Consumer‐mediated indirect interaction with a native plant lowers the fitness of an invasive competitor

1. Seed dispersal is a primary driver, via propagule pressure, of biological invasions. In animal-dispersed plants, theory predicts that indirect ecological interactions at the neighbourhood scale may determine landscape-scale patterns of seed dispersal and predation, and thus invasion. 2. Indirect interactions in seed dispersal may arise between two plant species if shared seed dispersal agents adjust their foraging decisions to the relative frequencies of co-occurring seeds. We used seed-tracking experiments and empirically parameterized simulations to test the hypothesis that an indirect interaction between two plant species, mediated by scatterhoarding rodents that both consume and disperse seeds, affects recruitment of an invasive tree. We predicted that acorns of Quercus rubra (invasive in Central Europe) would experience higher caching (and therefore survival) rates when they co-occurred with native Quercus petraea than when they occurred in isolation, that Q. petraea acorns would survive at lower rates with Q. rubra relative to isolation, and that this interaction would facilitate Q. rubra seedling recruitment in Q. petraea forests. 3. Contrary to expectations, the presence of Q. petraea reduced caching of Q. rubra acorns, and co-occurrence with Q. rubra increased caching of Q. petraea. Thus, a rodent-mediated indirect interaction may be slowing rather than facilitating the Q. rubra invasion. However, our simulations indicate that the magnitude of this indirect interaction will change nonlinearly as Q. rubra becomes more dominant on the landscape, and that the pace of invasion will accelerate as the alien species is released from the rodent-mediated effect of Q. petraea. 4. Synthesis. The presence of native Quercus petraea decreases recruitment rates of Quercus rubra, and consequently, slows down the invasion process. The strong dependence of pairwise interactions on the presence of a third-party species emphasizes the role of indirect interactions in the invasion processes.