Data from: How climate extremes—not means—define a species' geographic range boundary via a demographic tipping point

Species’ geographic range limits interest biologists and resource managers alike; however, scientists lack strong mechanistic understanding of the factors that set geographic range limits in the field, especially for animals. There exists a clear need for detailed case studies that link mechanisms to spatial dynamics and boundaries because such mechanisms allow us to predict whether climate change is likely to change a species’ geographic range and, if so, how abundance in marginal populations compares to the core. The bagworm Thyridopteryx ephemeraeformis (Lepidoptera: Psychidae) is a major native pest of cedars, arborvitae, junipers, and other landscape trees throughout much of North America. Across dozens of bagworm populations spread over six degrees of latitude in the American Midwest, we find latitudinal declines in fecundity and egg and pupal survivorship as you proceed towards the northern range boundary. A spatial gradient of bagworm reproductive success emerges, which is associated with a progressive decline in local abundance and an increase in the risk of local population extinction near the species’ geographic range boundary. We develop a mathematical model, completely constrained by empirically estimated parameters, to explore the relative roles of reproductive asynchrony and stage-specific survivorship in generating the range limit for this species. We find that overwinter egg mortality is the biggest constraint on bagworm persistence beyond their northern range limit. Overwinter egg mortality is directly related to winter temperatures that fall below the bagworm eggs’ physiological limit. This threshold, in conjunction with latitudinal declines in fecundity and pupal survivorship, creates a non-linear response to climate extremes that sets the geographic boundary and provides a path for predicting northward range expansion under altered climate conditions. Our mechanistic modeling approach demonstrates how species’ sensitivity to climate extremes can create population tipping points not reflected in demographic responses to climate means, a distinction that is critical to successful ecological forecasting.

物种地理分布边界始终是生物学家与资源管理者共同关注的核心科学问题；然而，当前学界对野外环境中决定物种分布边界的机制仍缺乏深入透彻的理解，针对动物类群的相关研究尤为薄弱。
因此，亟需开展将机制与空间动态及分布边界相结合的精细化个案研究——这类机制性研究可帮助我们预判气候变化是否会改变物种的地理分布范围，若发生改变，还能阐明边缘种群与核心种群的种群密度差异。
蓑蛾（bagworm）Thyridopteryx ephemeraeformis（鳞翅目（Lepidoptera）：蓑蛾科（Psychidae））是广泛分布于北美多数区域的本土重大害虫，寄主涵盖雪松、崖柏、桧柏及其他多种园林树木。我们针对美国中西部横跨6个纬度梯度的数十个蓑蛾种群开展研究，结果显示：随着向该物种的北部分布边界推进，其繁殖力（fecundity）、卵存活率与蛹存活率均呈现显著的纬度梯度下降趋势。
由此可见，蓑蛾的繁殖成功率呈现空间梯度分布特征，该特征与种群局部密度的逐步降低，以及临近物种地理分布边界时局部种群灭绝风险的升高密切相关。我们构建了一个完全基于经验估算参数的数学模型，用以探究繁殖不同步性（reproductive asynchrony）与各发育阶段存活率（stage-specific survivorship）在塑造该物种分布边界过程中的相对贡献。
研究发现，越冬卵死亡率（overwinter egg mortality）是限制蓑蛾在北部分布边界以外区域存续的最关键因子。越冬卵死亡率与冬季气温低于蓑蛾卵生理耐受阈值的程度直接相关。该耐受阈值结合繁殖力与蛹存活率的纬度梯度下降趋势，共同造就了对极端气候的非线性响应，进而划定了物种的地理分布边界，同时也为气候变化背景下预测物种向北扩张的范围提供了可行路径。
我们的机制性建模方法揭示了：物种对极端气候的敏感性如何催生种群临界点，而这类临界点无法通过种群对气候平均态的响应体现——这一差异对于精准的生态预测至关重要。