I made a lovely little model in Excel, letting me play with all kinds of variables. The most amazing thing is, no matter what I put in for those variables, there’s a tipping point, where just a few more spay/neuter surgeries per year will bring the population under control, and you only have to go back to maintenance level of s/n, while with less, it’s runaway growth.
Of course, the real world isn’t so precise. And there are a lot of factors that will put the brakes on the population explosion. I put a couple in: that population above a certain level increases infertility and kitten mortality and even higher, 1/4th of the population will die of starvation or disease. But even that wasn’t enough to slow things down much.
This model doesn’t make a distinction between owned and unowned cats. In the real world, owned cats are abandoned and unowned kittens are tamed and adopted. Besides, what difference does it really make in the population dynamics? A higher proportion of ferals would mean a lower life span and higher kitten mortality, and probably a lower initial % fixed at the start. All those are variables in this model.
It’s interesting that if shelter deaths are factored in, the tipping point goes down by only about half of shelter death rate.
The biggest factors affecting how many surgeries need to be done to reach the tipping point are the kitten mortality rate and number of litters per year. These values are probably very local, especially for the feral population, depending on weather and the activity of animal rescuers.
Note, this model assumes that there’s 10% of the unfixed population that will never be fixed. They could be ferals that can’t be trapped or cats whose people just refuse. And if a miracle happens (or a programming error, but that’s less likely) and they all get fixed, 10 pregnant cats will materialize out of thin air. (If you’ve ever tried to TNR a whole colony, you know it happens.)
I’ll be blogging more about this model in the future and I’ll even give you an equation to calculate the tipping point.