Abstract
We conducted a validation of the line transect technique to estimate densities of orangutan (Pongo pygmaeus) nests in a Bornean swamp forest, and compared these results with density estimates based on nest counts in plots and on female home ranges. First, we examined the accuracy of the line transect method. We found that the densities based on a pass in both directions of two experienced pairs of observers was 27% below a combined sample based on transect walks by eight pairs of observers, suggesting that regular line-transect densities may seriously underestimate true densities. Second, we compared these results with those obtained by nest counts in 0.2-ha plots. This method produced an estimated 15.24 nests/ha, as compared to 10.0 and 10.9, respectively, by two experienced pairs of observers who walked a line transect in both directions. Third, we estimated orangutan densities based on female home range size and overlap and the proportion of females in the population, which produced a density of 4.25–4.5 individuals/km2 . Converting nest densities into orangutan densities, using locally estimated parameters for nest production rate and proportion of nest builders in the population, we found that density estimates based on the line transect results of the most experienced pairs on a double pass were 2.82 and 3.08 orangutans/km2, based on the combined line transect data are 4.04, and based on plot counts are 4.30. In this swamp forest, plot counts therefore give more accurate estimates than do line transects. We recommend that this new method be evaluated in other forest types as well.
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Acknowledgements
We thank the BOS-Foundation for permission to work at Mawas, and its staff for the great support of all activities; the staff and students from Universitas Nasional, Jakarta (Tatang Mitra Setia, Imran Said L. Tobing, Didik Prasetyo, Dwi Mulyawati, Tirza Yohana, Lili Aries Sadikin, Adi Hadinata, Ari Meididit, Fikty Apirlinayati, Fitriah Basalamah, and Ika Mian Karlina) for participating in the line transect experiment; the Indonesian Institute of Sciences (LIPI) for permission to work in Indonesia; the L.S.B. Leakey Foundation for financial support of the Tuanan Orangutan Project; the Netherlands Organisation for Scientific Research for financially supporting S.A. Wich, and Meredith Bastian and Maria van Noordwijk for comments on the MS.
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van Schaik, C.P., Wich, S.A., Utami, S.S. et al. A simple alternative to line transects of nests for estimating orangutan densities. Primates 46, 249–254 (2005). https://doi.org/10.1007/s10329-005-0134-z
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DOI: https://doi.org/10.1007/s10329-005-0134-z