Counting gibbons: The evolution of sample methods
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Abstract
Estimating the population sizes of endangered species in the natural environment is a major activity of conservationists. Dierent types of animals require dierent methods of population census and sampling. Statistical standards of sampling populations have increased and new analytical software has reduced the time needed for manual computation, but increased the need for standardized data collection methods. Here I discuss the methodology used in gibbon population sampling and current discussions regarding the best method of data analysis. Gibbons are most effciently sampled by listening for their duetted songs given by mated reproductive pairs in small territorial groups. Several problems have to be overcome in field data collection and analysis: the decline in sound detection with distance, the possibility of groups singing more than once in a day, the probability of a group singing (or not singing) on a given day, and determination of the total area that the audible groups are occupying. The traditional way of dealing with these problems is by triangulating on singing groups from several simultaneous listening posts, and carefully mapping the locations of groups. There are biases present in most methods and ways overcoming bias using newer methods are discussed. Survey of gibbons and other animals is a constantly evolving process, and there is still no universally accepted methodology.
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