Data_Sheet_1_Demographic Tipping Points as Early Indicators of Vulnerability for Slow-Breeding Megafaunal Populations.ZIP

Decisions based on trends in population abundance and distribution may fail to protect populations of slow-breeding, long-lived megafauna from irrevocable decline if they ignore demographic constraints. For such taxa, we urge that effort be directed at understanding the interactions among vital rates governing population growth rates, rather than on predicting probabilities of extinction. The proximity of a population to demographic tipping points, i.e., where growth rate switches from positive to negative, can signal vulnerability to perturbation long before numbers drop below a point of no return. We define the “demographic safe space” as the combination of key vital rates that support a non-negative growth rate and illustrate this approach for Asian elephants. Through simulations, we find that even with optimal reproduction, Asian elephant populations cannot tolerate annual female mortality rates exceeding 7.5%. If adult mortality is very low (3%/year), populations can tolerate high annual mortality in calves below age 3 (up to 31.5%/year), or slow female reproduction (primiparity at 30 years or average inter-birth interval of up to 7.68 years). We then evaluate the potential impact of current threats, showing that near-optimal reproduction and high calf survival is necessary to offset even modestly increased mortality among adult female age classes. We suggest that rather than rely on simple counts or “viability” assessments, conservation planners for slow-breeding megafauna should consider demographic tipping points and strive to keep populations within their safe spaces.

基于人口丰度和分布趋势所做的决策，若忽视人口学限制，则可能无法有效保护繁殖速度缓慢、寿命较长的巨型动物种群免受不可逆转的衰退。对于此类物种，我们强烈建议将努力方向转向理解调控种群增长率的关键生命率之间的相互作用，而非预测灭绝的概率。种群临近人口学临界点，即增长率从正值转为负值时，可能在数量降至不可逆转的低点之前，就预示着对扰动的脆弱性。我们定义“人口学安全空间”为支持非负增长率的若干关键生命率的组合，并以亚洲象为例阐述了这一方法。通过模拟研究，我们发现即使在最佳繁殖条件下，亚洲象种群也无法承受年雌性死亡率超过7.5%。如果成年死亡率极低（3%/年），种群可以承受3岁以下幼崽的高年死亡率（高达31.5%/年），或者雌性繁殖速度缓慢（首次产仔年龄在30岁或平均产仔间隔达7.68年）。随后，我们评估了当前威胁的潜在影响，结果显示，为了抵消成年雌性群体中哪怕轻微增加的死亡率，接近最佳繁殖率和较高的幼崽存活率是必要的。我们建议，对于繁殖速度缓慢的巨型动物种群的保护规划者而言，不应仅仅依赖简单的计数或‘存活性’评估，而应考虑人口学临界点，并努力使种群保持在安全空间内。