Soundscape Ecology

Soundscape ecology is the study of sound within a landscape and its effect on organisms. Sounds may be generated by organisms (biophony), by the physical environment (geophony), or by humans (anthrophony). Soundscape ecologists seek to understand how these different sound sources interact across spatial scales and through time. Variation in soundscapes may have wide-ranging ecological effects as organisms often obtain information from environmental sounds. Soundscape ecologists use recording devices, audio tools, and elements of traditional ecological analyses to study soundscape structure. Increasingly, anthrophony, sometimes referred to in older, more archaic terminology as anthropogenic noise dominates soundscapes, and this type of noise pollution or disturbance has a negative impact on a wide range of organisms. The preservation of natural soundscapes is now a recognized conservation goal.

What is soundscape ecology?


Soundscape ecology is the bio- and geo-acoustic branch of ecology that studies sound within a landscape (the soundscape). The soundscape of a given region can be viewed as the sum of three separate sound elements: biophony (sound generated from non-human organisms), geophony (sound generated from wind, rain, or other natural physical processes), and anthrophony (sound generated from human activities). Biophony may consist of sound generated simply through organism movement but often represents vocalizations that are used for conspecific communication. In contrast, anthrophony from heavily populated urban regions usually contains information that was intentionally produced for communication with a sound receiver. Geophony consists of non-biological ambient sounds. The expression in various combinations of these acoustic features across space and time generate unique soundscapes.

Soundscape ecologists seek to investigate the structure of soundscapes, explain how they are generated, and study how they may affect organisms. A number of hypotheses have been proposed to explain the structure of soundscapes, particularly elements of biophony. For instance, an ecological theory known as the acoustic adaptation hypothesis predicts that acoustic signals of animals are altered in different physical environments in order to maximize their propagation through the habitat. In addition, acoustic signals from organisms may be under selective pressure to minimize their frequency (pitch) overlap with other auditory features of the environment. This acoustic niche hypothesis is analogous to the classical ecological concept of niche partitioning. It suggests that acoustic signals in the environment should display frequency partitioning as a result of selection acting to maximize the effectiveness of intraspecific communication for different species. Observations of frequency differentiation among insects, birds, and anurans support the acoustic niche hypothesis. Organisms may also partition their vocalization frequencies to avoid overlap with pervasive geophonic sounds. For example, territorial communication in some frog species takes place partially in the high frequency ultrasonic spectrum. Presumably this communication method represents an evolutionary adaptation to the frogs’ riparian habitat where running water produces constant low frequency sound. Invasive species that introduce new sounds into soundscapes can disrupt acoustic niche partitioning in native communities, a process known as biophonic invasion. Although adaptation to acoustic niches may explain the frequency structure of soundscapes, spatial variation in sound is likely to be generated by environmental gradients in altitude, latitude, or habitat disturbance. These gradients may alter the relative contributions of biophony, geophony, and anthrophony to the soundscape. For example, when compared with unaltered habitats, regions with high levels of urban land-use are likely to have increased levels of anthrophony and decreased physical and organismal sound sources. Soundscapes typically exhibit temporal patterns, with daily and seasonal cycles being particularly prominent. These patterns are often generated by the communities of organisms that contribute to biophony. For example, birds chorus heavily at dawn and dusk while anurans call primarily at night; the timing of these vocalization events may have evolved to minimize temporal overlap with other elements of the soundscape.

Contributions from other fields
As an academic discipline, soundscape ecology shares some characteristics with other fields of inquiry but is also distinct from them in significant ways. For instance, acoustic ecology is also concerned with the study of multiple sound sources. However, acoustic ecology, which derives from the founding work of R. Murray Schafer and Barry Truax, primarily focuses on human perception of soundscapes. Soundscape ecology seeks a broader perspective by considering soundscape effects on wildlife communities and potential interactions between sounds in the environment. Compared to soundscape ecology, the discipline of bioacoustics tends to have a narrower interest in individual species’ physiological and behavioral mechanisms of auditory communication. Soundscape ecology also borrows heavily from some concepts in landscape ecology, which focuses on ecological patterns and processes occurring over multiple spatial scales. Landscapes may directly influence soundscapes as some organisms use physical features of their habitat to alter their vocalizations. For example, baboons and other animals exploit specific habitats to generate echoes of the sounds they produce.

The function and importance of sound in the environment may not be fully appreciated unless one adopts an organismal perspective on sound perception, and, in this way, soundscape ecology is also informed by sensory ecology. Sensory ecology focuses on understanding the sensory systems of organisms and the biological function of information obtained from these systems. In many cases, humans must acknowledge that sensory modalities and information used by other organisms may not be obvious from an anthropocentric viewpoint. This perspective has already highlighted many instances where organisms rely heavily on sound cues generated within their natural environments to perform important biological functions. For example, a broad range of crustaceans are known to respond to biophony generated around coral reefs. Species that must settle on reefs to complete their developmental cycle are attracted to reef noise while pelagic and nocturnal crustaceans are repelled by the same acoustic signal, presumably as a mechanism to avoid predation (predator densities are high in reef habitats). Similarly, juvenile fish may use biophony as a navigational cue to locate their natal reefs. Other species’ movement patterns are influenced by geophony, as in the case of the reed frog which is known to disperse away from the sound of fire. In addition, a variety of bird and mammal species use auditory cues, such as movement noise, in order to locate prey. Disturbances created by periods of environmental noise may also be exploited by some animals while foraging. For example, insects that prey on spiders concentrate foraging activities during episodes of environmental noise to avoid detection by their prey. These examples demonstrate that many organisms are highly capable of extracting information from soundscapes.

Methods in soundscape ecology
Acoustic information describing the environment is the primary data required in soundscape ecology studies. Technological advances have provided improved methods for the collection of such data. Automated recording systems allow for temporally replicated samples of soundscapes to be gathered with relative ease. Data collected from such equipment can be extracted to generate a visual representation of the soundscape in the form of a spectrogram. Spectrograms provide information on a number of sound properties that may be subject to quantitative analysis. The vertical axis of a spectrogram indicates the frequency of a sound while the horizontal axis displays the time scale over which sounds were recorded. In addition, spectrograms display the amplitude of sound, a measure of sound intensity. Ecological indices traditionally used with species-level data, such as diversity and evenness, have been adapted for use with acoustic metrics. These measures provide a method of comparing soundscapes across time or space. For example, automated recording devices have been used to gather acoustic data in different landscapes across yearlong time scales, and diversity metrics were employed to evaluate daily and seasonal fluctuations in soundscapes across sites. Spatial patterns of sound may also be studied using tools familiar to landscape ecologists such as geographic information systems (GIS). Finally, recorded samples of the soundscape can provide proxy measures for biodiversity inventories in cases where other sampling methods are impractical or inefficient. These techniques may be especially important for the study of rare or elusive species that are especially difficult to monitor in other ways.

Insights from soundscape ecology: anthrophony
Although soundscape ecology has only recently been defined as an independent academic discipline (it was formalized in 2011), many earlier ecological investigations have incorporated elements of soundscape ecology theory. For instance, a large body of work has focused on documenting the effects of anthropogenic noise on wildlife. Anthropogenic noise (often used synonymously with noise pollution) can emanate from a variety of sources, including transportation networks or industry, and may represent a pervasive disturbance to natural systems even in seemingly remote regions such as national parks. A major effect of noise is the masking of organismal acoustic signals that contain information. Against a noisy background, organisms may have trouble perceiving sounds that are important for intraspecific communication, foraging, predator recognition, or a variety of other ecological functions. In this way, anthropogenic noise may represent a soundscape interaction wherein increased anthrophony interferes with biophonic processes. The negative effects of anthropogenic noise impact a wide variety of taxa including fish, amphibians, birds, and mammals. In addition to interfering with ecologically important sounds, anthropogenic noise can also directly affect the biological systems of organisms. Noise exposure, which may be perceived as a threat, can lead to physiological changes. For example, noise can increase levels of stress hormones, impair cognition, reduce immune function, and induce DNA damage. Although much of the research on anthropogenic noise has focused on behavioral and population-level responses to noise disturbance, these molecular and cellular systems may prove promising areas for future work.

Anthrophony and birds


Birds have been used as study organisms in much of the research concerning wildlife responses to anthropogenic noise, and the resulting literature documents many effects that are relevant to other taxa affected by anthrophony. Birds may be particularly sensitive to noise pollution given that they rely heavily on acoustic signals for intraspecific communication. Indeed, a wide range of studies demonstrate that birds use altered songs in noisy environments. Research on great tits in an urban environment revealed that male birds inhabiting noisy territories tended to use higher frequency sounds in their songs. Presumably these higher-pitched songs allow male birds to be heard above anthropogenic noise, which tends to have high energy in the lower frequency range thereby masking sounds in that spectra. A follow-up study of multiple populations confirmed that great tits in urban areas sing with an increased minimum frequency relative to forest-dwelling birds. In addition, this study suggests that noisy urban habitats host birds that use shorter songs but repeat them more rapidly. In contrast to frequency modulations, birds may simply increase the amplitude (loudness) of their songs to decrease masking in environments with elevated noise. Experimental work and field observations show that these song alterations may be the result of behavioral plasticity rather than evolutionary adaptations to noise (i.e., birds actively change their song repertoire depending on the acoustic conditions they experience). In fact, avian vocal adjustments to anthropogenic noise are unlikely to be the products of evolutionary change simply because high noise levels are a relatively recent selection pressure. However, not all bird species adjust their songs to improve communication in noisy environments, which may limit their ability to occupy habitats subject to anthropogenic noise. In some species, individual birds establish a relatively rigid vocal repertoire when they are young, and these sorts of developmental constraints may limit their ability to make vocal adjustments later in life. Thus, species that do not or cannot modify their songs may be particularly sensitive to habitat degradation as a result of noise pollution.



Even among birds that are able to alter their songs to be better heard in environments inundated with anthropogenic noise, these behavioral changes may have important fitness consequences. In the great tit, for example, there is a tradeoff between signal strength and signal detection that depends on song frequency. Male birds that include more low frequency sounds in their song repertoire experience better sexual fidelity from their mates which results in increased reproductive success. However, low frequency sounds tend to be masked when anthropogenic noise is present, and high frequency songs are more effective at eliciting female responses under these conditions. Birds may therefore experience competing selective pressures in habitats with high levels of anthropogenic noise: pressure to call more at lower frequencies in order to improve signal strength and secure good mates versus opposing pressure to sing at higher frequencies in order to ensure that calls are detected against a background of anthrophony. In addition, use of certain vocalizations, including high amplitude sounds that reduce masking in noisy environments, may impose energetic costs that reduce fitness. Because of the reproductive trade-offs and other stresses they impose on some birds, noisy habitats may represent ecological traps, habitats in which individuals have reduced fitness yet are colonized at rates greater than or equal to other habitats.

Anthrophony may ultimately have population- or community-level impacts on avian fauna. One study focusing on community composition found that habitats exposed to anthropogenic noise hosted fewer bird species than regions without noise, but both areas had similar numbers of nests. In fact, nests in noisy habitats had higher survival than those laid in control habitats, presumably because noisy environments hosted fewer western scrub jays which are major nest predators of other birds. Thus, anthropogenic noise can have negative effects on local species diversity, but the species capable of coping with noise disturbance may actually benefit from the exclusion of negative species interactions in those areas. Other experiments suggest that noise pollution has the potential to affect avian mating systems by altering the strength of pair bonds. When exposed to high amplitude environmental noise in a laboratory setting, zebra finches, a monogamous species, show a decreased preference for their mated partners. Similarly, male reed buntings in quiet environments are more likely to be part of a mated pair than males in noisy locations. Such effects may ultimately result in reduced reproductive output of birds subject to high levels of environmental noise.

Soundscape conservation
The discipline of conservation biology has traditionally been concerned with the preservation of biodiversity and the habitats that organisms are dependent upon. However, soundscape ecology encourages biologists to consider natural soundscapes as resources worthy of conservation efforts. Unaltered soundscapes have value for wildlife as demonstrated by the numerous negative effects of anthropogenic noise on various species. Organisms that use acoustic cues generated by their prey may be particularly impacted by human-altered soundscapes. In this situation, the (unintentional) senders of the acoustic signals will have no incentive to compensate for masking imposed by anthropogenic sound. In addition, natural soundscapes can have benefits for human wellbeing and may help generate a distinct sense of place, connecting people to the environment and providing unique aesthetic experiences. Because of the various values inherent in natural soundscapes, they may be considered ecosystem services that are provisioned by intact, functioning ecosystems. Targets for soundscape conservation may include soundscapes necessary for the persistence of threatened wildlife, soundscapes that are themselves being severely altered by anthrophony, and soundscapes that represent unique places or cultural values. Some governments and management agencies have begun to consider preservation of natural soundscapes as an environmental priority. In the United States, the National Park Service's Natural Sounds and Night Skies Division is working to protect natural and cultural soundscapes.