Page 36 - Acoustics Today

PASSIVE ACOUSTIC MONITORING FOR ESTIMATING ANIMAL DENSITY
Len Thomas
Centre for Research into Ecological and Environmental Modeling
University of St. Andrews, The Observatory, Buchanan Gardens, Fife KY16 9LZ, Scotland
and
Tiago A. Marques
Centre for Research into Ecological and Environmental Modeling
University of St Andrews, The Observatory, Buchanan Gardens, Fife KY16 9LZ, Scotland
and
Centro de Estatística e Aplicações da Universidade de Lisboa Bloco C6, Piso 4, Campo Grande, 1749-016 Lisboa, Portugal
One of the most fundamental “Although “how many?” is a pletely. If animals do not bear natural
questions we can ask about a
wildlife population is “How
many are there?” Estimates of popula-
tion size, or a related quantity popula-
tion density (i.e., animals per unit
area), are crucial for effective manage-
ment, whether the management goal is
conservation of a threatened or endan-
gered species, control of a pest species,
or optimal harvest of a species used for
food. Population estimates are used to
prioritize species of conservation con-
cern, to monitor the success of man-
agement 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 popu- lations 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 popula- tion size and density using passive acoustics.
The potential for passive acoustic density estimation
Traditional methods for estimating animal numbers most often rely on visual surveys, where animals need to be seen to be counted. Probably the most common is a visually- based distance sampling survey (Buckland et al., 2001), where observers visit a set of randomly placed transect lines or points, recording the distance to all detected animals of the target species. The distances are important because they allow us to estimate the probability of detecting animals, and hence account for the proportion missed (see later). Alternatively, if the animals have uniquely identifiable mark- ings, then an alternative method can be used, called mark- recapture (or capture-recapture; Williams et al., 2002). Here, observers visit the study area on a sequence of occasions, recording each animal detected and, if it was marked, which animal it was. The patterns of missing detections of marked animals can be used to estimate the probability of detection and hence infer how many animals have been missed com-
markings, they can in some studies be caught and given marks (such as tags).
These traditional visual methods have been used in thousands of studies
1
simple question to ask, it’s often a hard one to answer ...we introduce an emerging field with great potential: The estimation of wild animal population size and density using passive acoustics.”
covering every major taxonomic group. However, they do not work well in all circumstances. For one thing, some ani- mals are inherently hard to see, for example because they live underwater or in thick forest, or because they are small, well camouflaged, or only active at night. Many do not have readily dis- tinguishable markings and are hard to trap. In addition, traditional surveys can be very expensive, requiring trained observers and expensive survey vehicles
to operate for extended periods in often far-flung and inhos- pitable environments. Think what it would cost to undertake a visual survey of marine mammals in the Southern Ocean in winter. For these reasons, there has been a keen interest in developing alternatives.
Passive acoustic monitoring offers one such alternative. Many species of animal produce distinctive sounds, either as part of a social display (e.g., to mark a territory or attract mates) or as an aid in navigation and foraging (e.g., echolo- cation). In many cases, these acoustic signals are detectable at greater distances than visual cues. Indeed, in environments where light does not pass easily, and hence visual methods are ineffective, animals are more likely to use sound as a means of communication, making them ideally suited for acoustic methods. Acoustic methods are also potentially less affected by weather conditions, and can operate under vary- ing light levels, particularly at night. Another advantage is that the science of automated detection and classification of sounds is relatively well developed, as readers of this maga- zine will know, opening the possibility of automated process- ing of large volumes of remotely gathered data. By contrast, visual surveys are still almost universally performed by human observers, and even if digital imaging is used, classi- fication is almost exclusively performed by humans (e.g., Buckland et al., 2012).
Passive Acoustic Monitoring for Estimating Animal Density 35