Large seeds as a defensive strategy against partial granivory in the Fagaceae

Abstract

Large seeds interact with a wide range of animals (e.g. predators) and are dispersed via certain small animals' foraging behaviours, such as caching. Some of the most iconic species of large‐seeded plants have long fascinated ecologists studying biotic interactions, such as oaks and their relatives in the Fagaceae family. The Fagaceae acorns are dispersed through synzoochory, a specific dispersal mode in which animal partners act as both seed dispersers and granivores. Although granivory (i.e. seed herbivory) can profoundly impact the survival of plant offspring, partial damage on seed reserves is a prevalent phenomenon that does not always result in seed mortality. However, previous single‐species studies have resulted in mixed evidence across treatments and traits, leaving the impact of partial granivory on plant regeneration unclear. Using artificial granivory experiments on 1185 seeds of 20 Fagaceae species, here we quantify how partial loss of seed reserve affects seed germination, seedling growth and biomass allocation across a damage gradient from 0% to 96% biomass loss. We show that, although partial granivory reduces seedling growth (e.g. total biomass and number of leaves), it does not significantly affect seed germination or the overall biomass allocation of seedlings (e.g. leaf mass fraction and root:shoot biomass ratio). Seedlings from seeds more preyed upon have higher specific leaf area, indicating that they tend to grow larger but less protected leaves against herbivores, perhaps to compete for light. Synthesis. As seeds dispersed through scatter‐hoarding granivores have evolved relatively large sizes, like Fagaceae acorns, our findings demonstrate that this type of seeds may tolerate partial granivory in exchange for high dispersal efficiency. This study opens new perspectives to our understanding of seed size diversity and evolution. We conclude that seed size per se is a defensive trait, that large seeds counteract potential losses of seed reserve to escape full predation and allow germination.