Individual study: Using epizootics and poison baiting to eradicate European rabbits Oryctolagus cuniculus from Cabbage Tree Island, New South Wales, Australia
Priddel D., Carlile N. & Wheeler R. (2000) Eradication of European rabbits (Oryctolagus cuniculus) from Cabbage Tree Island, NSW, Australia, to protect the breeding habitat of Gould's petrel (Pterodroma leucoptera leucoptera). Biological Conservation, 94, 115-125
Largely due to deliberate introduction by humans beyond its native range, the European rabbit Oryctolagus cuniculus is now found on every continent, with the exception of Antarctica. They have also been introduced to a large number of islands, many of which have subsequently suffered severe ecological degradation through over-grazing and burrowing. Changes in vegetation cover and erosion often result, having a knock-on negative effect on native fauna and flora.
In 1906, rabbits were introduced to Cabbage Tree Island (Australia), where the endangered Gould's petrel Pterodroma leucoptera leucoptera nests. In 1953, this island supported the petrel's only known breeding colonies, nesting occuring within two deep rainforest gullies, nests being sited under fallen palm fronds and amongst rock scree. Cabbage Tree Islands was declared a nature reserve in order to protect the petrels.
However, the presence of rabbits on the island bought about a number of conservation problems. Removal of the forest understorey by rabbit grazing decreased cover for Gould's petrels making them more vulnerable to predation by pied currawong Strepera graculina (a native crow-like passerine). The petrels are also more likely to become ensnared in the sticky fruits of the bird-lime tree Pisonia umbellifera. Additionally, the presence of rabbits had increased palm seedling mortality, causing the age structure of some dominant protective forest canopy species to be skewed towards mature trees, hence potentially endangering the forest in the long term.
It was therefore decided to mount a rabbit eradication programme which was initiated in 1997.
Study site: Cabbage Tree Island is approximately 1 km long by 480 m wide at the widest point, and lies 1.4 km offshore from Yacaba Head, New South Wales. It has a maximum elevation of 123 m above mean sea level. The climate is warm temperate, with primarily winter rainfall. The predominant habitat is dry littoral rainforest.
Mortality agents: Three mortality agents were used consecutively in an attempt to eradicate the rabbits: myxomatosis, rabbit haemorrhagic disease (RHD) and a cereal-based bait (Talon® 20P) containing the poison brodifacoum. (Brodifacoum is an extremely potent anticoagulant, with near 100% kill rates for rabbits after only a single dose). To increase the effectiveness of the myxoma virus, European and Spanish rabbit fleas (Spilopsyllus cuniculi and Xenopsylla cunicularis respectively) were introduced beforehand to act as disease vectors.
Eradication programme: In May 1997, the programme to eradicate the population of an estimated 250 rabbits commenced, using the two mentioned epizootics, followed by aerial baiting. Seventy rabbits were captured, tagged, fitted with radio-transmitters (so locations of any survivors could be determined later by radio-tracking)and released. Live-trapping was carried out periodically in order to monitor the progression of myxomatosis and RHD. A monitoring period of six months after the application of the three mortality agents was undertaken, consisting of monitoring at stations located throughout the island and around warrens (using apples Malus domestica secured in a pipe to detect rabbit presence).
Application of mortality agent 1 - myxomatosis: Myxoma virus was introduced into the rabbit population in May 1997. Individuals that died from the virus were left in situ in the forest to increase breeding opportunities for blowflies, which could become vectors for RHD.
Application of mortality agent 2 - RHD: The haemorrhagic virus was introduced into the population in August 1997, soon after myxomatosis had diminished. Seven radio-tracked individuals (five males, two females) were inoculated intramuscularly (0.5 ml per individual) in the hind leg with live virus. Thirty-three of the radio-tracked rabbits were vaccinated against RHD to ensure some survived, thus allowing an assessment of poisoning effectiveness later in the programme. After post-mortems, diseased carcasses were discarded across the island to aid the spread of disease.
Application of mortality agent 3 - poisoned bait: Cereal pellets (20 mm long x 10 mm diameter) were dyed bright green to decrease attractiveness to birds, and brodifacoum incorporated into them at a concentration of 20 mg/kg. In September 1997, after signs of RHD had diminished, 300 kg of bait was dispersed over Cabbage Tree Island by helicopter.
Mortality agent 1 - myxomatosis: The myxomatosis virus persisted in the population throughout May and June 1997. Sightings of sick rabbits decreased into July, with the last recorded death occurring on 15 July. The results from trapping of tagged individuals suggested that the population had been reduced to approximately 100 individuals.
Mortality agent 2 - RHD: The seven rabbits inoculated with live RHD virus survived only 2–6 days. The first non-inoculated individual died four days after introduction of the virus, with all further deaths from RHD occurring before day 24 (see Figure 1, attached). Of the 70 radio-tracked rabbits, 29 were susceptible (i.e. non-vaccinated and non-inoculated), of which 16 (55%) died (excluding one rabbit taken by a raptor).
Mortality agent 3 - poisoned bait: The remaining known population consisted of 45 rabbits - 13 survived RHD and 32 of the 33 individuals were vaccinated against the second mortality agent (plus one taken by a raptor). Of the 45 rabbits, 42 are known to have been killed by the bait over a period of fifteen days (see Figure 2, attached). The remaining three deaths could not conclusively be attributed to poisoning. In spite of this, mortality was 100% following the third stage of the eradication programme.
Subsequent monitoring: Over the six month monitoring phase, beginning in late September 1997, only one rabbit carcass (a male) was found, this individual having died from brodifacoum poisoning. The island was then declared rabbit-free.
Conclusions: Since eradication of rabbits from the island in 1997, the number of breeding pairs, fledgling production and breeding success of Gould's petrel has continued to increase.
In 2000, there were estimated to be approximately 900 breeding pairs on the island, a great increase over the preceding decade thought to be attributable to the rabbit eradication. As a result of a significant increase in density of herbs and grasses, especially in the previously rabbit-disturbed sites, petrels are no longer so prone to currawong predation and the survival of the Gould's petrel population is no longer dependent on intervention management.
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