Quark_666
political cynic
Because of evolution. It happens all the time. A spiked population leads to crisis but not extinction. The power behind evolution is that populations receive their greatest handicaps when the spiked population has caused enough of a genetic variety that there is bound to be a portion of survivors...immune survivors. For example, it is possible for us to chemically wipe out more then 99% of an insect population in a given area, but as the poor farmers in the 50s and 60s discovered, there are survivors, they are immune, and their children are immune.
What are the threats? Sickness? Even if we had a series of plagues amounting to double the effect of the bubonic plague (assuming biotechnological engineers couldn't reduce the mortality rate), we'd still have a population of 4 billion people, with a wide range of the gene pool left. Nuclear warfare could wipe out major cities, and only small areas might be left. But even then, there would be those who adapted to the radioactive poisoning.
If the early stages of life can adapt to and benefit from electromagnetic radiation and survive the resulting presence of oxygen in vapor form (which was one of the most biologically toxic events suspected in the history of life), evolved life can survive particle radiation.
Besides, the threat of nuclear war is decreasing overtime, and has been since 1990. Yeah, North Korea, I know. But the truth is, they are trying to develop weapons with a geographic location as tough as the German location. Germany, as you know, simply could not do quality nuclear development in WW2 when they were under pressure from their rather defensive war opponents...and they have been masters of quality engineering for a century now. North Korea doesn't have much of a chance, trying to keep secrets from sniffer planes, satalytes, the entire UN, etc. Don't get me wrong...they are a threat. But I would give the human population much chance of going extinct any time soon.