Biodiversity changes in forest understory suggest that forests develop as a mosaic of patches over time and space. Ecological processes that determine plant and insect diversity in these patches are driven by complex direct and indirect forces of resource availability and trophic interactions. We investigated biodiversity fluxes associated with interactions among light, arthropods, and plants in forest understory patches, using path analysis on empirical measurements and manipulative experiments with top predators. In a three-year study of redwood forest of California, increases in canopy gap size from selective logging were associated with increases in exotic plant richness and insect herbivore richness in the understory, but no corresponding change in the diversity of native plants or carnivorous arthropods occurred. In three tropical wet forests of Costa Rica, generalist herbivore pressure, measured on Piper shrubs, was negatively associated with seedling species richness. We demonstrated how herbivore pressure in this system can be driven by the presence of top predators. The temperate case suggests that bottom-up forces drive an imbalance in herbivore to carnivore species richness as invasive plants increase in understory cover. The tropical case suggests that top-down forces can suppress plant species diversity in understory patches. Plant-insect interactions demonstrate how top-down and bottom-up forces affect biodiversity at the community level. Forest conservation and restoration efforts based on land acquisition or preservation of charismatic species could be misguided if complex ecological processes that affect biodiversity are not also taken into account.