PS 42-126
Granivorous birds drive seed bank dynamics of a dominant chaparral shrub
Faunal 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, as most dominant species of the chaparral solely rely on dormant, persistent soil seed banks to recruit after wildfires.
However, there are no reliable empirical data on the identity of seed predator taxa or functional groups, despite the fact that the differential impact of different taxa and functional groups, such as scatter-hoarding rodents versus granivorous birds, may have a defining impact on plant community structure in this system. Specifically, we asked if different taxa have a differential impact on the seed bank dynamics of a dominant post-fire shrub population of the Californian chaparral.
We used a series of exclusion experiments in combination with close video observation to determine if granivorous birds are more effective than rodents in removing seeds of Ceanothus papillosus. We furthermore used seed traps and germination experiments to quantify seed production and short-term seed bank dynamics of C. papillosus.
Results/Conclusions
We found that the average seed loss for C. papillosus between fruiting seasons can be up to 94%. This rate correlates with the 96% average predation rates of birds and rodents as found in three independent exclusion experiments. However, rodents alone were only able to remove on average between 4% and 44% of seeds.
We conclude in contrast to other studies, that granivorous birds, in addition to rodents, are major determinants of C. papillosus seed bank densities. Ecosystem-level implications of our findings include that C. papillosus is supporting a wider and more diverse food web then previously assumed. In return, the diversity in consumers would support a diverse assembly of carnivorous predators, some of which are rare or endangered throughout chaparral. Second, if the presence of caching rodents is limited and outweighed by an abundance of granivorous birds, seed bank densities may decrease to a point where post-fire regeneration is at risk. Consequences of failed post-fire regeneration of woody chaparral shrub species can have such dramatic effects as loss of biodiversity, loss of critical habitat, impaired protection from erosion, and even type conversion to alien-dominated annual grasslands.