FOXES, RABBITS, ALTERNATIVE PREY AND RABBIT CALICIVIRUS DISEASE - CONSEQUENCES OF A NEW BIOLOGICAL-CONTROL AGENT FOR AN OUTBREAKING SPECIESIN AUSTRALIA

1. Rabbit calicivirus disease (RCD; also known as rabbit haemorrhagic disease) has been introduced recently as a biocontrol agent for rabbit s in Australia. The consequences for fox populations that use rabbits as primary prey, for populations of alternative native prey, and for p astures, were examined using a model for rabbit- and fox-prone areas o f semi-arid southern Australia. 2. Existing data were used to quantify the interactions of foxes, rabbits and pasture. A generic model for p redation on native herbivores was constructed by modifying the density -dependent (Type III) functional response of foxes to rabbits to a dep ensatory (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 con sequences of differing predation. In the absence of quantitative data for Australian conditions, the epidemiology of RCD was simulated empir ically to mimic a range of potential patterns of occurrence. 3. For se mi-arid Australia the model predicts that as the frequency and intensi ty of RCD epizootics increases: (i) the mean abundance of rabbits will decline, as will the frequency of eruptions of rabbits; (ii) there ma y be little increase in mean pasture biomass and a small decrease in p eriods 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 ab undance of alternative prey and in the proportion of time their densit y exceeds a threshold comparable to that currently required for erupti ons of rabbits.