Understanding the impacts of global change on mutualistic networks is a major challenge in modern ecology. The gain or loss of particular species and the disruption of key interactions are both consequences and drivers of global change that can lead to the disassembly of complete mutualistic webs. We explored whether the disruption of a hummingbird-mistletoe-marsupial keystone interaction by the invasion of non-native species can have cascading effects on both pollination and seed dispersal networks in the temperate forest of Patagonia. We found that the disruption of the keystone interaction resulted in diverse indirect effects that led to less complex pollination and seed dispersal networks. Similarly, the disruption of the keystone interaction decreased the functional redundancy among generalist plants and pollinators. Our results demonstrate how the disruption of a keystone interaction can have cascading effects through the community, reducing the complexity and stability of ecological networks.