Acorn removal and dispersal in Californian oak savannas: A mutualism-antagonism continuum
Oak-dominated systems are currently suffering a continuous failure in tree recruitment. Acorn are highly valuable food items that are consumed by many different animal species. Unsuccessful seed dispersal is usually considered a bottleneck in the regeneration of oaks since acorns suffer high levels of fatal predation. However, some animals may contribute to seed dispersal (potential mutualists) whereas many others may only prey upon the seeds (antagonists). Very few studies have examined the whole animal community and the antagonism-mutualism continuum that may exist across all acorn foragers and across different microsites. Here we offered Quercus agrifolia and Quercus lobata acorns in different microsites (under tree, open, under palatable and non-palatable shrubs) and tracked their fate for each of the main acorn foragers using video recordings and acorn tagging. We also buried some seeds simulating scatter-hoarding to monitor their survival, germination and establishment.
We found that acorns were mostly removed by scrub jays, deer mice, ground squirrels, mule deer, woodrats and tree squirrels. Based on the ability of each species to cache the acorns (survival), the average dispersal distance, the recovery rates and the embryo damage we were able to establish an antagonism-mutualism gradient from exclusive seed predators to effective seed dispersers. Overall, scrub jays were the most effective seed disperser, followed by tree squirrels and deer mice whereas woodrats, ground squirrels and mule deer where at the opposite end of the gradient, behaving mostly as acorn predators. Quercus lobata acorns were bigger and preferred to Quercus agrifolia and acorns that were buried and found in open areas survived in higher proportion. This study highlights the importance of considering the whole animal community and their behavior in order to understand the ecology of acorn dispersal and, thus, foster the regeneration of the threatened oak savannas.