In grassland ecosystems, consumers such as herbivores and pathogens help shape plant community composition and influence whether introduced plant species become invasive and displace resident species. When consumers negatively influence hosts, their presence may limit host population growth. In other cases, consumer interactions may benefit introduced hosts and contribute to their invasiveness. In contemporary California grasslands, for example, evidence suggests that at least two pathogen groups may benefit invasive annual grasses, which have displaced native perennial species throughout much of the state over the last several centuries. One example is Ulocladium atrum, a dematiaceous hyphomycetes that infects seed heads of Aegilops triuncialis (barbed goatgrass), which is a rapidly spreading noxious invader in the region. This fungus substantially enhances Aegilops’ establishment and growth, most likely by degrading seed head structures and speeding seed germination. A second example are the Barley and cereal yellow dwarf viruses (Luteoviridae: BYDVs and CYDVs, here referred to collectively as BYDVs), which are aphid-vectored pathogens of grass and cereal species. Evidence suggests that BYDV-mediated apparent competition disadvantages native perennials in California grasslands and contributes to the non-natives’ invasiveness and continued dominance (the disease facilitation hypothesis). Field studies indicate that non-native invaders amplify spring aphid populations and increase BYDV infection in natives, which in turn suffer substantially reduced survivorship when infected. Interactions with other consumers influence the extent of virus effect; in some situations herbivory increases the survivorship of infected native seedlings. Complementary historical analyses and modeling studies indicate that BYDVs likely influenced early stages of the California grassland invasion as well and may have been critical factors in facilitating the region’s dramatic landscape conversion. These examples highlight the importance of considering multitrophic interactions in ecosystem management and restoration.