Declining Orangutan Encounter Rates from Wallace to the Present Suggest the Species Was Once More Abundant

BackgroundBornean 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 FindingsWe 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/SignificanceThis 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.

背景：当前婆罗洲猩猩（Bornean orangutan, Pongo pygmaeus）的种群密度极低，野外偶遇野生个体实属罕见。与当下极低的猩猩偶遇率形成鲜明对比的是，阿尔弗雷德·拉塞尔·华莱士（Alfred Russel Wallace）这类历史狩猎采集者，曾可在数周乃至数月内每日持续射杀大量猩猩。这一差异着实令人讶异：这是否意味着，约150年前的猩猩偶遇率或种群密度，较当下更高？
方法与主要结果：我们通过量化狩猎记录、馆藏标本记录及近期野外研究所得的偶遇率，分析其随时间推移是否存在下降趋势。对数据的逻辑回归（Logistic regression）分析证实，19世纪中期至今，婆罗洲的猩猩偶遇率确实存在下降趋势。即便在控制调查与狩猎团队规模、考察时长等变量后，在几乎无森林干扰的区域，日均偶遇率仍下降了约6倍。
结论与意义：该发现对我们理解婆罗洲猩猩具有重要启示，因为它表明该物种历史上的种群密度曾更高。我们探讨了潜在的解释因素——栖息地丧失与退化、狩猎及疾病，并认为狩猎最符合观测到的种群变化模式。这意味着，狩猎作为影响猩猩种群密度与分布的关键驱动因素，其作用曾被低估；且仅基于栖息地丧失估算的种群下降幅度，实则远低于实际情况。我们的研究结果或需重新审视猩猩的生物学特征，因为当前多数生态学认知，均基于对种群密度低于历史水平的野外个体的研究。我们通过历史数据量化物种种群偶遇率的方法证实，该手段可获取物种过去的生态学与种群密度信息，为物种保护需求的研究提供全新视角。