An integrated population model reveals source-sink dynamics for competitively subordinate African wild dogs linked to anthropogenic prey depletion

Many African large carnivore populations are declining due to decline of the herbivore populations on which they depend. The densities of apex carnivores like the lion and spotted hyena correlate strongly with prey density, but competitive subordinates like the African wild dog benefit from competitive release when the density of apex carnivores is low, so the expected effect of a simultaneous decrease in resources and dominant competitors is not obvious. Wild dogs in Zambia's Luangwa Valley Ecosystem occupy four ecologically similar areas with well-described differences in the densities of prey and dominant competitors, due to spatial variation in illegal offtake. We used long-term data to fit a Bayesian integrated population model (IPM) of the demography and dynamics of wild dogs in these four regions. The IPM used Leslie projection to link a Cormack-Jolly-Seber model of area-specific survival (allowing for individual heterogeneity in detection), a zero-inflated Poisson model of area-specific fecundity, and a state-space model of population size that used estimates from a closed mark-capture model as the counts from which (latent) population size was estimated. The IPM showed that both survival and reproduction were lowest in the region with the lowest density of preferred prey (puku, Kobus vardonii, and impala, Aepyceros melampus), despite little use of this area by lions. Survival and reproduction were highest in the region with the highest prey density, and intermediate in the two regions with intermediate prey density. The population growth rate (λ) was positive for the population as a whole, strongly positive in the region with the highest prey density, and strongly negative in the region with the lowest prey density. It has long been thought that the benefits of competitive release protect African wild dogs from the costs of low prey density. Our results show that the costs of prey depletion overwhelm the benefits of competitive release and cause local population decline where anthropogenic prey depletion is strong. Because competition is important in many guilds and humans are affecting resources of many types, it is likely that similarly fundamental shifts in population limitation are arising in many systems.诸多非洲大型食肉动物种群正随其赖以生存的草食动物种群衰减而持续下降。狮、斑鬣狗等顶级食肉动物的种群密度与猎物密度显著相关；而非洲野犬这类竞争从属物种，在顶级食肉动物密度较低时可通过竞争释放获得生存优势。因此，资源与优势竞争者同时减少所带来的预期效应并不明确。栖息于赞比亚卢安瓜河谷生态系统的非洲野犬，分布在4个生态特征相似的区域；由于非法捕猎的空间异质性，这些区域的猎物与优势竞争者密度存在显著且已被充分研究的差异。本研究利用长期监测数据，针对这4个区域的非洲野犬种群统计特征与动态，构建了贝叶斯集成种群模型（Bayesian Integrated Population Model, IPM）。该IPM采用莱斯利投影矩阵，将区域特异性生存的科马克-乔利-西伯模型（Cormack-Jolly-Seber Model，允许个体检测异质性）、区域特异性繁殖力的零膨胀泊松模型，以及种群规模状态空间模型进行耦合；其中状态空间模型以闭合标记重捕模型的估计值作为计数数据，以此估算（潜在）种群规模。该IPM结果显示，尽管狮极少利用该区域，但在首选猎物（普库羚Kobus vardonii与黑斑羚Aepyceros melampus）密度最低的区域，非洲野犬的生存率与繁殖率均为最低。在猎物密度最高的区域，野犬的生存率与繁殖率均达到最高；而在另外两个猎物密度中等的区域，相关指标则处于中等水平。整体种群的增长率（λ）为正值；在猎物密度最高的区域，增长率显著为正，而在猎物密度最低的区域，增长率则显著为负。长期以来，学界普遍认为竞争释放带来的生存优势，可使非洲野犬免受低猎物密度带来的生存成本压力。本研究结果表明，在人为导致猎物大量耗竭的区域，猎物匮乏带来的生存成本远超竞争释放的收益，最终引发局域种群衰退。由于种间竞争在诸多生物功能群中均发挥关键作用，且人类活动正持续影响多种类型的生物资源，因此诸多生态系统中很可能正出现类似的种群限制机制的根本性转变。