Seed predation is one of the most important biotic filters that determines whether an exotic species can become part of the community. Rodents are generalist seed consumers whose feeding preferences can affect the exotic species recruitment. Their foraging decisions are often modulated by seed traits, abundance, and the environmental context in which rodents encounter seeds. Besides, the consumption preference for exotic seeds may depend on how different they are from natives. We evaluated the combined effect of species-specific seed size, their relative abundance, and vegetation structure on predation rates of native and exotic plant species in a Patagonian forest, where two species of contrasting seed sizes are naturalized (Prunus cerasus and Rubus ideaeus). We performed a cafeteria experiment in which we offered seeds of native and exotic species in paired plots located in microhabitats with shrub cover or open. In plots located under shrub cover seed predation rates were 2.3 times higher than in open areas. Feeding preferences of rodents were driven by seed size, selecting seeds of intermediate sizes. As result, they strongly avoided large exotic Prunus seeds, but consumed intermediate Rubus seeds at similar rates to natives. Thereby, in our system, size is a strong modulator of post-dispersal predation of exotic seeds and rarity does not confer any advantage to exotics. Low predation rates in open areas suggest that they could be regeneration foci for all species. Future work quantifying recruitment across microhabitats will allow to evaluate if patterns here found have an imprint in the seedling community.