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Counting orangutans: the best way to survey the great apes

How do you count orangutans when they are difficult to spot in the wild given that they are shy, arboreal, and few and far between? To find a solution, biologists have turned to estimating orangutan populations by counting their nests, which the great apes make anew every night. In order to make the most accurate count possible, researchers have studied the different factors that could impact the success, or lack thereof, of nest-counters in’s open access journal Tropical Conservation Science.

“We analyze the effect of several factors (distance to [line] transect, height, decay stage, and observer experience) on the probability that a [Sumatran] orangutan (Pongo abelii) nest is found along a line transect. The results indicate that all factors significantly influence nest detection,” the researchers write. Line transect is a simple survey technique to measure abundance along a specified ‘line’ in the forest, in this case the line measured 1.5 kilometers of land in Sumatra’s Gunung Leuser National Park.

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Orangutan nest in Gunung Leuser National Park. Photo by: Serge Wich.

The study followed six teams’ results in counting nests, and found that on average teams missed 11% of the total nests. Not surprisingly, the most experienced teams sighted the most nests, while the higher the nest in the canopy the more likely it would be missed.

The study suggests that one of the best ways to avoid errors in density is to decrease the width of the line transect.

“Restricting the transect width could have effects similar to using much more experienced sampling teams, and the surveys may, in some circumstances, be much easier to do,” the authors conclude.

CITATION: Wich, S. A. and Boyko, R. H. 2011. Which factors determine orangutan nests’ detection probability along transects? Tropical Conservation Science Vol. 4 (1):53-63.

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