Diversity within species can have community‐level effects similar in magnitude to those of species diversity. Intraspecific diversity in producers and consumers has separately been shown to affect trophic interactions, yet we have little understanding of how variation at these two levels could simultaneously affect trophic interactions. Salt marshes dominated by Spartina alterniflora are an ideal system in which to ask this question as this plant exhibits substantial genetically based trait variation. Further, herbivores can have sizable impacts on Spartina, but the impact of herbivore trait variation is not well understood. We conducted an experiment in a Massachusetts salt marsh to determine how herbivorous crab (Sesarma reticulatum) size diversity and Spartina genotypic diversity affect the plant community. Herbivore effects on plant traits varied by herbivore size, with large crabs generally having stronger impacts on plants. At times, the effect of small crabs on plant traits depended on plant genotypic diversity. The effects of crab size diversity (i.e., small and large crabs combined) were most often predicted by the independent effects of each size class, though there were synergistic effects on stem density, flowering stems, and mean stem height. Finally, we tested whether herbivore size or size diversity could have reciprocal effects on plant genotypic diversity. Small‐ and mixed‐crab treatments promoted plant genotypic richness, whereas large crabs did not. Our results demonstrate that intraspecific diversity at multiple trophic levels can have simultaneous and sometimes interactive effects on species interactions, highlighting the importance of variation within species for understanding species interactions and community processes.

中文翻译：

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两个营养级的种内多样性影响植物-草食动物的相互作用

物种内的多样性在社区层面上的影响与物种多样性相似。生产者和消费者的种内多样性已分别显示出影响营养相互作用，但我们对这两个水平的变异如何同时影响营养相互作用的了解很少。以互花米草（Spartina alterniflora）为主的盐沼是一个理想的系统，因为该植物表现出大量的基于遗传的性状变异，因此可以提出这个问题。此外，食草动物会对斯巴达虫有相当大的影响，但食草动物性状变异的影响尚不十分清楚。我们在马萨诸塞州的盐沼中进行了一项实验，以确定草食蟹（Sesarma reticulatum）的大小多样性和Spartina基因型多样性影响植物群落。草食动物对植物性状的影响因草食动物的大小而异，大蟹一般对植物的影响更大。有时，小螃蟹对植物性状的影响取决于植物的基因型多样性。尽管对茎密度，花茎和平均茎高有协同作用，但通常通过每种大小类别的独立影响来预测蟹的大小多样性（即，大小蟹合并）的影响。最后，我们测试了草食动物的大小或大小多样性是否会对植物基因型多样性产生相互影响。小螃蟹和混合螃蟹的处理促进了植物基因型的丰富性，而大螃蟹则没有。