Unusually large invasive seeds are spared by rodents in a Patagonian forest

Seed predation by rodents can act as a recruitment barrier during community assembly, limiting the establishment of exotic species. Predation rates of exotic seeds may depend on their attractiveness, determined by their traits, and how different they are from natives. Additionally, at the naturalization stage of the invasion process, exotic seeds may escape post-dispersal predation because they are rare in the community. To test these ideas, we assessed granivory in a Patagonian forest, where two species with contrasting seed sizes are naturalized. Rubus idaeus seeds are of similar size of natives, whereas Prunus cerasus seeds are four times larger. The relative abundance of their seeds within the landscape is low compared to native seeds. Throughout the fruiting season, we offered seeds from all species present in the community (native and exotics), whenever they were available in the landscape. To consider the effects of vegetation structure on rodent foraging behavior, we offered seeds in areas with and without understory cover. We found a hump-shaped relationship between predation rates and seed size. Consequently, rodents strongly avoided large exotic Prunus seeds, but removed Rubus seeds at similar rates to natives. Contrary to our expectations, seed abundance did not affect predation, and hence, rarity did not confer an advantage to exotic seeds. The presence of shrub cover increased 2.3 times the removal rates compared to open areas. We suggest that the dissimilarity in seed size compared to native species and the presence of shrub cover influenced predation pressure on exotic species within our community.