Herbivory rates have classically been hypothesized to decrease from the tropics towards higher latitudes because the more benign abiotic conditions in tropical systems foster greater ecosystem complexity including greater intensity of biotic interactions. However, attempts to quantify latitudinal patterns of herbivory often fail to support this hypothesis. While biases have been offered as explanations for null results, here, we argue that framing the question of latitudinal variation in herbivory around nutrient and energetic constraints of insect herbivores and plants may provide mechanistic explanations of latitudinal herbivory patterns. As a case study, we focused on sodium as an uncoupled nutrient between herbivore and plant communities: sodium is a key limiting micronutrient for herbivore neural and muscular development while present at orders of magnitude lower concentrations in plants. We compared sodium deposition with latitude, mean annual temperature (MAT) and actual evapotranspiration (measure of primary productivity, AET) in their ability to predict consumed percentage leaf area from published datasets. Leaf percent herbivory increased with sodium deposition and MAT and decreased with latitude but was unrelated to AET. Sodium had comparable effect size and predictive ability to either MAT or latitude. Additionally, herbivory was highest in locales with both high sodium deposition and high MAT. Our hypothesis that geographic variation in herbivory is driven by an interaction of unrestrictive temperature environments (high MAT) and limiting nutrient supply to herbivores (high sodium deposition) was strongly supported. We propose that greater generality, predictability and theoretical development on geographic variation in herbivory will arise from a refocus on the biophysical constraints (e.g. productivity, micronutrient availability, leaf mass consumed) that ultimately control consumer interactions rather than latitude per se. This refocus is likely to open new hypotheses for the evolution of defense syndromes across plant populations and communities based on the specific geography of limiting nutrients.

中文翻译：

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不仅仅是他们吃的东西：不耦合的生物物理限制是食草动物地理模式的基础

传统上假设食草率从热带向高纬度地区下降，因为热带系统中更良性的非生物条件促进了更大的生态系统复杂性，包括更大强度的生物相互作用。然而，量化草食性纬度模式的尝试往往无法支持这一假设。虽然偏差已被提供作为无效结果的解释，但在这里，我们认为围绕昆虫食草动物和植物的营养和能量限制来构建食草动物的纬度变化问题可能会提供纬度食草动物模式的机械解释。作为案例研究，我们将钠作为食草动物和植物群落之间的一种非偶联营养素：钠是食草动物神经和肌肉发育的关键限制性微量营养素，而在植物中的浓度要低几个数量级。我们比较了钠沉降与纬度、年平均温度 (MAT) 和实际蒸散量（初级生产力的测量值，AET）在预测已发布数据集中消耗的叶面积百分比方面的能力。叶食草百分比随着钠沉积和 MAT 的增加而增加，并随着纬度的增加而减少，但与 AET 无关。钠具有与 MAT 或纬度相当的效应大小和预测能力。此外，食草动物在高钠沉积和高 MAT 的地区最高。我们的假设是食草动物的地理变异是由不受限制的温度环境（高 MAT）和限制食草动物的营养供应（高钠沉积）的相互作用驱动的，这一假设得到了强烈支持。我们建议，重新关注最终控制消费者互动而非纬度的生物物理限制（例如生产力、微量营养素可用性、消耗的叶量）将产生食草动物地理变异的更大普遍性、可预测性和理论发展本身。这种重新聚焦可能会基于限制营养素的特定地理条件，为跨植物种群和群落的防御综合症的进化提出新的假设。