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Declining orangutan encounter rates from Wallace to the present suggest the species was once more abundant.

Meijaard, E and Welsh, A and Ancrenaz, M and Wich, SA and Nijman, V and Marshall, AJ (2010) Declining orangutan encounter rates from Wallace to the present suggest the species was once more abundant. PLoS One, 5 (8). ISSN 1932-6203

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Abstract

BACKGROUND: Bornean orangutans (Pongo pygmaeus) currently occur at low densities and seeing a wild one is a rare event. Compared to present low encounter rates of orangutans, it is striking how many orangutan each day historic collectors like Alfred Russel Wallace were able to shoot continuously over weeks or even months. Does that indicate that some 150 years ago encounter rates with orangutans, or their densities, were higher than now? METHODOLOGY/PRINCIPAL FINDINGS: We test this hypothesis by quantifying encounter rates obtained from hunting accounts, museum collections, and recent field studies, and analysing whether there is a declining trend over time. Logistic regression analyses of our data support such a decline on Borneo between the mid-19th century and the present. Even when controlled for variation in the size of survey and hunting teams and the durations of expeditions, mean daily encounter rates appear to have declined about 6-fold in areas with little or no forest disturbance. CONCLUSIONS/SIGNIFICANCE: This finding has potential consequences for our understanding of orangutans, because it suggests that Bornean orangutans once occurred at higher densities. We explore potential explanations-habitat loss and degradation, hunting, and disease-and conclude that hunting fits the observed patterns best. This suggests that hunting has been underestimated as a key causal factor of orangutan density and distribution, and that species population declines have been more severe than previously estimated based on habitat loss only. Our findings may require us to rethink the biology of orangutans, with much of our ecological understanding possibly being based on field studies of animals living at lower densities than they did historically. Our approach of quantifying species encounter rates from historic data demonstrates that this method can yield valuable information about the ecology and population density of species in the past, providing new insight into species' conservation needs.

Item Type: Article
Uncontrolled Keywords: MD Multidisciplinary
Subjects: A General Works > AM Museums (General). Collectors and collecting (General)
A General Works > AM Museums (General). Collectors and collecting (General)

Q Science > QH Natural history
Q Science > QL Zoology
Divisions: Natural Sciences and Psychology
Publisher: Public Library of Science
Related URLs:
Date Deposited: 27 Sep 2016 11:18
Last Modified: 27 Sep 2016 11:18
DOI or Identification number: 10.1371/journal.pone.0012042
URI: http://researchonline.ljmu.ac.uk/id/eprint/4201

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