Nutrient enrichment is widespread throughout grassland systems and expected to increase during the Anthropocene. Trophic interactions, like aboveground herbivory, have been shown to mitigate its effect on plant diversity. Belowground herbivory may also impact these habitats’ response to nutrient enrichment, but its influence is much less understood, and likely to depend on factors such as the herbivores’ preference for dominant species and the symmetry of belowground competition. If preferential toward the dominant, fastest growing species, root herbivores may reduce these species’ relative fitness and support diversity during nutrient enrichment. However, as plant competition belowground is commonly considered to be symmetric, root herbivores may be less impactful than shoot herbivores because they do not reduce any competitive asymmetry between the dominant and subordinate plants. To better understand this system, we used an established, two‐layer, grassland community model to run a full‐factorially designed simulation experiment, crossing the complete removal of aboveground herbivores and belowground herbivores with nutrient enrichment. After 100 yr of simulation, we analyzed communities' diversity, competition on the individual level, as well as their resistance and recovery. The model reproduced both observed general effects of nutrient enrichment in grasslands and the short‐term trends of specific experiments. We found that belowground herbivores exacerbate the negative influence of nutrient enrichment on Shannon diversity within our model grasslands, while aboveground herbivores mitigate its effect. Indeed, data on individuals’ above‐ and belowground resource uptake reveals that root herbivory reduces resource limitation belowground. As with nutrient enrichment, this shifts competition aboveground. Since shoot competition is asymmetric, with larger, taller individuals gathering disproportionate resources compared to their smaller, shorter counterparts, this shift promotes the exclusion of the smallest species. While increasing the root herbivores’ preferences toward dominant species lessens their negative impact, at best they are only mildly advantageous, and they do very little reduce the negative consequences of nutrient enrichment. Because our model’s belowground competition is symmetric, we hypothesize that root herbivores may be beneficial when root competition is asymmetric. Future research into belowground herbivory should account for the nature of competition belowground to better understand the herbivores’ true influence.

中文翻译：

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嫩芽草食动物减少，而根草食动物增加养分富集对草地模型多样性的影响

在整个草地系统中，养分丰富，并且在人类世期间有望增加。营养相互作用，如地上食草动物，已显示出减轻其对植物多样性的影响。地下食草动物也可能影响这些生境对养分富集的反应，但是其影响却鲜为人知，并且可能取决于诸如食草动物对优势物种的偏好以及地下竞争的对称性等因素。如果优先选择生长最快的优势物种，则根食草动物可能会降低这些物种的相对适应度，并在营养富集期间支持多样性。但是，由于地下竞争通常被认为是对称的，根食草动物的影响可能不如芽食草动物，因为它们不会减少优势植物和次要植物之间的竞争性不对称性。为了更好地理解该系统，我们使用了已建立的两层草地群落模型来进行全要素设计的模拟实验，将地上食草动物和地下食草动物的完全去除与营养富集相交。经过100年的模拟，我们分析了社区的多样性，个体层面的竞争以及他们的抵抗和恢复。该模型再现了在草地上观察到的养分富集的一般效果以及特定实验的短期趋势。我们发现地下草食动物加剧了我们模型草原内养分富集对香农多样性的负面影响，而地上食草动物会减轻其影响。的确，有关个人地上和地下资源吸收的数据表明，食草根减少了地下资源的限制。与营养丰富一样，这将竞争转移到了地上。由于新梢竞争是不对称的，与较小，较短的同龄人相比，较大，较高的个体收集了不成比例的资源，因此这种转变促进了最小种的排斥。虽然增加根草食动物对优势物种的偏好可以减轻其负面影响，但充其量只是在一定程度上有利，而它们却很少减少养分富集的负面影响。因为我们模型的地下竞争是对称的，所以我们假设当根竞争不对称时，根草食动物可能是有益的。