Nutrient enrichment and loss of herbivores are assumed to cause plant diversity loss in grassland ecosystems because they increase plant cover that decreases understory light. Empirical tests of the role of competition for light in natural systems are based on indirect evidence and have contributed to strong debates over the last 40 years. Using illumination by LED-lamps, we demonstrate that experimentally restoring light to understory plants in a natural grassland mitigated the loss of plant diversity caused either by nutrient enrichment or the absence of mammalian herbivores. The initial effect of light addition on restoring diversity under fertilization was transitory and outweighed by the greater effect of herbivory on light levels, highlighting herbivory as a major factor controlling diversity, partly via light. Our results provide the first direct experimental demonstration in a natural system that competition for light is a major mechanism contributing to biodiversity loss under cessation of mammalian herbivory. Our results also demonstrate that herbivore effects can outpace fertilization effects on competition for light. Management practices that target maintaining grazing by native or domestic herbivores may have applied utility for protecting biodiversity in grassland ecosystems because they alleviate competition for light in the understory.

All statistical analyses were carried out using R Statistical program version 4.0.0 (https://www.R-project.org/). We used the 'nlme' package for linear mixed effect models (version 3.1.147), the 'lme4' package for generalized linear mixed effects models (version 1.1.23), and the 'car' package for P-values (version 3.07).Funding provided by: Academy of FinlandCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100002341Award Number: 297191

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