Recent estimate of Asian elephants in Borneo reveals a smaller population

Asian elephants occurring in northern Borneo form a geographically isolated and genetically distinct population. Of this, the subpopulation of Central Sabah holds the greatest opportunity for long-term survival, due to a relatively large population size and occurrence over a vast, contiguous, and protected habitat. We surveyed this subpopulation in 2015 using advanced methods to obtain a population size estimate. We used the distance-sampling framework and laid out transects following a stratified random design for counting elephant dung piles; measured dung decay following the ‘retrospective’ method; and used Bayesian analysis to estimate dung decay rate and dung pile density. Thus, we estimated a posterior mean dung decay rate of 212 days (95% BCI: 133–319), an overall elephant density of 0.07 per km2 (95% BCI: 0.03–0.11), and a population size of 387 elephants (95% BCI: 169–621). These estimates were far lower than the population size of 1132 individuals and density of 1.18 per km2 estimated in 2008. It is unlikely that there has been a steep population decline, as there were no drastic land-use changes between 2008 and 2015, nor were there other identifiable causes for a population decline. Therefore, it appears that the methodological and analytical flaws in the previous estimate are the most plausible reason for this observed difference. Given that the new estimate suggests a much smaller population, it is prudent and precautionary to use the new estimate as the basis for all policy decisions and conservation actions for elephants in Sabah.

栖息于婆罗洲北部的亚洲象构成了一个地理隔离且遗传独特的种群。其中，沙巴中部的亚种群拥有最佳的长期生存前景——其种群规模相对较大，且栖息于广袤连续、受保护的生境中。我们于2015年针对该亚种群开展调查，采用先进方法获取种群规模估算数据：采用距离抽样（distance-sampling）框架，按照分层随机设计布设样线以计数象粪堆；通过“回顾性”方法测定粪堆衰变率；并使用贝叶斯分析（Bayesian analysis）估算粪堆衰变速率与粪堆密度。据此，我们得到后验均值粪堆衰变率为212天（95%贝叶斯置信区间（BCI）：133–319），整体象密度为0.07头/平方公里（95% BCI：0.03–0.11），种群规模为387头（95% BCI：169–621）。上述估算结果远低于2008年得到的1132头个体的种群规模与1.18头/平方公里的密度估算值。由于2008至2015年间未发生剧烈土地利用变化，也未发现其他可识别的种群下降诱因，因此种群骤降的可能性极低。由此看来，此前估算中存在方法学与分析缺陷，是造成这一差异的最合理解释。鉴于新估算显示种群规模显著更小，将该新估算作为沙巴州象类所有政策制定与保护行动的依据，是审慎且具备前瞻性的做法。