Just to throw some stats into the equation, I foster for a feral cat charity in the UK, and we produce a leaflet about spaying. They did a table about how many kittens one female can produce, they based it on a low rate of only 2 litters a year (although they could have 3 or 4), only 2 male and 2 female kittens per litter and dont take the death rate into account. They worked out that just one female can be responsible for 10,924 kittens in 6 years (based on additional female cats giving birth, it would work out at 2731 females).
Plus the health risks due to fighting, FIV, FeLV, abscesses
Plus the health risks due to fighting, FIV, FeLV, abscesses