Population dynamics of brushtail possums subject to fertility control.
Reducing the fertility of pest species using immunocontraceptive vaccines holds promise for the humane and effective control of vertebrate pest populations. However, despite much research into the development of contraceptive vaccines, there are few data illustrating the effectiveness of fertility control for managing wild populations of vertebrate pests. In New Zealand, introduced brushtail possums Trichosurus vulpecula have been the focus for the development of contraceptive vaccines to provide a possible alternative to lethal control techniques. The effect of fertility control on the population dynamics of brushtail possums was investigated experimentally using surgical sterilization to suppress breeding on six 12-ha study sites between January 1996 and October 2000. Sterilization treatments, consisting of 0%, 50% and 80% of resident females sterilized, were applied at a particular site, with adjustments made annually on recruits to maintain levels of sterility. Mark-recapture trapping was used to estimate age-specific demographic rates and their relative contribution to age-specific population growth rates (λ). Sterility treatments resulted in reductions to per capita rates of local recruitment (surviving young per adult female) of 60% and 74% on average for 50% and 80% levels of sterility, respectively. However, immigration by yearling-aged possums compensated for reductions in local recruitment because of sterility treatments, leading to the stable (λ=1) population trajectories on sterility treatment sites. There was some evidence that sterilized females had higher annual survival rates than fertile females, but otherwise there was no evidence of any other compensatory responses to imposed sterility. However, additional studies are needed to clarify possible density-dependent responses that could compensate for the effects of fertility control. Synthesis and applications. Demographic compensation by pest populations can negate the effects of fertility control. Results from this study suggest that immigration can easily compensate for fertility control in pest species where adult survival is high and fecundity is low. This suggests that fertility control will need to be applied at large spatial scales to reduce this effect. Where adult survival makes a relatively high contribution to the population growth rate, the benefits of fertility control would be enhanced if integrated with conventional control to provide rapid initial population reduction.