Above- and belowground herbivores promote plant diversity when selectively feeding on dominant plant species, but little is known about their combined effects. Using a model system, we show that neutral effects of an aboveground herbivore and positive effects of a belowground herbivore on plant diversity became profoundly negative when adding these herbivores in combination. The non-additive effects were explained by differences in plant preference between the aboveground- and the belowground herbivores and their consequences for indirect interactions among plant species. Simultaneous exposure to aboveground- and belowground herbivores led to plant communities being dominated by a few highly abundant species. As above- and belowground invertebrate herbivores generally differ in their mobility and local distribution patterns, our results strongly suggest that aboveground-belowground interactions contribute to local spatial heterogeneity of diversity patterns within plant communities.