A dynamic simulation model of the dog population was created to aid in understanding the impact of spay/neuter and other potential methods of addressing companion animal overpopulation. The model was calibrated based on a survey of residents in the Capital Region of Upstate New York. Although the model was specifically set up to reflect parameters for dogs in one particular region, the results below are expected to be equally applicable to other region, and in some cases to cat populations as well.

The results of the simulation support the importance of spay/neuter programs. Even if spay/neuter programs only reach a small portion of the population, they can still have a powerful impact. Figure 1 compares the reduction in the population who does not

spay/neuter their animal with the resulting reduction in the euthanasia rate. The solid line indicates the simulation results while the dotted line is a straight line with identical start and end points for comparison purposes. As indicated, if the number of people who do not spay/neuter their animal is reduced by slightly less than 50 percent, this would be enough to reduce the euthanasia rate to zero.

The shape of the curve of actual data points shows evidence of what economists often call "diminishing returns". As the spay/neuter rate increases, the slope of the line becomes less steep, indicating there is less change in the euthanasia rate from the same amount of change in the spay/neuter rate. The further the curve deviates from a straight line, the greater the diminishing returns are.

It is important to note that this graph gives the "long-term" change in euthanasia rate from a permanent shift in the spay/neuter rate. However, just what "long-term" means here may be quite surprising to many. Figure 2 shows the time pattern for a spay/neuter program that results in a 50 percent reduction in euthanasia. As the chart indicates, it takes about 8 years to get half of the benefit of a one-time shift in the spay/neuter rate and as much as sixty years for the euthanasia rate to stabilize at a new lower rate. It should be noted that this long time scale is not due to the time it takes to implement the spay/neuter program. The results here assume that there is an instantaneous and permanent one-time shift in the spay/neuter rate. The long time scale shown can be interpreted as an "adjustment period". In other words, a spay/neuter program reduces populations not only by eliminating the direct offspring of the spayed/neutered animal, but the benefits also multiply over time by preventing the birth of future generations from those offspring. Eventually, the benefits diminish and the population stabilizes at a new equilibrium as indicated by the chart.

The surprisingly long adjustment period for a spay/neuter program can be thought of in both positive and negative terms. From a negative perspective, it is true that a spay/neuter program takes a long time to reach its full impact. On the other hand, this is due in part to the great power of spay/neuter programs. Furthermore, it also suggests that most attempts to evaluate the impact of spay/neuter probably greatly underestimate their impact. If a spay/neuter program’s impact is evaluated a year after it is implemented, it will have only had about 10 percent of its full effect. Even after two years, the program will have reached less than 20 percent of its full effect.

The study also found some interesting results regarding the effect of combining programs. For example, combining a spay/neuter program with an adoption campaign could result in the two programs somehow interfering with each other, or it could result in the two programs having synergies (i.e. enhancing each other’s effectiveness). This question can be addressed by using the economic concept of a production possibilities frontier (PPF). A PPF curve shows all the combinations of two inputs that can be used to achieve a certain level of output. Figure 3 shows a PPF curve with the two axes representing spay/neuter and adoption programs. The curve represents the amount needed for each program to reduce euthanasia by 50% over a 30-year horizon.

The solid curve represents the actual data, while a straight dotted line is shown for comparison purposes. The fact that the curve lies below the line indicates that the two programs show "synergies" when combined. However, the fact that the curve lies close to the line suggests that the synergies are not very large.

The results here have important implications for programs nationwide. Although specific numerical results will vary by region general conclusions, such as the level of magnitude of the time scale needed to reach full effectiveness, are quite likely applicable to most U.S. dog populations and possibly to cat populations as well.