Consumer pressure, seed versus safe-site limitation, and plant population dynamics

Plants often suffer reductions in fecundity due to insect herbivory. Whethe r this loss of seeds has population-level consequences is much debated and often unknown. For many plants, particularly those with long-lived seedbank s, it is frequently asserted that herbivores have minimal impacts on plant abundance because safe-site availability rather than absolute seed number d etermines the magnitude of future plant recruitment and hence population ab undance. However, empirical tests of this assertion are generally lacking a nd the interplay between herbivory, spatio-temporal variability in seed- or safe-site-limited recruitment, and seedbank dynamics is likely to be compl ex. Here we use a stochastic simulation model to explore how changes in the spatial and temporal frequency of seed-limited recruitment, the strength o f density-dependent seedling survival, and longevity of seeds in the soil i nfluence the population response to herbivory. Model output reveals several surprising results. First, given a seedbank, herbivores can have substanti al effects on mean population abundance even if recruitment is primarily sa fe-site-limited in either time or space. Second, increasing seedbank longev ity increases the population effects of herbivory, because annual reduction s in seed input due to herbivory are accumulated in the seedbank. Third, po pulation impacts of herbivory are robust even in the face of moderately str ong density-dependent seedling mortality. These results imply that the cond itions under which herbivores influence plant population dynamics may be mo re widespread than heretofore expected. Experiments are now needed to test these predictions.