Root hemiparasitic plants both compete with and extract resources from host plants. By reducing the abundance of dominant plants and releasing subordinates from competitive exclusion, they can have an outsized impact on plant communities. Most research on the ecological role of hemiparasites is manipulative and focuses on a small number of hemiparasitic taxa. Here, we ask whether patterns in natural plant communities match the expectation that hemiparasites affect the structure of plant communities. Our data were collected on 129 national park units spanning the continental United States. The most common hemiparasite genera were Pedicularis, Castilleja, Krameria, and Comandra. We used null models and linear mixed models to determine whether hemiparasites were associated with changes in community richness and evenness. Hemiparasite presence did not affect community metrics. Hemiparasite abundance was positively associated with increasing evenness of herbaceous species, but not with species richness. The associations that we observed on a continental scale are consistent with evidence that the impacts of root hemiparasitic plants on evenness can be substantial and abundance dependent but that effects on richness are less pronounced. Hemiparasites mediate competitive exclusion in communities to facilitate species coexistence and merit consideration of inclusion in ecological theories of coexistence.

中文翻译：

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根系半寄生植物与北美更均匀的社区有关

根半寄生植物与寄主植物竞争并从中提取资源。通过减少优势植物的数量并将下属从竞争排斥中释放出来，它们可以对植物群落产生巨大影响。大多数关于半寄生虫生态作用的研究都是操纵性的，并且侧重于少数的半寄生虫类群。在这里，我们询问天然植物群落的模式是否符合半寄生虫影响植物群落结构的预期。我们的数据收集于横跨美国大陆的 129 个国家公园单位。最常见的半寄生虫属是Pedicularis、Castilleja、Krameria和Comandra. 我们使用空模型和线性混合模型来确定半寄生虫是否与群落丰富度和均匀度的变化有关。半寄生虫的存在不影响社区指标。半寄生虫丰度与草本物种的均匀度增加呈正相关，但与物种丰富度无关。我们在大陆范围内观察到的关联与根系半寄生植物对均匀度的影响可能很大且依赖丰度但对丰富度的影响不太明显的证据是一致的。半寄生虫介导社区中的竞争排斥，以促进物种共存，并值得考虑纳入生态共存理论。