Modelling the relative efficacy of culling and sterilisation for controlling populations

Simple logistic models are used to investigate and compare the effects of continuous control by culling or sterilisation on population density. The models consider populations regulated by density-dependent mortality or density-dependent recruitment, with monogamous or polygamous mating systems, and with one or both sexes sterilised. For the first time, an analytical solution is presented for the effect of sterilisation on density-dependent population growth. The models suggest that the mating system has a considerable effect on the relative efficacy of sterilisation, and that previous models for sterilisation may have overestimated the impact of sterilisation by assuming idealised monogamous mating. In general, and except for populations with monogamous mating or 'harem' systems and both sexes sterilised, culling gives a more rapid reduction in density than does sterilisation. However, the long-term degree of suppression obtained with the same levels of control applied is likely to be similar. Populations with density-dependent mortality will be reduced by sterilisation more quickly than those regulated by density-dependent recruitment, and the effect on the steady-state density of a given proportion that is sterile is greater in the first case than in the second. However, the effect of a given continuous sterilisation rate (e.g. baiting effort) on density is the same in both cases. We compare our model results with those of others and question previous conclusions about the circumstances under which sterilisation is most effective.