Granivory from native rodents and competition from an exotic invader strongly and equally limit the establishment of native grasses

Seed predation and resource competition are fundamental biotic filters that affect the assembly of plant communities, yet empirical studies rarely assess their importance relative to one another. Here, we used rodent exclosures and experimental seed additions to compare how rodent granivory and resource competition affected the net establishment of an exotic invader (Bromus tectorum) and two native bunchgrasses (Pseudoroegneria spicata and Elymus elymoides) in the Great Basin Desert, USA. Rodent granivory limited the establishment of both native grasses, but had no significant effect on B. tectorum. Competition from B. tectorum limited the establishment of both native grasses, but neither native grass imposed a significant competitive effect on B. tectorum. Interestingly, we found that rodent granivory and B. tectorum competition limited the establishment of native grasses to the same extent, suggesting that these biotic interactions may impose equally important barriers to the local establishment of P. spicata and E. elymoides. By evaluating the strength of multiple biotic interactions in simultaneous, coordinated experiments, we can understand their relative contributions to community-level patterns.