The result of not sterilizing: Dr. Brenda Griffin of Cornell University estimates: "If one unspayed female cat produces two litters per year, and two kittens per litter survive to reproduce, and none of these cats is ever spayed or neutered, the total population multiplies in five years, or ten generations, to 59,049"
(Statistics from Helping Hands Humane Society's webpage, showing significant reduction in the numbers of incoming cats and kittens into shelter since the inception of TCCF in 2014)