Rattus exulans (mammal)
Physical: Control on mainland sites predominantly consists of snap-trapping.
Chemical: Over the last fifteen years, Pacific rats (Rattus exulans) have been eradicated from increasingly larger New Zealand offshore islands. To date, the largest eradication has been from Raoul Island (2938 ha), although confirmation of eradication from the larger Little Barrier Island (3083 ha) is due this year. Eradication of R. exulans populations on islands is achieved using chemical poisons. In New Zealand compound 1080 has not proved effective against R. exulans, but they are susceptible to anticoagulant poisons such as brodifacoum and bromadialone. Recent successful eradication campaigns have all sown Talon 20 P baits aerially by helicopter. Talon 20 P is a cereal-based (pollard) pellet of approximately 0.8 g containing the anticoagulant toxin brodifacoum at 20 ppm. Currently this is applied at 15kg/ha at a cost of ~$75US/ha (Atkinson and Towns 2001).
Fisher et al. (2004) suggest that diphacinone especially, and also coumatetralyl and warfarin, should be evaluated in field studies as alternative rodenticides in New Zealand. Brodifacoum, the most widely used rodenticide in New Zealand currently, can acquire persistent residues in non-target wildlife. Mineau et al. (2004) presented a risk assessment of second generation rodenticides at the 2nd National Invasive Rodent Summit. O'Connor and Eason (2000) discusses the variety of baits which are available for use on offshore islands in New Zealand.
Biological: Monitor lizards and mongooses were introduced to Pacific islands in early attempts to control R. exulans.
Contraceptive methods of control are currently experimental, but the potential for effective control using contraceptive methods is promising. National Wildlife Research Center (USA) scientists are working on several possible formulations that may make effective oral immunisation possible (Nash and Miller, 2004).
Guidelines for the Eradication of Rats From Islands Within the Falklands Group offers guidelines for the eradication of rats from islands, based on the experiences in eradicating rats from the Falklands group.
A study (Pierce, 2002) was carried out after the eradication of the Pacific rats, to test the following hypotheses: (1) Breeding success of the two seabird species (Pycroft’s petrel (see Pterodroma pycrofti in IUCN Red List of Threatened Species) and the winter-nesting little shearwater Puffinus assimilis haurakiensis (see Puffinus assimilis in IUCN Red List of Threatened Species) is not limited by Pacific rats presence, (2) breeding success is not limited by the presence of tuatara (see Sphenodon punctatus in IUCN Red List of Threatened Species) an endemic predatory reptile, (3) and the two seabird species are not in competition with each other. The study found that the breeding success was significantly lower when the study islands contained Pacific rats: little shearwaters averaged a 16% breeding success in the presence of Pacific rats and 61% in the absence of Pacific rats, while Pycroft’s petrels averaged a 33% breeding success in the presence of Pacific rats and 57% in the absence of Pacific rats (Thomas and Taylor, 2002).
In June 2006, Little Barrier Island in the Hauraki Gulf was declared rat free. This follows a two-year pest eradication programme by the Department of Conservation and means endangered species will now have a fighting chance of survival there. It is hoped the same success will also be experienced on the island's of Motutapu and Rangitoto where more than NZD 500,000 has been allocated for similar pest eradication programmes. The New Zealand Department of Conservation's Island Biodiversity Manager, Richard Griffiths, says having no rats on Little Barrier island is already making a big difference, with 90% of Cook's petrel (see Pterodroma cookii in IUCN Red List of Threatened Species) now producing chicks. Griffiths, says tuatara (see Sphenodon punctatus in IUCN Red List of Threatened Species), currently held in cages on the island, will be released into the wild at the end of the year (TVNZ, 2006).
Towns and Broome (2003) outline the history of rat eradications in New Zealand.
Poison was used at remote anchorages of southern Stewart Island in spring and summer of 1984/85 to reduce the likelihood of ship rats (Rattus rattus), Norway rats (R. norvegicus) and kiore (R. exulans) boarding fishing boats heading for the Snares Islands. Poison baits were removed at successively slower rates, probably because poisoning had reduced rat numbers. The effectiveness of poisoning was tested by (i) live-capturing and tracking marked rats at a simulated anchorage near Halfmoon Bay, (ii) poisoning there as in southern Stewart Island, and (Hi) monitoring the survival and responses of the marked population. Population density approximated 2.0-2.5 ship rats per hectare before poisoning. The minimum monthly home range of ship rats averaged 0.54 ha (mean range length 142 m), which is much larger than previously recorded for ship rats in New Zealand. Neither Norway rats nor ship rats were restricted to the shoreline or along creeks. Poisoning caused a 93% reduction in an index of rat numbers in a 0.69 ha poisoning zone over 16 days, and a 76% reduction over the larger 10.4 ha effective trapping area including the poison zone. Poisoning reduces the risk of rats boarding boats, and can protect endangered plants and animals on infested islands (Summary from Hickson et al. 1986).
The Viwa Stakeholder Committee (made up of local people, government officials and eradication experts) decided the vertebrate pest eradication should be conducted first with progress carefully monitored as the operation proceeds. During the vertebrate eradication, cane toad eradication techniques will be tested and refined. The mammalian eradication operation will help demonstrate eradication techniques to the villagers and will determine if the island eradication team can maintain a long-term eradication project around day-to-day village life and tasks. It will also remove any of potential interference of Pacific rats (Rattus exulans), cats (Felis catus) and dogs (Canis lupus) from the cane toad eradication.
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