Understanding the role that native biodiversity plays in controlling exotic species invasion is a critical goal in ecology. In terrestrial plant communities, most research has focused on the effects of native vascular plants on invasion by exotic vascular plants. However, in many ecosystems, native bryophytes and other non-vascular plants are common and can affect the establishment, survival and growth of vascular plants. A more complete picture of how native biodiversity affects exotic plant invasion, demands that more studies measure the effects of native bryophytes on exotic vascular plants. Moreover, there is growing realization that the effects of native species on invaders can range from negative to positive and that a complete picture of interactions between native and exotic plants requires measuring interactions in multiple environments. We used both observational and experimental studies to quantify the effects of native moss on two exotic annual grass species along a 200-m environmental gradient in a coastal dune in northern California. We found the effects of bryophytes to be species-specific and to vary with environmental context. Bryophytes facilitated the survival of one exotic grass species at both ends of the environmental gradient. For the other exotic grass species, bryophytes reduced survival at one end of the environmental gradient and had no effect at the other end. Our findings provide an important test of the effects of native bryophytes on exotic vascular plant invasion, and importantly show that these effects can vary dramatically even across local environmental gradients.