stimuli we receive through all our senses are the signatures of the natural world. However, as in light pollution, the presence of anthropogenic light in the night environment, sound pollution – noise – is also carelessly damaging of wild nature, and in a less obvious way. It reminds me of one of the premises that John Davis, erstwhile editor of Wild Earth and with the Rewilding Institute now, believed found agreement among wildland advocates (30):
“Anthropocentrism is ethically repugnant and biologically indefensible. All creatures have equal rights to their natural places on earth. Humans need to adopt an ecocentric paradigm and cease dominating the world”

Mark Fisher 4 September 2018

(1) Rumination, mindfulness and Awe Walks, Self-willed land June 2017

(2) Wilderness uncovered - the past and future of drowned lands, Self-willed land November 2016

(3) Soule, M.E. (1993) Biophilia: Unanswered questions. In Kellert, S.R. and Wilson (eds) The Biophilia Hypothesis. Island Press

(4) Natural Sounds, National Park Service

(5) Resource Management, NPS Essentials

(6) Natural Resource Management, Chapter Four, Management Policies 2006: The Guide to Managing the National Park System, National Park Service, US Department of the Interior

(7) Why Sounds Matter, Natural Sounds, National Park Service

(8) Sound Library, Rocky Mountain National Park, National Park Service

(9) Sound Library, Yellowstone National Park, National Park Service

(10) Grizzly Bear Eating, Sound Library - Grizzly Bear, Yellowstone National Park, National Park Service

(12) Wolves, Sound Library – Wolves, Yellowstone National Park, National Park Service

(13) Soundscape - Lake (bull elk bugling) Yellowstone National Park, National Park Service

(14) Habitat fragmentation and the ecology of artefacts, Self-willed land January 2015

(15) Effects of Noise on Wildlife, Natural Sounds, National Park Service

(16) Noise, Natural Sounds, National Park Service

(17) Shannon, G., McKenna, M.F., Angeloni, L.M., Crooks, K.R., Fristrup, K.M., Brown, E., Warner, K.A., Nelson, M.D., White, C., Briggs, J. and McFarland, S., 2016. A synthesis of two decades of research documenting the effects of noise on wildlife. Biological Reviews, 91(4), pp.982-1005.

(18) Barber, J. R., Crooks, K. R., & Fristrup, K. M. (2010). The costs of chronic noise exposure for terrestrial organisms. Trends in ecology & evolution, 25(3), 180-189

(19) Buxton, R. T., McKenna, M. F., Mennitt, D., Fristrup, K., Crooks, K., Angeloni, L., & Wittemyer, G. (2017). Noise pollution is pervasive in US protected areas. Science, 356(6337), 531-533

(20) Ware, H. E., McClure, C. J., Carlisle, J. D., & Barber, J. R. (2015). A phantom road experiment reveals traffic noise is an invisible source of habitat degradation. Proceedings of the National Academy of Sciences, 112(39), 12105-12109.

(21) Krause, B. (1987). Bioacoustics, habitat ambience in ecological balance. Whole Earth Review, 57, 14-18.

(22) Krause, B. L. (1993). The niche hypothesis: a virtual symphony of animal sounds, the origins of musical expression and the health of habitats. The Soundscape Newsletter, 6, 6-10.

(23) Krause, B. (1999). Loss of natural soundscapes within the Americas. The Journal of the Acoustical Society of America, 106(4), 2201-2201

(24) Krause, B. L. (2002). Wild soundscapes in the national parks: an educational program guide to listening and recording. National Park Service, USA.

(25) Krause, B. L., & Gage, S. (2003). Testing biophony as an indicator of habitat fitness and dynamics. SEKI Natural Soundscape Vital Signs Pilot Program Report, 18

(26) Krause, B., Gage, S. H., & Joo, W. (2011). Measuring and interpreting the temporal variability in the soundscape at four places in Sequoia National Park. Landscape ecology, 26(9), 1247

(27) Pijanowski, B.C., Villanueva-Rivera, L.J., Dumyahn, S.L., Farina, A., Krause, B.L., Napoletano, B.M., Gage, S.H. and Pieretti, N., 2011. Soundscape ecology: the science of sound in the landscape. BioScience, 61(3), pp.203-216.

(28) Pijanowski, B. C., Farina, A., Gage, S. H., Dumyahn, S. L., & Krause, B. L. (2011). What is soundscape ecology? An introduction and overview of an emerging new science. Landscape ecology, 26(9), 1213-1232.

(29) The voice of the natural world, Bernie Krause TED Talk 2013

(30) Davis, J. (1994). A Sidelong Glance at The Wildlands Project. Burks, D. C. (ed). Place of the wild: a wildlands anthology. Island Press.236-245