An omnivore vigor hypothesis? Nutrient availability strengthens herbivore suppression by omnivores across 48 field sites

Nutrients regulate herbivore growth from the ‘bottom-up’ via improved plant vigor and food quality. Nitrogen also affects ‘top-down’ control of herbivores by moderating attraction of predators and the rates at which they consume herbivorous prey.
Tri-trophic consequences of nitrogen availability are more challenging to predict among omnivorous natural enemies who feed on both plants and herbivores, limiting our ability to predict net outcomes of nutrient availability in food webs.
In a two-year field survey of insects on zucchini host plants at 48 sites, I predicted that both herbivores and foliar-feeding omnivores would increase with nutrient availability, while predators would not.
My results revealed positive relationships between omnivores and foliar nitrogen concentrations, while predators had neutral responses to foliar N. Surprisingly, herbivores declined with increasing foliar N across the field sites.
Greenhouse experiments reinforced these patterns, as herbivore growth invers..., Field survey
To examine how natural populations of herbivores, omnivores and generalist predators respond to plant quality along a nutrient gradient, I performed observational field surveys. Between 2020 and 2021, my students and I sampled arthropods, soils and foliar tissues at 56 farm fields that varied in fertility management practices and formed a gradient of nutrient availability across Georgia, South Carolina, and North Carolina, USA (Appendix Fig. S1, Appendix Tables 1,2). The number of sites surveyed in 2020 (18) were considerably fewer than those sampled in 2021 (38) due to restricted mobility during the Covid-19 pandemic.
At each site, we randomly selected 10 unique zucchini plants along a transect spanning each field and visually surveyed arthropods on six haphazardly selected leaves (two new leaves, two middle-aged leaves, and two old leaves), stems, and in all open flowers. Herbivores and natural enemies were counted and identified on-site with the finest resolution possib..., Microsoft Excel, program R

营养物质可通过提升植物活力与食物品质，以“下行调控（bottom-up）”路径调控植食动物的生长。氮元素亦可通过调节捕食者的寄主吸引力及其捕食植食性猎物的速率，影响对植食动物的“上行调控（top-down）”控制。对于既取食植物又取食植食动物的杂食性天敌而言，氮素有效性的三营养级效应更难预测，这限制了我们对食物网中营养物质有效性最终结果的预测能力。本研究在48个样地开展了为期两年的西葫芦寄主植物昆虫野外调查，最初假设植食动物与取食叶片的杂食性动物的种群数量会随营养有效性提升而增加，而捕食者则不会。研究结果显示，杂食性动物种群与叶片氮浓度呈正相关关系，而捕食者对叶片氮含量的响应则为中性。令人意外的是，在所有野外样地中，植食动物种群数量随叶片氮含量升高而下降。温室实验进一步验证了这些模式，植食动物的生长呈负相关…… 野外调查为探究植食动物、杂食动物与广食性捕食者的自然种群沿营养梯度对植物品质的响应模式，本研究开展了野外观测调查。2020年至2021年间，我与学生在美国佐治亚州、南卡罗来纳州与北卡罗来纳州的56块肥力管理方式各异、形成营养有效性梯度的农田中，采集了节肢动物、土壤与叶片组织样本（附录图S1、附录表1、表2）。受新冠疫情期间出行限制影响，2020年调查的样地数量（18个）远少于2021年（38个）。在每个样地，我们沿每条田块样带随机选取10株独立的西葫芦植株，对随机选取的6片叶片（2片新叶、2片成熟叶与2片老叶）、茎秆以及所有开放花朵上的节肢动物进行目视调查。现场对植食动物与天敌进行鉴定与计数，尽可能达到最高分类分辨率…… 所用工具包括Microsoft Excel与program R。