Can foxes be controlled by reducing their fertility?

A model based on data from research in New South Wales conducted by the Cooperative Research Centre for the Biological Control of Vertebrate Pest Populations suggests that the effectiveness of fertility control in reducing the abundance of foxes (Vulpes vulpes) can be strongly influenced by environmental variability. The model includes age-specific recruitment and survival as functions of resources indexed by rainfall. It is assumed that fertility control will affect only female foxes and that the use of a baiting regime to deliver a contraceptive agent will result in fixed proportional changes in pregnancy rates. By comparing the variability in the rate of increase of treated and untreated fox populations, the model predicts that: (i) frequent baiting, every one or two years, will be more effective than applications of baits at longer time intervals; (ii) the abundance of foxes will decline more rapidly, with higher levels of fertility control; (iii) infertility which lasts for only one breeding season is less effective than permanent sterility which allows for accumulation of sterile animals in the population; and (iv) highly variable results are likely to be the outcome of low-frequency baiting with an agent that produces only temporary infertility.