Tropical rainforests are biodiversity hotspots that provide a variety of ecosystem functions and services. Seed dispersal by fruit-eating birds is an important ecosystem process in the regeneration of tropical rainforests, which is increasingly threatened by widespread deforestation. In particular, the expansion of agricultural land often leads to forest fragmentation, which can have a negative impact on the interactions between plants and frugivores and thus on seed dispersal. However, little is known about how forest fragmentation affects the structure and robustness of plant-frugivore interaction networks. Here, we examined the effects of forest fragmentation on species richness of frugivorous birds interacting with focal tree species, and the structure and robustness of plant-frugivore interaction networks in the tropical lowland forests of northern Costa Rica. Species richness of frugivorous birds at the forest edges increased with fragment size and forest cover in the surrounding landscape, as well as with local fruit availability. Modularity and robustness of plant-frugivore networks increased with enhanced fragment size and forest cover, while network specialization (H2´) increased only with greater forest cover. Additionally, the three common tanager species (Ramphocelus passerinii, Thraupis palmarum, and Thraupis episcopus) were identified as key bird species for network functioning by promoting among-module and within-module connectivity. Conservation measures should therefore not only focus on threatened specialist species, but more on the key species that enhance network structure and consequently increase the robustness of these trophic interaction networks. Ultimately, our study demonstrates that tropical forest fragmentation simplifies network structure, making these interactions more vulnerable to anthropogenic disturbances.