Aloha Johanna!

You are mistaken.  What you are thinking about are very short term reductions in colony size, but they fail to take into account the inevitable population rebound that actually increases the size of colonies.  Plenty of studies show long term reductions using TNR, and I have used TNR - it does work, and I have seen it used all around the world and it works.  What doesnt work is mass trapping and killing.  Makes people feel better to kill, but it increases the population in just a few months.