Ecology and acoustics at a large scale

There is likely no silent place on Earth. Sounds produced by inanimate sources, as well as by plants, animals and humans, permeate all environments from the deep-sea to the aerosphere, 24 h a day (Stocker, 2013). Eavesdropping on animal vocal communications has been an active field of research for more than 60 years (Busnel, 1963) and has, for example, alerted us to the possible deleterious effects of anthropogenic noise on animal physiology and behavior (Barber et al., 2011; Brumm and Slabbekoorn, 2005). Such research is the domain of bioacoustics, the study of the emission, propagation and reception of sound by animals (Bradbury and Vehrencamp, 1998). Bioacoustics is necessarily interdisciplinary, with links to ethology, physiology, neurobiology, biomechanics and evolution, but it tends to focus on the acoustic behavior of individuals, groups, or populations almost always with reference to the biological concept of a species. In other words, it is a species-centered discipline. However, species do not live in closed systems— rather they are part of larger hierarchical structures such as guilds, communities, ecosystems and landscapes. Acoustic interactions between species and between species and their environment may impose important constraints on the structure of these higher-level systems and their development through time. The justification for this special edition on ecological acoustics is the need for investigations of the role of acoustics at higher levels of biological organization. Such investigations link bioacoustics to ecology and point to newways of understanding both animal sounds and ecosystem processes. First attempts to link acoustics with ecology involved the development of acoustic diversity indices as indicators of biodiversity (Pieretti et al., 2011; Sueur et al., 2008) andwith the formalization of soundscape ecology (Pijanowski et al., 2011a). Acoustic diversity indices were inspired by classical indices used in biodiversity assessment with the community level as the unit of sampling and analysis. Soundscape ecology is “the study of sound in landscapes based on an understanding of how sound, from various sources — biological, geophysical and anthropogenic, can be used to understand coupled natural–human dynamics across different spatial and temporal scales” (Pijanowski et al, 2011b) (Fig. 1). Scaling up to community or landscape level would help to address three important challenges in ecology: (1) monitoring animal diversity, (2) understanding the interactions between animal species and (3) measuring andmitigating human noise pollution. Monitoring global changes in biodiversity due to urbanization, ecosystem fragmentation, climate change etc. will require techniques that can scale massively. Acoustic monitoring techniques offer this possibility and a key objective of this special edition is to report on acoustic monitoring techniques that can scale. For example, what kind of acoustic-sensing networks are required to monitor oceans, landscapes and cities? Can acoustic diversity indices act as proxies for biodiversity? Can acoustic indices be used to