Plant viral richness correlates with both host richness and biomass at local and global scales in Mediterranean, temperate, and tropical grasslands
Understanding the drivers of species richness is a central goal in ecology, yet parasitic microorganisms -particularly viruses - remain understudied. Using standardized metagenomic inventories of unmanaged grasslands worldwide, we develop a macroecological perspective on how host plant community features predict plant virus richness from local to global scales. We show that virus presence is highly stochastic at host level, but consistent communities of viruses and plants can be defined at a 100m2 scale. In these communities, virus richness is strongly associated with both host species richness and biomass, and more weakly related to the broader environment. Notably, heterogeneity in plant cover - species diversity weighted by their proportional soil cover - outperforms simple taxonomic richness. We interpret this to mean that virus richness is shaped by two main ecological drivers: niche diversity, which increases with host richness, and metapopulation persistence, where stochastic transmission success increases with host abundance. Our results underscore the need for multi-scale studies that allow to disentangle these viral metacommunity processes, with vector communities as a key missing piece to understand transmission dynamics from single host to whole landscape scale.