Alien species pose a significant threat to global biodiversity and alien plants are a particularly serious threat given their ability to displace native plants and animals. Therefore to understand how alien plants impact biodiversity and shape ecological interactions is a vital step to correctly prioritize invaded areas for habitat restoration and to monitor restoration success. Working in the Island of São Miguel in the Azores, we studied the impact of alien plants on a community of seed feeders, including the rare endemic bird, the Azores Bullfinch. The research focused on four trophic groups: seeds, birds (as seed predators and dispersers), seed-feeding insects and their parasitoids. In order to understand the real impact of alien plants on the community it was necessary to consider the network of interactions rather than simply concentrating on focal species. In this talk I will first present food webs along a gradient of invasion by alien plants, then use a field experiment to assess whether alien plants are appropriating the seed disperser's of native plants and finally consider how alien plants can reduce the overall biomass/abundance of invertebrates in a habitat. We conclude that the usually overlooked impact of aliens through the disruption of mutualistic interactions can be critical to native species at several trophic levels, particularly by deleting native mutualisms and reducing total insect biomass.