Toward a Synthetic Acoustic Ecology: Sonically Situated, Evolutionary Agent Based Models of the Acoustic Niche Hypothesis Alice Eldridge1;2 and Chris Kiefer1 1Emute Lab, Department of Music, University of Sussex, UK 2Peck Labs, Evolution, Behaviour & Environment, University of Sussex, UK [email protected] Abstract lective creature choruses as more powerful, informative nar- ratives in soundscape composition (Monacchi, 2013; Bar- We introduce the idea of Synthetic Acoustic Ecology (SAC) as clay and Gifford, 2018). However, there is a paucity of co- a vehicle for transdisciplinary investigation to develop meth- herent theory addressing the ecological significance of the ods and address open theoretical, applied and aesthetic ques- global soundscape, lack of effective computational tools for tions in scientific and artistic disciplines of acoustic ecology. Ecoacoustics is an emerging science that investigates and in- ecological monitoring (Sueur et al., 2008) and many latent terprets the ecological role of sound. It draws conceptually creative applications, for example in musical composition or from, and is reinvigorating the related arts-humanities dis- game world design. Just as Alife modelling has potential to ciplines historically associated with acoustic ecology, which mediate theoretical and empirical biology (Wheeler et al., are concerned with sonically-mediated relationships between 2002), we propose that a sonically situated flavour of Alife, human beings and their environments. Both study the acous- tic environment, or soundscape, as the literal and concep- which we call Synthetic Acoustic Ecology (SAC), may be a tual site of interaction of human and non-human organisms. productive vehicle for investigation and a nexus of exchange However, no coherent theories exist to frame the ecological between the scientific, artistic and technological facets of role of the soundscape, or to elucidate the evolutionary pro- acoustic ecology. In doing to we hope to advance our under- cesses through which it is structured. Similarly there is a lack standing and appreciation of soundscape as an interface of of appropriate computational methods to analyse the macro soundscape which hampers application in conservation. We human and natural systems which both reflects and affects propose that a sonically situated flavour of Alife evolutionary our coupled environments. agent-based model could build a productive bridge between the art, science and technologies of acoustic ecological in- vestigations to the benefit of all. As a first step, two simple Soundscape, Ecoacoustics and Acoustic Ecology models of the acoustic niche hypothesis are presented which The term ‘soundscape’ has been used by a variety of dis- are shown to exhibit emergence of complex spectro-temporal soundscape structures and adaptation to and recovery from ciplines to describe the relationship between a landscape noise pollution events. We discuss the potential of SAC as a and the composition of its sound in both real and virtual lingua franca between empirical and theoretical ecoacoustics, worlds (Grimshaw and Schott, 2007). Originally coined and wider transdisciplinary research in ecoacoustic ecology. in the context of urban design (Southworth, 1967), sound- scape was later used by a group of environmentally-aware radio artists and sonic sociologists to describe ‘the acousti- Introduction cal characteristics of an area that reflect natural processes’ There is increasing interest across ecological science (Pi- (Schafer, 1977). Within the scientific discipline of sound- janowski et al., 2011; Sueur and Farina, 2015), arts (Bar- scape ecology a framework has been proposed to investi- clay and Gifford, 2018; Monacchi and Krause, 2017; Mc- gate soundscape in terms of the causes and consequences Cormack et al., 2009) and humanities (Turner et al., 2003) of the biological (biophony), geophysical (geophony), and in listening to, recording, investigating and interpreting the human-produced (anthrophony) sounds that emanate from acoustic environment - or soundscape - as the interface of a landscape (Pijanowski et al., 2011). The emerging inter- human and natural systems. If bioacoustics can be charac- disciplinary science of Ecoacoustics subsumes both sound- terised as the study of the isolated duets of vocalising crit- scape ecology and bioacoustics (Sueur and Farina, 2015) to ters, then the nascent field science of ecoacoustics is con- study the ecological role of sound. There is a growing im- cerned with investigating and interpreting the ecological rel- petus to develop acoustic ecology as a truly interdisciplinary evance of the strains of full orchestra. A similar shift is evi- endeavour (Barclay and Gifford, 2018), bridging traditional dent in the arts; representation of biophonies in field record- disciplinary divides. This is where we position Synthetic ings and music has broadened from individual voices to col- Acoustic Ecology. Whereas bioacoustics infers behavioural information from intra- and interspecific signals, ecoacoustics inves- tigates the ecological role of sound at higher ecological and evolutionary organisational units - from population and community up to landscape scales. Sound is understood as a core ecological component (resource) and ipso facto, due to structuring by competition, an indicator of ecological sta- tus (source of information). The field has been substantially bolstered by the increasing availability and decreasing costs of automated recording devices (Acevedo and Villanueva- Rivera, 2006; Farina et al., 2018), cheap storage and de- velopments in acoustic data processing (Truskinger et al., 2014). However, whilst it has drawn from theories of related ecological disciplines including bioacoustics, and landscape ecology (Turner et al., 2001), there is an absence of coher- ent theory regarding the ecological significance of the macro soundscape. This not only constrains theoretical advances, but hampers potential applications such as environmental monitoring and prediction. Ecoacoustics is born of Bioa- coustic Big Data, but lacks coherent theories and computa- Figure 1: Spectrogram (0 − 22:5 kHz) of a field record- tional tools for effective development and application. ing made in the Ecuadorian amazon showing inter-taxon fre- quency partitioning of the acoustic environment. The vocal- Acoustic Niche Hypothesis isations of each taxa are bandlimited, minimising frequency Three hypotheses underpin ecoacoustics. The morphologi- overlap between species; quasi temporal partitioning can cal adaptation hypothesis (MAH) and the acoustic adapta- also be observed in anuran species, although it is not clear tion hypothesis (AAH) are borrowed by bioacoustics and here if this is conspecific or heterospecific. describe how signals evolve through ecological feedback; the acoustic niche hypothesis is core to ecoacoustics (and is also the hardest to evaluate) and describes the evolution of (Chek et al., 2003). soundscape complexity. The MAH focuses on the ‘sender’ The more significant and controversial prediction follows and hypothesises that the embodied form (body size, trachea that soundscape structure is a proxy for ecological integrity: length, beak shape etc.) will shape potential range of signals less-disturbed habitats with unaltered species assemblages (Bennet-Clark, 1998). The AAH (Morton, 1975) predicts will exhibit higher levels of coordination between inter- that acoustic properties of an environment can influence the specific vocalizations than more heavily disturbed habitats, evolution of vocalizations in certain species. where species assemblages are in rapid flux. Likewise, inva- In his formulation of the ANH, musician-turned bioacous- sive species could create biophonic disturbances, thereby al- tician Bernie Krause pointed out that both morphological tering natural acoustic partitioning (Pijanowski et al., 2011). and behavioural adaptations can also be triggered by inter- This implies that if we listen in the right way we can hear specific interference when organisms’ calls contain similar the health of an ecosystem. frequency and timing features (Krause, 1993). The ANH ANH is foundational to ecoacoustic theory and has ma- was inspired by observation of complex arrangements of jor implications for ecological monitoring and prediction, dense, non-overlapping signals in recordings of soundscapes however empirical validation and development of applica- across multiple pristine habitats. Krause postulated that this tion is hard: firstly because it is not clear exactly what it could be explained by evolutionary pressure to minimize means for a soundscape to exhibit ‘higher levels of coordina- spectral or temporal overlaps in interspecific vocalizations. tion’; secondly because measurement of ecological integrity The ANH expands Hutchinson’s ecological niche concept and even biodiversity remains contentious (Hillebrand et al., (Hutchinson, 1957), which is foundational to modern ecol- 2018); thirdly we lack appropriate computational meth- ogy, by adding a sonic dimension to evolutionary ecospace. ods for community-level machine listening (Eldridge et al., That vocalising species partition acoustic space to minimise 2016). Whilst research into bioacoustically motivated ma- interference from sympatric species has long been recog- chine listening algorithms for automated species detection is nised (Duellman and Pyles, 1983) in bioacoustics. As illus- well developed (e.g. Stowell and Plumbley (2014)), commu- trated in Figure 1, frequency partitioning across major taxa