Improving the Estimation of Calling Probability and Correction Factors in Gibbon Monitoring Using the Auditory Point Count Method
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Estimating the population size of primate species is very important in identifying appropriate conservation actions and measuring their effectiveness. All 17 species of gibbons recognized by the IUCN are considered threatened with extinction. Thus, there is a great need to determine and monitor their status, to conduct effective conservation interventions. Gibbons can be detected by their loud and long song bouts, and surveys usually rely on auditory point count methods. However, gibbon groups do not call daily, so we need a correction factor based on the daily calling probability to estimate gibbon population size accurately. Several studies have estimated such correction factors, but these estimates might be negatively biased if gibbon groups move beyond the maximum hearing distance or are far from the listening post and their calls are faint. We aimed to address this issue and estimate the calling probability and correction factor accurately. We collected data on southern yellow-cheeked crested gibbon (Nomascus gabriellae) in Cat Tien National Park, Vietnam, from July to October 2016, using an auditory point count method. We explored the influence of the maximum distance between the listening post and the gibbons on estimates of the calling probability, correction factor, and population size. We detected 44 gibbon groups during the survey. The calling probability and correction factor decreased with the maximum distance from the listening post to the gibbon groups. If we used all the gibbon groups to estimate the correction factor, population estimates were 15% and 18% higher than those estimated using only gibbon groups detected within 700 m and 500 m, respectively. Based on these findings, we recommend that researchers calculate the calling probability and correction factor for several maximum distances and determine the percentage overestimation of the gibbon population. They should then choose the correction factor so as to balance sample size and the risk of overestimation of gibbon population size.
KeywordsHylobatidae Correction factor Gibbon Mark-recapture Nomascus
We thank the Vietnam National Foundation for Science and Technology (NAFOSTED) for support given to this project (Contract number 06-NN.06-2015.37). Our gratitude also extends to the forest rangers in Cat Tien National Park for permitting us to conduct the survey. We also thank all the field assistants for helping us with the field survey. Finally, we thank the reviewers and editors for their insightful comments, suggestions, and corrections.
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