Avian dispersal of an invasive oak is modulated by acorn traits and the presence of a native oak

Successful invasions of non-native plants often depend on establishing mutualistic interactions with native organisms. Animal seed dispersers can greatly enhance the spread of invasive plants. Emerging seed dispersal mutualisms might also result in indirect interactions between non-native and native plants, mediated by shared dispersal agents. We investigated tripartite interactions between Eurasian jays (Garrulus glandarius), non-native northern red oaks (Quercus rubra), and native pedunculate oaks (Q. robur) in a lowland forest of Central Europe. We estimated the probability of acorn removal for both oak species in single vs. mixed treatment, and the effects of acorn length, width, and weight. We also radio-tracked removed acorns to determine their fate. While jays preferred to harvest the native acorns, they also removed almost all offered acorns of the invader. The removal rate of non-native acorns increased when the two oak species co-occurred, and was modulated by acorn traits: jays selected red oak acorns that were long and narrow. In comparison to pedunculate oak acorns, those of red oak were transported shorter distances and were more likely to be consumed rather than cached. Altogether, dispersal of red oaks by jays was less intense and brought smaller benefits relatively to the dispersal of pedunculate oak. Yet, these differences were moderate, and jays provided effective dispersal of both oaks. Our results highlight the role of keystone native seed dispersers in the expansion of non-native plants. Furthermore, they illustrate how dispersal quantity of non-native plants can be facilitated by the presence of native plants that attract shared seed dispersers.