Conclusion
One or a few species and their direct and indirect interactions can be critical to maintain the structure of communities and ecosystem processes. Thus, identifying which species or interaction to conserve is critical to prevent cascading effects in the community. Here, we have demonstrated the role of a hummingbird-mistletoe-marsupial mutualism as a keystone interaction in Patagonian temperate forest and the cascading impacts of the disruption of this interaction by non-native ungulates. We analyzed interaction motifs as the building blocks of pollination and seed dispersal networks, and found that the hummingbird-mistletoe-marsupial mutualism increases complexity on pollination and seed dispersal networks, supporting a high diversity and frequency of complex motifs and improving the persistence of the community. In addition, the keystone interaction can increase the robustness of pollination networks to generalist species extinction due the high number of plant and pollinator species at generalist motif positions. Moreover, we found that non-native ungulates disrupt this keystone interaction, affecting the ecological importance of the mistletoe and marsupial and reducing the complexity and stability of pollination networks and, marginally, of seed dispersal networks. Recognizing the keystone interactions is essential to achieve the conservation of communities due its role to maintain the structure and functionality of them, especially in the current scenario of global biodiversity loss and spread of invasive species. Moreover, incorporate indirect effects in ecological studies is necessary to increase the realism of global changes impacts on communities, widely underestimated when direct effects are only considered.