Population responses to sterility imposed on female European rabbits

Additional methods are needed in Australia to control the European rabbit Oryctolagus cuniculus, which continues to destroy valued native flora. A control option under development, immunocontraception, is intended to suppress the rabbit's high fertility. It would spread contagiously via genetically modified myxoma virus and European rabbit fleas Spilopsyllus cuniculi. An experiment with field populations of rabbits assessed whether suppressing fertility reduces their abundance. In south-eastern Australia, four treatments in three replicates were applied to 12 subpopulations of rabbits. The treatments were surgical sterilization of 0%, 40%, 60% and 80% of the adult and juvenile females trapped before the annual breeding seasons of 1993-96. The sterilized populations produced fewer young but the average adult population size remained unchanged in all treatments. Immigration was minimal in all treatments. Sterilized adult female rabbits survived much better than fertile females, indicating a high cost of reproduction. Immature rabbits and unsterilized adults of both sexes also survived better in the sterilization treatments. The improved survival in all rabbit classes compensated for reduced reproductive output. Fleas were fewer on both adult females and males in the sterilized populations but this did not impede transmission of myxomatosis. Synthesis and applications. Imposing sterility on rabbit populations reduces breeding-season peaks of abundance. Improved survival compensates for the sterility of up to 80% of females and sustains populations, even in the presence of drought and myxomatosis. Immunocontraception alone has poor prospects for controlling rabbits. Cost-effective rabbit control requires multiple, integrated forms of attrition.