Effects of surgically imposed sterility on free-ranging rabbit populations

1. Demographic changes in response to surgically imposed female sterility w ere monitored in 12 free-ranging rabbit populations in south-western Austra lia over a 4-year period. This was part of a research programme aimed at ex amining the potential for virally vectored immunocontraception to limit the abundance of rabbits (e.g. using a recombinant myxoma virus) and other mam malian pests. Sterility levels were 0%, 40%, 60% and 80% of all females in year 1, with a similar proportion of female recruits sterilized surgically in subsequent years. 2. There was a significant decrease in rabbit productivity with increasing sterility level. This was overcome by increased survival of kittens and adu lts on the high-sterility sites, such that the base-level numbers of rabbit s were maintained, and mean annual rates of increase (r) were near zero for all treatments in all years. However, in the high-sterility populations th is compensation was insufficient to overcome the effects of sterility total ly, and there was a marked decrease in the seasonal peaks in rabbit abundan ce for these treatments. 3. Survival and recruitment were dependent upon the level of sterility, and consequently the density of rabbits, with greatest survival of adult rabbi ts occurring on the 80% sites. Survival of sterile females was greater than that of other adults, probably because of their increased ability to maint ain body condition during times of low pasture biomass (summer drought). Th us two density-dependent processes were identified: the first was operating through increased survival of juvenile rabbits, the second through increas ed adult survival, particularly sterilized females. 4. Because the proportional impact of immigration was greater (i.e. immigra nts constituted a greater proportion of the population) and emigration was less, from the 80% sites, the effects of sterility may have been underestim ated on these sites. 5. The abundance of European rabbit fleas, a vector of myxomatosis, was sig nificantly lower on the 80% sites, but this did not appear to affect the tr ansmission of myxoma. Myxomatosis occurred as an annual epizootic in three of four years, with > 90% of rabbits on site after each epizootic testing p ositive for myxoma antibodies. 6. To achieve a sustained long-term reduction in rabbit abundance, 60-80% o f female rabbits would need to be prevented from breeding. This could be ac hieved by a recombinant strain of myxoma provided the strain retained good transmissibility and all infected rabbits became sterile for life.