Tracking orangutans from the sky

From the hundreds of thousands of orangutans that once ranged throughout southeast Asia, only two orangutan species now inhabit just two countries: Indonesia and Malaysia. The Sumatran orangutan is listed as critically endangered, the Bornean, endangered. In a new study published in the open access journal PLoS Biology, Marc Ancrenaz and colleagues report an innovative method for reliably counting orangutan numbers from the sky, and estimate that the entire population in Sabah has dropped by 35% in the past 20 years.

Conservation efforts depend on having reliable data on population size, density, and distribution, but estimates of orangutan numbers in Sabah--which range from 2,000 to 20,000--are outdated. Orangutans are tough to spot directly, so researchers traditionally count the nests they sleep in at night, which is time-consuming, especially when faced with the hip-deep muck and steep slopes of the rainforest floor. Instead, Ancrenaz and colleagues have developed a survey method by which estimates of orangutan numbers can be made from helicopters. By comparing ground survey data, collected over two years, with aerial counts garnered in only 72 hours the authors calibrated and refined their aerial survey results and were able to directly assess the distribution and size of orangutan populations throughout the whole of Sabah (an area of 72,000 square kilometers).

The authors attribute the 35% decline in Sabah orangutan numbers primarily to habitat loss from agricultural development. While the authors acknowledge the density estimates could be more precise, their survey reveals crucial information on orangutan numbers and distribution, and their results suggest that orangutans may adapt better to degraded forests than previously thought--provided illegal hunting and agricultural conversion are controlled. Having reliable estimates of orangutan numbers is vital to conservation efforts. These aerial surveys will speed that work, and help sustain orangutans' refuge in the treetops, above their human relatives.

Citation: Ancrenaz M, Gimenez O, Ambu L, Ankrenaz K, Andau P, et al. (2004) Aerial surveys give new estimates for orangutans in Sabah, Malaysia. PLoS Biol 3 (1): e3.

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