ReviewThe emerging significance of bioacoustics in animal species conservation
Introduction
Communication, the way organisms convey information to each other, is the gel that holds animal societies together: it facilitates reproduction, provides information on individual identity, status, mood and intentions (Bradbury and Vehrencamp, 1998). As it includes a substantial proportion of the behavioural repertoire of animal species, communication behaviour can become an important driver of several aspects of species biology, affecting the evolution of life histories and genes.
Along with several other animals, humans share the use of sounds as the principal means of exchanging information. Many vertebrates (bony fishes, amphibians, reptiles, birds, mammals) and invertebrates (insects, spiders, crustaceans, nematodes) make sounds (or vibrations) for a variety of reasons, mostly for courtship and agonistic behaviours, but also for more complex social communication (Hauser, 1997, Owings et al., 1998). Many birds (oscines, some sub-oscines, trochilids and psittacines) and some mammals (cetaceans, primates, bats) may acquire important components of their acoustic repertoires by copying others, while this behaviour is thought to be innate in the other taxonomic groups (Kroodsma and Baylis, 1982, Janik and Slater, 1997). As an example, birdsong was the first ‘cultural’ trait (i.e. acquired through social learning) to be described in non-human animals, based on evidence dating back to Aristotle (Laland and Galef, 2009).
Acoustic signals are particularly well suited for studying the evolution of animal communication because of the relative ease with which sounds can be recorded and analyzed, synthesised and played back with efficiency (Gerhardt and Huber, 2002). Animal sounds have indeed served as models to address essential evolutionary questions, such as the way sexual selection operates and intervenes in speciation processes and the way natural selection shapes animal interactions (Kroodsma and Miller, 1996). In spite of being the target of many evolutionary studies, the role of animal vocalizations has been less significant in applied ecological research (Terry et al., 2005). Until the last decade, their use has been limited to acoustic surveys and censuses to detect vocal species of birds, mammals, amphibians and insects. Bioacoustics has also been used to generate basic demographic variables through the vocal identification of individuals, or estimate species occurrence and richness in those cryptic taxa characterized by species-specific acoustic signals (see also Caro, 1998, Vaughan et al., 1997, Gaunt and McCallum, 2004).
More recently, bioacousticians have begun to tackle the questions of how human activities challenge the communication systems of animal species, what are the stochastic or deterministic mechanisms involved (natural, sexual or social selection processes), and what information of conservation significance can be derived by studying animal sounds (Rabin and Greene, 2002, Slabbekoorn and Ripmeester, 2008, Laiolo et al., 2008). A similar drive determined the development of ‘Conservation Behaviour’, a discipline that combines applied and baseline research to address the behavioural mechanisms that influence the fate of populations and species (Curio, 1996, Buchholz, 2007, Caro, 2007).
The aim of this review is to collect recent literature on the impact of human activities on animal communication, and provide an overview of the potential of bioacoustics in conservation science. Based on published evidence, I discuss the type of information that could be extracted from animal sounds which may be relevant to species conservation and population ecology, and highlight a series of troublesome cases, in which acoustic variation may cause conservation problems and affects population persistence. Finally, I discuss how acoustic signals can be used in conservation studies as early-warning indicators of ongoing human-driven perturbations or to monitor population processes.
Section snippets
Bibliographic search
The overview is based on a Thompson’s ISI Web of Science search of journals within the subject categories of ‘Zoology’, ‘Ecology’, ‘Multidisciplinary Sciences’, ‘Behavioural Science’, ‘Acoustics’, ‘Biology’, ‘Marine and Freshwater Biology’, ‘Evolutionary Biology’, ‘Ornithology’, and ‘Environmental Science’ from 1970 to 2009. As a variety of human impacts has been proven to affect animal communication and no single search term could define them, I started with a broad search of the terms CALL or
Results
I found that 53 papers explicitly focused on human-driven alterations (excluding review papers). For simplicity, I refer to these studies as ‘Conservation Bioacoustics’ papers. In the remaining titles of the search, I paid special attention to those of a more descriptive nature, which dealt with intra-specific acoustic variation. I searched here for inadvertent comparisons among natural and anthropogenic habitats, populations separated by anthropogenic barriers or differently affected by human
Discussion
In the following sections (4.1–4.7) I consider each anthropogenic vector of acoustic change and discuss the consequences for individuals and populations. When the species response (or lack of response) critically affects the fate of populations living in human-transformed ecosystems, the acoustic shift has great conservation significance. When human-driven acoustic variation is not directly affecting individual fitness or population viability, it could still serve in a conservation context, as
Conclusions
A variety of human impacts can affect animal communication systems by triggering stochastic or deterministic forces of evolutionary or ecological change. The findings of this overview show that communication may become a mechanism that negatively affects population persistence in some instances, resulting in a conservation dilemma. All acoustic traits associated with (or mediating) aspects of reproductive success, survivorship and/or recruitment can indirectly affect population growth rates and
Acknowledgements
I am very grateful to F. Sergio, A.S. Pullin and two anonymous referees for providing useful comments and suggestions. While writing, I was supported by the Project CGL2008-02749 of the Spanish Ministry of Science and Innovation.
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