Quantifying lost opportunities to hear natural sounds

Abstract

Hearing provides an omnidirectional alerting sense for wildlife that seems to be universal: No deaf vertebrate species are known and invertebrates display a remarkable diversity of hearing mechanisms. Anthropogenic noise elevates ambient sound levels, which masks natural sounds that would otherwise be heard. The costs of this masking can be assessed by calculating the loss of listening area or alerting distance that results. Listening area metrics are appropriate when a search function might be affected (e.g., foraging), while alerting distance metrics are appropriate when the distance to the sound source mediates the function (e.g., avoiding predation). Analytical approaches for calculating loss of listening area and alerting distance should incorporate available hearing data to account for the effects of hearing thresholds and critical bandwidths. A range of models for masking can be used. Very simple models may sacrifice accuracy to suggest metrics that are readily calculated using existing noise models. More complex models can capture the idiosyncrasies of each species hearing capabilities to render more detailed results. Examples of applying these metrics to National Park Service contexts are discussed, to illustrate the use of these concepts to render environmental acoustic data for resource managers and NPS leadership.