Watson Fred, Becker Matthew S, Smit Daan, Droge Egil, Mukula Teddy, Martens Sandra, Mwaba Shadrach, Christianson David, Creel Scott, Brennan Angela, M'soka Jassiel, Gaylard Angela, Simukonda Chuma, Nyirenda Moses, Mayani Bridget
California State University Monterey Bay Seaside California USA.
Zambian Carnivore Programme Mfuwe Zambia.
Ecol Evol. 2022 Oct 17;12(10):e9414. doi: 10.1002/ece3.9414. eCollection 2022 Oct.
Large herbivore migrations are imperiled globally; however the factors limiting a population across its migratory range are typically poorly understood. Zambia's Greater Liuwa Ecosystem (GLE) contains one of the largest remaining blue wildebeest () migrations, yet the population structure, vital rates, and limiting factors are virtually unknown. We conducted a long-term demographic study of GLE wildebeest from 2012 to 2019 of 107 collared adult females and their calves, 7352 herd observations, 12 aerial population surveys, and concurrent carnivore studies. We applied methods of vital rate estimation and survival analysis within a Bayesian estimation framework. From herd composition observations, we estimated rates of fecundity, first-year survival, and recruitment as 68%, 56%, and 38% respectively, with pronounced interannual variation. Similar rates were estimated from calf-detections with collared cows. Adult survival rates declined steadily from 91% at age 2 years to 61% at age 10 years thereafter dropping more sharply to 2% at age 16 years. Predation, particularly by spotted hyena, was the predominant cause of death for all wildebeest ages and focused on older animals. Starvation only accounted for 0.8% of all unbiased known natural causes of death. Mortality risk differed substantially between wet and dry season ranges, reflecting strong spatio-temporal differences in habitat and predator densities. There was substantial evidence that mortality risk to adults was 27% higher in the wet season, and strong evidence that it was 45% higher in the migratory range where predator density was highest. The estimated vital rates were internally consistent, predicting a stable population trajectory consistent with aerial estimates. From essentially zero knowledge of GLE wildebeest dynamics, this work provides vital rates, age structure, limiting factors, and a plausible mechanism for the migratory tendency, and a robust model-based foundation to evaluate the effects of potential restrictions in migratory range, climate change, predator-prey dynamics, and poaching.
大型食草动物的迁徙在全球范围内受到威胁；然而，限制一个种群在其迁徙范围内活动的因素通常却鲜为人知。赞比亚的大刘瓦生态系统（GLE）拥有现存规模最大的黑尾牛羚（ ）迁徙活动之一，但该种群的结构、生命率和限制因素实际上却无人知晓。我们在2012年至2019年期间对GLE的牛羚进行了一项长期的种群统计学研究，涉及107只佩戴项圈的成年雌性及其幼崽、7352次兽群观察、12次空中种群调查以及同步的食肉动物研究。我们在贝叶斯估计框架内应用了生命率估计和生存分析方法。从兽群组成观察中，我们估计繁殖率、一岁存活率和补充率分别为68%、56%和38%，且年际变化明显。通过对佩戴项圈母牛所产幼崽的监测也得出了类似的比率。成年存活率从2岁时的91%稳步下降到10岁时的61%，此后在16岁时急剧下降至2%。捕食，尤其是斑鬣狗的捕食，是所有年龄段牛羚死亡的主要原因，且主要集中在老年动物身上。饥饿仅占所有无偏倚已知自然死亡原因的0.8%。干湿季活动范围内的死亡风险存在显著差异，这反映出栖息地和捕食者密度在时空上的强烈差异。有大量证据表明，成年牛羚在湿季的死亡风险高出27%，且有充分证据表明，在捕食者密度最高的迁徙范围内，其死亡风险高出45%。估计的生命率在内部是一致的，预测出与空中估计一致的稳定种群轨迹。这项工作从对GLE牛羚动态几乎一无所知的基础上，提供了生命率、年龄结构、限制因素以及迁徙倾向的合理机制，并为评估迁徙范围潜在限制、气候变化、捕食者 - 猎物动态和偷猎影响提供了一个基于模型的坚实基础。
