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Theoretical models suggest that species abundance plays a crucial role in mutualism; high densities can lead to overexploitation. Additionally, mutualistic benefits are expected to increase under abiotic stress. We investigated the interplay between density dependence and abiotic factors in conditional mutualism, focussing on the interactions between yellow‐necked mice (Apodemus flavicollis) and common oaks (Quercus robur) across 3 years. We controlled for seed abundance, while mouse densities varied. Our approach included monitoring mouse abundance, measuring seed removal, determining the fate of harvested seeds and conducting seedling emergence trials to assess the advantages of seeds caching. We found that mouse abundance exerted a strong influence on all studied aspects of seed dispersal. Higher densities led to increased seed removal and greater dispersal distances. However, they also decreased the probability of seed caching and increased seed consumption, degrading dispersal quality and shifting the interaction towards antagonism. Furthermore, the reliance of seedling recruitment on burial varied over time, likely becoming more critical during dry conditions. This indicates that plants face the worst conditions when high abundance of mice coincides with abiotic stress. Synthesis: Our results supports the notion that increased rodent abundance reduces plant recruitment, revealing the conflicting interests of the interacting species. However, the dynamics of rodent–oak interactions are shaped not only by the density‐dependent foraging decisions of rodents, but also by the benefits of seed burial under stress. These findings illustrate how the interplay between population density and abiotic factors jointly dictate the costs and benefits of mutualistic interactions. [ABSTRACT FROM AUTHOR]

Seeds produced by individual plants often vary substantially in size. Typically, larger seeds produce seedlings that have higher chances of establishment and survival relative to seedlings produced by smaller seeds. However, larger seeds are also preferred by granivores due to larger caloric content. While choosing the patch to forage, granivores might avoid the ones with smaller, less preferred seeds. We tested a novel hypothesis that the production of different size seeds by a plant may be a strategy to decrease predation by granivores. We conducted a 3-year seed removal experiment. We presented Quercus robur acorns in forests in three configurations: large acorns alone, medium acorns alone, and large acorns mixed with small ones. The impact of seed size on seed survival was inconsistent: in the first year of the study, survival probability for seeds in the mixed treatment was significantly higher than survival probability of large seeds alone, supporting our hypothesis. However, in the following years, results were non-significant, probably because of reduced granivore selectivity in poor seed crop years. Our study demonstrated that the impact of neighbourhood of different size seeds seed survival varied over time in Q. robur. This provides limited evidence that intraspecific variation in seed size could evolve to shift the interaction between trees and scatter hoarders away from predation and towards mutualism. [ABSTRACT FROM AUTHOR]