In many areas, wildlife populations have increased substantially in their local density because of a loss of natural controls or some artificially supplemented resource. These populations are often managed to avoid harmful effects on other wildlife species and human-wildlife conflicts. Many species are managed using lethal population reduction, but in those that are resistant to these means or where the method is unpalatable due to public concern, fertility control is becoming increasingly common. This method seeks to reduce the population size of some target problem species by capturing, sterilizing, and releasing individuals back into their habitat. Fertility control is often paired with vaccination programs because each has synergistic effects. Sterilization reduces the population size, making it easier to achieve a higher vaccination proportions for herd immunity. However, these programs have uncertain effects on both the basic biology, population demographics, and disease epidemiology. The current literature makes strongly countered species-specific conclusions. It is also unclear if fertility control is an effective method at reducing the population size in an economically viable way, compared to lethal removal.
Here I use computer simulations, cross sectional surveys, and long-term monitoring of two populations, the street dogs (Canis lupus familiaris) of Rajasthan, India, and the raccoons (Procyon lotor) of the Columbus Zoo and Aquarium, to investigate what impact fertility control makes on the populations it targets. In Chapter 2, I exposed replicate simulated populations to various control schemes to see which most lowered the population size and increased vaccination coverage. In Chapter 3, I report the results of surveys of dogs from several real world Indian cities with varied histories of fertility control for several diseases. In Chapters 4 and 5, I report the results of a randomized control study on raccoons, which measured differences in parasite load and survival among control, vaccinated and vaccinated/sterilized individuals.
My work demonstrates that fertility control programs can be more effective than lethal control, although the methods used to locate sexually intact individuals for treatment can significantly affect the results. In Chapter 3, I found that intact dogs living in cities with more fertility control had significantly lower prevalence of several diseases compared to those dogs living in cities with less fertility control. This is especially significant because the interventions only vaccinated against rabies, meaning that the fertility control affected local disease epidemiology. This indicates that the sterilization program buffered treated individuals’ ability to resist or spread disease enough to lower exposure to non-treated individuals. I found that sterilization and vaccination in raccoons did not affect the apparent monthly survival rates, but lowered parasite prevalence in males. However, female raccoon parasite prevalence was negatively affected by sterilization. I suggest that the sterilization method used does not eliminate hormone production, causing females to increase the length or intensity of their reproductive seasons. As a whole, this work highlights the importance of understanding the secondary effects of intervention policies. I show that altering reproductive behavior can cause dramatic changes to population dynamics and epidemiology.