People and Nature Consulting International, Jakarta, Indonesia.
PLoS One. 2010 Aug 11;5(8):e12042. doi: 10.1371/journal.pone.0012042.
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.
婆罗洲猩猩（Pongo pygmaeus）目前的密度较低，看到野生猩猩是一件罕见的事情。与现在低的猩猩偶遇率相比，历史上的收藏家，如阿尔弗雷德·拉塞尔·华莱士（Alfred Russel Wallace），能够在数周甚至数月内连续射杀多少只猩猩，这令人震惊。这是否表明，大约 150 年前，猩猩的偶遇率或密度比现在更高？
方法/主要发现：我们通过量化狩猎记录、博物馆收藏和最近的实地研究中获得的偶遇率来检验这一假设，并分析是否存在随时间下降的趋势。对我们的数据进行逻辑回归分析支持了这种在 19 世纪中叶到现在的婆罗洲猩猩数量的下降趋势。即使控制了调查和狩猎队的规模以及探险的持续时间的变化，在森林干扰较少或没有森林干扰的地区，平均每日偶遇率似乎下降了约 6 倍。
结论/意义：这一发现可能对我们对猩猩的理解产生潜在影响，因为它表明婆罗洲猩猩曾经的密度更高。我们探讨了潜在的解释——栖息地的丧失和退化、狩猎和疾病，并得出结论认为，狩猎最符合观察到的模式。这表明，狩猎作为猩猩密度和分布的关键因果因素被低估了，而且物种的种群下降比仅基于栖息地丧失所估计的更为严重。我们的发现可能需要我们重新思考猩猩的生物学，我们的许多生态理解可能是基于对历史上密度较低的动物进行实地研究的基础上。我们从历史数据中量化物种偶遇率的方法表明，这种方法可以为过去物种的生态学和种群密度提供有价值的信息，为物种的保护需求提供新的见解。
