Robson Ashley S, Trimble Morgan J, Purdon Andrew, Young-Overton Kim D, Pimm Stuart L, van Aarde Rudi J
Conservation Ecology Research Unit (CERU), Department of Zoology and Entomology, University of Pretoria, Pretoria, South Africa.
Nicholas School of the Environment, Duke University, Durham, North Carolina, United States of America.
PLoS One. 2017 Apr 17;12(4):e0175942. doi: 10.1371/journal.pone.0175942. eCollection 2017.
Savannas once constituted the range of many species that human encroachment has now reduced to a fraction of their former distribution. Many survive only in protected areas. Poaching reduces the savanna elephant, even where protected, likely to the detriment of savanna ecosystems. While resources go into estimating elephant populations, an ecological benchmark by which to assess counts is lacking. Knowing how many elephants there are and how many poachers kill is important, but on their own, such data lack context. We collated savanna elephant count data from 73 protected areas across the continent estimated to hold 50% of Africa's elephants and extracted densities from 18 broadly stable population time series. We modeled these densities using primary productivity, water availability, and an index of poaching as predictors. We then used the model to predict stable densities given current conditions and poaching for all 73 populations. Next, to generate ecological benchmarks, we predicted such densities for a scenario of zero poaching. Where historical data are available, they corroborate or exceed benchmarks. According to recent counts, collectively, the 73 savanna elephant populations are at 75% of the size predicted based on current conditions and poaching levels. However, populations are at <25% of ecological benchmarks given a scenario of zero poaching (967,000)-a total deficit of ~730,000 elephants. Populations in 30% of the 73 protected areas were <5% of their benchmarks, and the median current density as a percentage of ecological benchmark across protected areas was just 13%. The ecological context provided by these benchmark values, in conjunction with ongoing census projects, allow efficient targeting of conservation efforts.
稀树草原曾经是许多物种的活动范围，但如今人类的 encroachment 已将其分布范围缩减至先前的一小部分。许多物种仅在保护区内生存。偷猎行为致使稀树草原大象数量减少，即便在保护区内也是如此，这可能对稀树草原生态系统造成损害。虽然投入了资源来估算大象数量，但缺乏一个用于评估数量的生态基准。了解大象的数量以及偷猎者捕杀的数量固然重要，但仅凭这些数据缺乏背景信息。我们整理了来自非洲大陆 73 个保护区的稀树草原大象数量数据，据估计这些保护区拥有非洲约 50%的大象，并从 18 个大致稳定的种群时间序列中提取了密度数据。我们以初级生产力、水资源可利用性以及偷猎指数作为预测因子对这些密度进行建模。然后，我们利用该模型根据当前状况和偷猎情况预测了所有 73 个种群的稳定密度。接下来，为了生成生态基准，我们预测了零偷猎情况下的此类密度。在有历史数据可用的地方，这些数据证实或超过了基准。根据最近的统计，总体而言，这 73 个稀树草原大象种群的数量仅为根据当前状况和偷猎水平预测数量的 75%。然而，在零偷猎的情况下（约 96.7 万头），种群数量仅为生态基准的不到 25%，总共短缺约 73 万头大象。在 73 个保护区中，30%的保护区内大象数量不到其基准数量的 5%，而整个保护区当前密度作为生态基准百分比的中位数仅为 13%。这些基准值所提供的生态背景，结合正在进行的普查项目，能够有效地确定保护工作的重点。
