Abstract
Abstract Non-native rats ( Rattus spp. ) threaten native island species worldwide. Efforts to eradicate them from islands have increased in frequency and become more ambitious in recent years. However, the long-term success of some eradication efforts has been compromised by the ability of rats, particularly Norway rats ( Rattus norvegicus ) which are good swimmers, to recolonize islands following eradications. In the Falkland Islands, an archipelago in the South Atlantic Ocean, the distance of 250 m between islands (once sug-gested as the minimum separation distance for an effective barrier to recolonization) has shown to be in-sufficient. Norway rats are present on about half of the 503 islands in the Falklands. Bird diversity is lower on islands with rats and two vulnerable passerine species, Troglodytes cobbi (the only endemic Falkland Islands passerine) and Cinclodes antarcticus , have greatly reduced abundances and/or are absent on islands with rats. We used logistic regression models to investigate the potential factors that may determine the presence of Norway rats on 158 islands in the Falkland Islands. Our models included island area, distance to the nearest rat-infested island, island location, and the history of island use by humans as driving vari-ables. Models best supported by data included only distance to the nearest potential source of rats and island area, but the relative magnitude of the effect of distance and area on the presence of rats varied depending on whether islands were in the eastern or western sector of the archipelago. The human use of an island was not a significant parameter in any models. A very large fraction (72%) of islands within 500 m of the nearest potential rat source had rats, but 97% of islands farther than 1,000 m away from potential rat sources were free of rats.
Highlights
Introduced species can have detrimental consequences for native island communities (Wiles et al 2003, Kurle et al 2008) and rats are among the most problematic species introduced to islands (Hilton and Cuthbert 2010)
The probability of rat occupancy decreased with distance to the nearest rat source, increased with island area, and was higher in the western sector
Since there was a significant effect of the geographical sector in the model, we separated the islands into two sectors and ran the models separately for each of them
Summary
Introduced species can have detrimental consequences for native island communities (Wiles et al 2003, Kurle et al 2008) and rats are among the most problematic species introduced to islands (Hilton and Cuthbert 2010). Three species of rats (Rattus norvegicus, R. rattus, and R. exulans) live as commensals of humans. Rats can cause population declines and even the extinction of many species of island birds, amphibians, and reptiles (Towns et al 2006, Cree et al 1985). Eradicating rats from islands can lead to the recovery of native species and many of the ecological processes that they mediate. Rats disperse to islands with human assistance, but they can move among islands by themselves (Russell et al 2010, Russell and Clout 2004). Norway rats (R. norvegicus) are good swimmers and can reinvade eradicated islands (Russell et al 2008). To eradicate rats successfully and permanently, it is necessary to understand the factors that determine their capacity to colonize islands (Fewster et al 2011)
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