Responses of wild rabbit populations to imposed sterility

The efficacy of reducing fertility in wild rabbit populations as a means of reducing rabbit abundance is being examined in two similar experiments in south-eastern and south-western Australia, each on 12 discrete populations. In the first year, 0%, 40%, 60% or 80% of all females in the respective populations were sterilised randomly by surgical ligation of fallopian tubes. In the second year the female recruits were sterilised at the same levels. Results from the first year of sterility treatments indicate that density-dependent responses in survival of juveniles sustained rabbit abundance and compensated for reduced production of young, and environmental factors constrained population abundance. Young were produced in direct proportion to the level of fertility in the populations, indicating that Fertile females did not respond to the sterility of other Females or the presence of reduced numbers of young, the rate of production of young per fertile Female was similar among all sterility treatments. The populations with higher levels of sterility recruited higher proportions of the Fewer young produced. A reduction in mortality of juveniles compensated for the reduced production of young caused by sterilisation. Al the end of the First breeding season, the populations with higher levels of sterility contained Fewer rabbits, but abundances became similar among treatment levels after the period of summer mortality of young. There was a near-significant trend For reduced numbers of young recruited into the 80% sterility treatment, the reduced mortality of young in these populations did not compensate fully for the imposed sterility. The survival, loss and immigration of adult rabbits were unaffected by the level of sterility in the treated populations. Within populations, the sterilised Females seemed to survive longer than the fertile females in the First year. However, any responses in survival and abundance of adult rabbits to sterility are likely to be more protracted than for kittens and are yet to be documented. The experiment needs to proceed For another two years before ii will be possible to assess fertility control as a means of reducing abundance of wild rabbits.