Data from: Biotic interactions contribute to the geographic range limit of an annual plant: herbivory and phenology mediate fitness beyond a range margin

Species’ geographic distributions have already shifted during the
Anthropocene. However, we often do not know what aspects of the
environment drive range dynamics, much less which traits mediate
organisms’ response to these environmental gradients. Most studies focus
on possible climatic limits to species’ distributions and have ignored the
role of biotic interactions, despite theoretical support for their
importance in setting distributional limits. We used field experiments and
simulations to estimate contributions of mammal herbivory to a range
boundary in the California annual plant Clarkia xantiana ssp. xantiana. A
steep gradient of increasing probability of herbivory occurred across the
boundary, and a reanalysis of prior transplant experiments revealed that
herbivory drove several-fold declines in lifetime fitness at and beyond
the boundary. Simulations showed that populations could potentially
persist beyond the range margin in the absence of herbivory. Using data
from a narrowly sympatric subspecies, C. x. parviflora, we also showed
that delayed phenology is strongly associated with C. xantiana ssp.
xantiana’s susceptibility to herbivory and low fitness beyond its border.
Overall, our results provide some of the most comprehensive evidence to
date of how the interplay of demography, traits, and spatial gradients in
species interactions can produce a geographic range limit, and lend
empirical support to recent developments in range limits theory.

人类世（Anthropocene）期间，物种的地理分布已发生变迁。然而，我们往往不清楚环境的哪些方面驱动了分布范围动态，更遑论哪些性状介导了生物对这些环境梯度的响应。多数研究聚焦于气候对物种分布可能构成的限制，却忽略了生物相互作用（biotic interactions）的角色——尽管理论研究已证实其在设定分布边界（range boundary）中的重要性。我们通过野外实验与模拟，评估了哺乳动物食草行为对加州一年生植物Clarkia xantiana ssp. xantiana分布边界的贡献。跨越该边界时，食草行为发生概率呈陡峭上升梯度；对先前移植实验（transplant experiments）的再分析显示，在边界及边界以外区域，食草行为导致终生适合度（lifetime fitness）下降数倍。模拟结果表明，若不存在食草行为，种群或可在分布边缘以外持续存在。利用近缘同域亚种（sympatric subspecies）C. x. parviflora的数据，我们还发现延迟物候（phenology）与Clarkia xantiana ssp. xantiana对食草行为的易感性及边界外适合度低下密切相关。总体而言，我们的研究为“种群统计学、性状与物种相互作用的空间梯度如何共同塑造地理分布边界”这一问题提供了迄今为止最为全面的证据之一，并为分布边界理论的最新进展提供了实证支持。