Wildlife Contraception, Individuals, and Populations: How Much Fertility Control is Enough?

Abstract

Author(s): Cowan, David P.; Massei, Giovanna | Abstract: The resolution of conflicts between human and wildlife interests often involves lethal control to reduce problem wildlife populations. However, lethal control has always had its limitations, the acceptable methods are becoming fewer, and public opposition is on the increase. Fertility control offers a potential alternative approach that is widely regarded as being inherently more benign. Furthermore, in some circumstances fertility control may have specific advantages over culling. The development of “single-shot” injectable immunocontraceptive vaccines that inhibit the fertility of individual animals for several years is leading to practical applications that exploit this novel technology. Further advances can be expected to lead to the emergence of a new generation of wildlife management tools. A key issue in this process is predicting what the population consequences will be for a particular species, given a specific level of induced infertility. Here, we use population modelling techniques to explore how much fertility control is enough to achieve different levels of population reduction, how long it will take to realise these reductions, and to understand how these effects are shaped by the population biology of the target species. This offers some generic conclusions, with low levels of infertility having little impact on species with high population turnover rates, while modest levels of infertility may yield useful population reductions for species with low intrinsic rates of increase, although such effects take longer to be realised in long-lived species. We also observed that there is potential for optimising the intensity of induced infertility, in terms of the proportion of breeding animals rendered infertile, and the frequency of application; so, for instance, biennial application could be more efficient than annual application for some forms of fertility control in certain species. There is increasing evidence from field studies that the survival of infertile animals is enhanced, probably because they do not incur the costs of reproduction. Our model predicts that this effect will be of limited importance for short-lived species with high intrinsic rates of increase, but it is more likely to compromise population reduction in long-lived species. We suggest that the generic modelling approach can help develop an evidence-based platform for discussing when fertility control can be regarded as a feasible, desirable, and sustainable option to manage problem wildlife.