Differential post-dispersal seed predation drives chaparral seed bank dynamics

Abstract

Animal community structure influences plant community structure in many ways, one of which is varying post-dispersal seed predation rates by different, sometimes distantly related animal taxa. In fire-prone mediterranean-climate vegetation, such as California chaparral, rodents are commonly assumed to be the most effective post-dispersal seed predators, which would render them the main driver for soil seed bank dynamics. This is a critical issue because the most dominant species of the chaparral rely on dormant, persistent soil seed banks to recruit after wildfires. Here, we used a series of exclusion experiments in combination with close video observation to show that granivorous birds are more effective than rodents in removing seeds of Ceanothus papillosus, a fire-dependent obligate seeder shrub of the California chaparral. We furthermore used seed traps and germination experiments to show that C. papillosus can have extremely high seed production and expresses strong intra and inter-annual seed bank dynamics. We conclude in contrast to other studies, that granivorous birds, in addition to rodents, are major determinants of C. papillosus seed bank densities. We also found that seed bank density increased between years, despite high seed predation rates. We conclude that high seed production in combination with small and dark seed design may help some seeds to stay undetected, thus allowing C. papillosus to build a sufficiently dense enough seed bank to regenerate after wildfire. Our results indicate that the ratio of granivorous birds to rodents has the potential to play a major role in shaping chaparral community structure by differentially impacting soil seed bank densities.