Community and ecosystem resilience following introduction and eradication of invasive species (Rattus norvegicus)
Invasive species are among the greatest threats to biodiversity, but their introduction and removal can be used to advance ecological knowledge. In the Falkland Islands, the presence of invasive rats (Rattus norvegicus) is associated with reduced diversity and altered community structure of native passerine birds. Two native Falkland Island passerines (Troglodytes cobbi and Cinclodes antarcticus) are the most abundant and prevalent on rat-free islands but they are also the species that are the most vulnerable to the presence of rats. To protect native passerines, conservation managers in the Falklands have eradicated rats from 66 islands in the archipelago. We surveyed 230 islands (85 with rats present, 108 with rats historically absent, and 37 with rats successfully eradicated at the time of survey). We used bird detection data to build occupancy models and estimate species-area curves. We used count data to estimate each species’ relative abundance and the rate of energy used by the passerine community. We predicted that if community properties were resilient following eradication, we would find similar values on historically rat-free islands and islands on which rats had been eradicated. We also extended the results from these analyses to other invasive species, i.e., feral swine (Sus scrofa).
Despite reduced species richness on islands with rats, richness was nearly identical on eradicated and historically rat-free islands; species richness appeared to be resilient following the removal of rats. However, community structure on eradicated islands was more similar to that of rat-infested islands than to that of islands historically free of rats. Passerine communities on islands with rats used about an order of magnitude less energy than on historically rat-free islands. The rate of energy used by passerines on eradicated islands was intermediate between the high value on historically rat-free islands and the low value on rat-infested islands. Species richness appears to be resilient following the removal of invasives in the Falklands, but community structure and energy flow are not. We hypothesize that this discrepancy is the result of the slow recovery of the two species that are dominant on historically rat-free islands (Cinclodes and Troglodytes). Other species are better at colonizing islands and thus lead to an increase in species richness, but they do not compensate for the contribution of these dominant species to community structure and energy flow. These results lend support to the hypothesis that species are functionally complementary rather than redundant.