The vacuum effect occurs when community cats are trapped and taken out of the territory they reside in and protect, leaving a void. This misguided way of thinking is called trap and kill. These cats are killed simply because they are deemed unadoptable and not socialized to humans. Through this practice, the number of community cats will decrease temporarily.
This is known as the vacuum effect and is well documented. During trappings, there will always be trap savvy, unaltered community cats that will hide out until the traps and other cats have been removed.
Leaving one unaltered cat behind will start the cycle again as a male will seek out a female in heat, as will a female seek out a male. An unspayed female cat's function is to reproduce. More and more cats will also move into the area where the others were removed.
The only real and humane way to deal with the community cats issue is through TNVR (trap-neuter-vaccinate-return). It really does work in humanely reducing the number of community cats.