Summary 0[ Rabbit calicivirus disease "RCD^also known as rabbit haemorrhagic disease# has been introduced recently as a biocontrol agent for rabbits in Australia[ The consequences for fox populations that use rabbits as primary prey\ for populations of alternative native prey\ and for pastures\ were examined using a model for rabbit! and fox!prone areas of semi!arid southern Australia[ 1[ Existing data were used to quantify the interactions of foxes\ rabbits and pasture[ A generic model for predation on native herbivores was constructed by modifying the density!dependent "Type III# functional response of foxes to rabbits to a depen! satory "Type II# response that is appropriate for alternative prey[ Similar dependence on pasture biomass was assumed for the dynamics of both rabbits and alternative prey in order to identify clearly the consequences of di}ering predation[ In the absence of quantitative data for Australian conditions\ the epidemiology of RCD was simulated empirically to mimic a range of potential patterns of occurrence[ 2[ For semi!arid Australia the model predicts that as the frequency and intensity of RCD epizootics increases] "i# the mean abundance of rabbits will decline\ as will the frequency of eruptions of rabbits^"ii# there may be little increase in mean pasture biomass and a small decrease in periods of very low pasture biomass when competition between herbivores is most intense^"iii# the mean abundance of foxes will decline^"iv# there will be a reduced frequency of occasions when rabbit density is low but fox density is high due to a lag in the response of predator populations^and "v# there is potential for an increase in the mean abundance of alternative prey and in the proportion of time their density exceeds a threshold comparable to that currently required for eruptions of rabbits[

Summary 1.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. 2. 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. 3. The sterilized populations produced fewer young but the average adult population size remained unchanged in all treatments. Immigration was minimal in all treatments. 4. 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. 5. Fleas were fewer on both adult females and males in the sterilized populations but this did not impede transmission of myxomatosis. 6. Synthesis and applications . Imposing sterility on rabbit populations reduces breedingseason 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.

A model based on data from research in New South Wales conducted by the Cooperative Research Centre for the Biological Control of Vertebrate Pest Populations suggests that the effectiveness of fertility control in reducing the abundance of foxes (Vulpes vulpes) can be strongly influenced by environmental variability. The model includes age-specific recruitment and survival as functions of resources indexed by rainfall. It is assumed that fertility control will affect only female foxes and that the use of a baiting regime to deliver a contraceptive agent will result in fixed proportional changes in pregnancy rates. By comparing the variability in the rate of increase of treated and untreated fox populations, the model predicts that: (i) frequent baiting, every one or two years, will be more effective than applications of baits at longer time intervals; (ii) the abundance of foxes will decline more rapidly, with higher levels of fertility control; (iii) infertility which lasts for only one breeding season is less effective than permanent sterility which allows for accumulation of sterile animals in the population; and (iv) highly variable results are likely to be the outcome of low-frequency baiting with an agent that produces only temporary infertility.

Summary 1.Research is currently underway to develop genetically engineered viruses that can sterilize pest animals. The technique, known as viral-vectored immunocontraception (VVIC), promises to control mammalian pests such as the European rabbit, the house mouse and domestic cats. 2. Using host±parasite models we explored the degree of control of the host population that can be attained when hosts that recover from infection become permanently infertile. The models assume some demographic compensation for reduced fertility in the host population, and are tailored to address issues raised by the use of the myxoma virus as an agent to sterilize rabbits. A`pay-o' function is developed, which de®nes the degree to which host density is suppressed by a sterilizing agent. 3. The results show that sterilizing viruses can reduce host abundance, and that hosts with low birth rates and moderate mortality rates are the best targets for VVIC. High transmissibility increases the pay-o from VVIC, but because virulent parasites kill most of the hosts that they infect, the pay-o is highest if benign parasites are used as the vector of contraceptive antigens. We argue that appropriate pay-o functions should be developed as a basis for research and development on genetically modi®ed organisms. 4. The host±parasite models are extended to include a competing strain of virus that does not sterilize the host. We analysed these models using a general approach to the analysis of competition, which has not often been applied to epidemiological models. The extended model shows that host sterilization per se does not aect the competitive ability of viruses, a result that applies to a broad class of models in which the per capita growth rates of competing parasites are linear functions of limiting competitive factors.