One of the most fundamental questions we can ask about a wildlife population is “How many are there?” Estimates of population size, or a related quantity population density (i.e., animals per unit area), are crucial for effective management, whether the management goal is conservation of a threatened or endangered species, control of a pest species, or optimal harvest of a species used for food. Population estimates are used to prioritize species of conservation concern, to monitor the success of management programs, and to set limits on harvest or incidental bycatch. Although “how many?” is a simple question to ask, it is often a hard one to answer, given that many populations are patchily distributed over very large areas and their lifestyle can make them quite cryptic to human observers. In this article, we introduce an emerging field with great potential—the estimation of wild animal population size and density using passive acoustics.

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