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In the Falkland Islands, islands with invasive rats have fewer passerine species compared to islands without rats. On islands on which rats have been eradicated, passerine species richness is indistinguishable from that found on islands historically free of rats, but community composition differs between these two island types. In particular, the most dominant species on historically rat-free islands, Cinclodes antarcticus, is less abundant and prevalent on eradicated islands. We compared passerine energy flow on islands with rats, islands from which rats have been eradicated, and islands on which rats were historically absent. Passerine communities on islands historically without rats used nine times more energy than on islands with rats present. Despite equivalent passerine species richness, passerine energy flow was approximately half on islands from which rats had been eradicated compared to historically rat-free islands. Because passerine energy flow was determined by community composition and not by species richness, passerine species within this community appear to be functionally complementary and not functionally redundant. At least one species, C. antarcticus, plays an irreplaceable role. Our results also document the dramatic and lingering effects of invasive species following their eradication, and the importance of species complementarity for the resilience of community properties.