Seed predation has the potential to drive a rare plant to extinction

1. Pre-dispersal seed predation is sometimes considered unlikely to dramatically affect plant population growth because plants are generally expected to produce more seeds than there are safe sites for germination. Lupinus constancei is a rare herb of limited distribution, with fewer than 400 reproductive individuals restricted to a single square kilometre of northwestern California, USA. In addition to the vulnerability resulting from its extremely small population size, L. constancei faces heavy seed prédation by small mammals. 2. As a stop-gap measure to prevent population decline, managers began covering a large number of the reproductive plants with herbivory exclosures in 2003, but the population-level effects of seed predation and the effectiveness of this caging treatment were unknown. We used 10 years of demographic data to compare the population dynamics of plants inside herbivory exclosures with those sustaining ambient rodent seed predation. 3. We found that the stochastic population growth rate would be robust without seed predation (λs = 1.17), but without continued human intervention (i.e. use of exclosures), the current rate of predation would result in a decline towards extinction (λs = 0.92). 4. After our study concluded, high mortality due to two extreme winter droughts followed by a wildland fire reduced the number of reproductive plants to ~ 103, making extinction of L. constancei more likely. 5. Synthesis and applications. The prevalence of consumer-driven population decline is largely unknown, but this study demonstrates that pre-dispersal seed predation by rodents can have powerful population-level effects, and represents one set of conditions under which consumer pressure has the potential to drive plant extinction. However, with continued management to limit the effects of seed predation in the short-term and investigation into the ultimate drivers of this high seed predation rate in the long term, the Lassies lupine population could be restored to a robust rate of growth.