Data from: Pre-dispersal seed predation and pollen limitation constrain population growth across the geographic distribution of Astragalus utahensis

1. A central focus of ecology is to understand the conditions under which biotic interactions affect species’ abundance and distribution. Classic and recent studies have shown that biotic interactions can strongly impact local or regional patterns of species abundance, but two fundamental questions remain largely unaddressed for non-competitive biotic interactions. First, do the effects of these interactions on population performance change predictably with environmental context? Second, to what extent do population-scale effects contribute to limiting species’ geographic distributions? 2. To address these questions, we experimentally assessed the extent to which pollen limitation and insect seed predators affected the fecundity and projected population growth rate (λ) of the native forb Astragalus utahensis. We studied populations at the center and northern edge of the latitudinal range of A. utahensis that occur across a gradient in abiotic harshness characterized primarily by declining mean annual precipitation. 3. Supplementing pollen and suppressing pre-dispersal seed predators increased seed production similarly within A. utahensis populations at the center and northern edge of the range. Integral projection population models revealed that relaxing these checks on seed production tended to increase λ in most populations, regardless of their location within the range. 4. Synthesis. Our results suggest that pollen limitation and insect herbivores limit population growth in A. utahensis similarly across the center-to-north portion of its latitudinal distribution. However, because A. utahensis population growth barely reaches the level of replacement at the northern range edge, the reduction in λ resulting from these interactions may contribute to limiting expansion at the northern edge of A. utahensis’ latitudinal range.1. 生态学的核心研究目标之一是阐明生物相互作用影响物种丰度与分布的具体条件。经典与近期研究均证实，生物相互作用可显著塑造物种丰度的局域或区域分布格局，但针对非竞争性生物相互作用的两个核心问题仍未得到充分解答：其一，此类相互作用对种群表现的效应是否会随环境背景呈现可预测的变化？其二，种群尺度的效应在多大程度上会限制物种的地理分布范围？ 2. 为解答上述问题，我们通过实验评估了花粉限制与昆虫种子捕食者对本土草本植物犹他黄芪（Astragalus utahensis）的繁殖力及种群投影增长速率（λ）的影响程度。我们选取了该物种纬度分布中心与北部边缘的种群，这些种群沿以年平均降水量逐步降低为主要特征的非生物环境严苛性梯度分布。 3. 在犹他黄芪分布中心与北部边缘的种群中，人工补充花粉与抑制扩散前种子捕食者对种子产量的提升效果相当。积分投影种群模型结果显示，解除对种子生产的这两类限制后，多数种群的λ均呈现上升趋势，且与其在分布区内的具体位置无关。 4. 综合分析：我们的研究结果表明，在犹他黄芪纬度分布从中心到北部的区域内，花粉限制与昆虫植食者对其种群增长的限制作用相似。然而，由于该物种在分布北部边缘的种群增长速率仅勉强达到种群更替水平，此类相互作用导致的λ降低可能会限制犹他黄芪在其纬度分布北部边缘的扩张范围。