Effects of surgically imposed sterility on free‐ranging rabbit populations

Abstract

Summary1. Demographic changes in response to surgically imposed female sterility were monitored in 12 free-ranging rabbit populations in south-western Australia over a 4-year period. This was part of a research programme aimed at examining the potential for virally vectored immunocontraception to limit the abundance of rabbits (e.g. using a recombinant myxoma virus) and other mammalian 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 signi®cant decrease in rabbit productivity with increasing sterility level. This was overcome by increased survival of kittens and adults on the highsterility sites, such that the base-level numbers of rabbits were maintained, and mean annual rates of increase (r) were near zero for all treatments in all years. However, in the high-sterility populations this compensation was insucient to overcome the eects of sterility totally, and there was a marked decrease in the seasonal peaks in rabbit abundance 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 rabbits occurring on the 80% sites. Survival of sterile females was greater than that of other adults, probably because of their increased ability to maintain body condition during times of low pasture biomass (summer drought). Thus two density-dependent processes were identi®ed: the ®rst was operating through increased survival of juvenile rabbits, the second through increased adult survival, particularly sterilized females. 4. Because the proportional impact of immigration was greater (i.e. immigrants constituted a greater proportion of the population) and emigration was less, from the 80% sites, the eects of sterility may have been underestimated on these sites. 5. The abundance of European rabbit¯eas, a vector of myxomatosis, was signi®-cantly lower on the 80% sites, but this did not appear to aect the transmission of myxoma. Myxomatosis occurred as an annual epizootic in three of four years, with > 90% of rabbits on site after each epizootic testing positive for myxoma antibodies. 6. To achieve a sustained long-term reduction in rabbit abundance, 60±80% of female rabbits would need to be prevented from breeding. This could be achieved by a recombinant strain of myxoma provided the strain retained good transmissibility and all infected rabbits became sterile for life.

Fertility control to mitigate human–wildlife conflicts: a review

“…For example, sterilised feral Soay rams showed increased food consumption and survival compared with control rams, ultimately leading to increased animal numbers and impact on the plant community (Jewell 1986). PZP-based immunocontraceptives increased lifespan and body conditions of mares Kirkpatrick and Turner 2007), tubal ligation increased survival in rabbits (Twigg et al 2000;Williams et al 2007) and GonaCon improved body condition of deer (Gionfriddo et al 2011b). Conversely, Saunders et al (2002) observed no differences in survival, dispersal or territory size of surgically sterilised foxes compared with fertile foxes, although sterilised vixens were more likely than fertile females to share their territories with each other.…”

“…When the size of a population is suddenly reduced, compensatory density-dependent processes may act to return the population to its previous level (Bomford 1990;Barlow 1994;Twigg et al 2000;Sinclair 2003;Ramsey 2005). In short-lived species, fecundity can make a greater proportional contribution than survival to population growth, and the reverse occurs in long-lived species (Sibly and Hone 2002).…”

“…These conclusions have been challenged by models showing that large, Fertility control for human-wildlife conflicts Wildlife Researchlong-lived species might be easier to manage with fertility control than are smaller, shorter-lived ones because a lower proportion of the population would need to be treated (Hone 1999), particularly if lifelong contraceptives were employed (Hobbs et al 2000; Table 2). However, in long-lived species, the benefits of using fertility control to decrease population size are accrued in the long term (Twigg et al 2000;Cowan and Massei 2008;Kirkpatrick and Turner 2008). Others have suggested that contraception is likely to be better than culling for controlling species with medium to high instantaneous rates of population increase, but equivalent to culling for species with low instantaneous rates of population increase (Zhang 2000).…”

“…Compensatory population changes that may occur in response to fertility control are likely to be less pronounced than those following population reduction by lethal methods, depending on whether populations are regulated by density-dependent mortality or recruitment (Johnson and Tait 1983;Bomford 1990;Bomford and O'Brien 1997). For instance, in populations of mice and rabbits, a compensatory response in female productivity did not offset the effects of sterilisation when 60-80% of the females were made infertile (Chambers et al 1999;Twigg and Williams 1999;Twigg et al 2000) (Table 2).…”

Introduction - virally vectored immunocontraception in Australia

“…In the first, molecular, immunological and reproductive biology studies commenced immediately to isolate and characterise rabbit reproductive antigens, initially sperm surface antigens, as candidates for gene cloning. Concurrently, studies were conducted in the field by surgically ligating the oviducts so that the rabbit's endocrine condition was not altered (Twigg et al 2000;Williams et al 2007). Further work continued to define…”