Rodent consumers and differential establishment of native grasses

Spatial variation in the level of herbivory may have an important role in the process of biological invasions in California grasslands. We examined the role of rodent consumers in generating the seed limitation thought to inhibit the establishment of native grasses, and whether the effect of rodent consumers on native plants varies among native and exotic habitats. We performed a seed addition experiment with Bromus carinatus, a native grass that is not present in either habitat type where our study was conducted, in combination with rodent exclosures in native and exotic habitats. Results from this study confirmed that B. carinatus is seed limited and that rodents contribute to the reduction of seed abundance. Moreover, rodent reduction of B. carinatus recruitment was greatest in native habitats compared to exotic habitats. These findings suggest that consumer pressure is greater in native habitats than in exotic habitats. Such spatial variation in consumer pressure could hinder restoration efforts as well as facilitate the dominance of invasive plants by reducing the success of native plant restoration as well as promoting loss of B. carinatus where it is currently present.