Biophony

Terminology

Soundscape ecologists seek to investigate the structure of soundscapes, explain how they are generated, and study how organisms interrelate acoustically. 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. 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.