Data from: Toward a geography of omnivory: omnivores increase carnivory when sodium is limiting

1. Toward understanding the geography of omnivory, we tested three hypotheses that predict the proportion of animal tissue consumed: The Sodium Limitation Hypothesis predicts that omnivores increase animal consumption in Na-poor environments because Na bioaccumulates from plants to predators; thus, heterotrophs are Na-rich sources. The Nitrogen Limitation and Habitat Productivity Hypotheses use the same logic to predict more animal consumption in N-poor and productive environments respectively. 2. Omnivory is a common trophic strategy, but what determines the balance of plant and animal tissue omnivores consume is relatively unexplored. Most of what we know comes from single populations at local scales. Here we quantitatively test these three hypotheses at a large geographic scale and across 20 species of omnivorous ants. 3. We tested each hypothesis using N stable isotopes (δ 15 N) to quantify the degree of carnivory in ant populations in 20 forests that span 12° latitude from Georgia to Maine, USA. We used the difference in δ 15 N between 20 ant conspecifics in 10 genera between two paired forests (10 pairs of 20 forests) that consisted of a coastal and inland forests on the same latitude to determine if the proportion of animal tissue consumed could be predicted based on Na, N or NPP. 4. Sodium gradients accounted for 18% of the variation in δ 15 N, 45% if one outlier ant species was omitted. In contrast, the Nitrogen Limitation and Habitat Productivity Hypotheses, which predict more animal consumption in N-poor and more productive environments respectively, failed to vary with δ 15 N. 5. Our results reveal a geography of omnivory driven in part by access to Na.1. 为解析杂食性的地理分布格局，我们检验了3个可预测杂食动物动物性组织摄入比例的假说：钠限制假说（Sodium Limitation Hypothesis）提出，在钠匮乏环境中，杂食动物会提升动物性食物摄入比例，因为钠会沿植物到捕食者的路径发生生物富集，而异养生物正是钠富集的来源；氮限制假说（Nitrogen Limitation Hypothesis）与生境生产力假说（Habitat Productivity Hypothesis）则沿用该逻辑，分别预测在氮匮乏环境与高生产力生境中，杂食动物的动物性食物摄入比例会更高。 2. 杂食性是一类广泛存在的营养策略，但目前学界对决定杂食动物动植物食物摄入比例的机制仍知之甚少，现有认知大多仅来自局域尺度下的单一种群研究。本研究在大地理尺度下，针对20种杂食性蚂蚁，对上述3个假说开展定量检验。 3. 我们利用氮稳定同位素（N Stable Isotopes, δ¹⁵N）量化美国佐治亚州至缅因州（跨12个纬度）的20个森林样地中蚂蚁种群的肉食性程度，以此检验各假说。我们选取隶属于10个属的20种蚂蚁，在10对同纬度沿海-内陆配对森林（合计20个森林样地）中比较其δ¹⁵N值差异，以判断动物性食物摄入比例是否可通过钠、氮或净初级生产力（Net Primary Productivity, NPP）进行预测。 4. 钠梯度可解释δ¹⁵N值18%的变异，当剔除1个异常蚂蚁物种后，该解释率提升至45%。与之相反，分别预测氮匮乏环境与高生产力生境中动物性食物摄入比例更高的氮限制假说与生境生产力假说，其预测结果与δ¹⁵N值无显著关联。 5. 本研究结果揭示了部分由钠的可获得性驱动的杂食性地理分布格局。