Rodent‐limited establishment of bush lupine: field experiments on the cumulative effect of granivory

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Abstract
Summary: Plants often suffer substantial loss of seeds to consumers. However, because the seed‐to‐seedling transition is frequently ignored, quantitative estimates of the effects of seed consumers on plant population dynamics are rare. We examined how post‐dispersal seed predation by rodents affected seedling emergence and subsequent adult plant abundance of bush lupine (Lupinus arboreus), a large N‐fixing shrub common to coastal dunes in California. We monitored patterns of seedling emergence and survival over 3 years for seeds sown into exclosed and control plots. We sowed additional cohorts of seeds in the second and third years and compared interannual variation in emergence patterns. Rodent exclusion substantially reduced seedling emergence, with an average of 109 seedlings emerging over 3 years from 476 seeds sown in rodent exclusion plots vs. 26 from control plots. The intensity of granivory, however, varied between years, with rodent exclusion increasing emergence from seeds sown in year one, but not in year two. Winter seedling mortality, due to cutworm herbivory, was similarly high in rodent‐free and control plots, and its net impact was to reduce the difference in seedling abundance. Thus, by mid‐summer in each of the three years, there were only marginally more seedlings in rodent‐excluded vs. control plots. The cumulative effect of protecting seeds, was, however, large. After 3 years, an average of four adult lupines were established in rodent‐free plots, whereas only 0.5 were found in control plots and lupine biomass was more than 5‐fold higher in exclusion plots. Taken together, the results indicate that rodents play a critical role by limiting the abundance and biomass of a large N‐fixing shrub in dunes.