Biotic resistance: exclusion of native rodent consumers releases populations of a weak invader

1. Biotic resistance is a commonly invoked hypothesis to explain why most exotic plant species naturalize at low abundance. Although numerous studies have documented negative impacts of native consumers on exotic plant performance, longer-term multi-generation studies are needed to understand how native consumer damage to exotics translates to their population-level suppression over large landscapes. 2. We used rodent exclosures and embedded seed-addition experiments to evaluate the effects of rodent herbivory and granivory on populations of Tragopogon dubius, a Eurasian aster that typically occurs at low abundance across North America. 3. Vegetation surveys encompassing 20 000 km of west-central Montana grasslands established that T. dubius is regionally well distributed, occurring in 81% of 16 grasslands sampled. However, it consistently occurred at low local abundance, averaging < 0.5% cover in 1 m plots. 4. In large rodent exclosures, T. dubius attained population densities that averaged over five times higher than in paired rodent-access control plots at 11 grassland sites spread over 750 km. This pattern was six times stronger in older compared with newer rodent exclosures, indicating enclosed populations were still increasing. Floral herbivory, which reduces or prevents reproduction in this biennial plant, decreased by 99% in rodent exclosures compared with control plots. Additionally, seedling establishment was nearly eight times higher in seed-addition subplots within versus outside of rodent exclosures. 5. Synthesis. Our findings illustrate how biotic resistance from granivory and florivory of native generalist rodents can provide an important ecosystem service by strongly limiting the local abundance of a widely distributed exotic weed.