Crash of fish populations?

I have read several times now that fish populations can crash but I don't understand the concept. I understand that the population becomes too big at a certain moment, but then what causes the crash? Lack of food, high levels of ammonia, or a suicide mechanism like with the lemmings that throw themselves over a cliff and drown themselves?

Hmmm, a fish that drowns itself.... Don't see how that happens...
Anybody can enlighten me? And does it only happen with livebearers or also with other fish?

I believe that it comes down to sex ratio, if the last few surviving members of a species have fry of only one sex, there are no members to keep the line going.

Population crashes as I've experienced them take more than one form.
The worst is when you come to the tank in the morning, and the entire tank is dead. Why? There are no symptoms, no marks or lesions on the fish, but no living fish. It's an extreme I've mostly seen with softwater fish - Apistogramma a couple of times, and with Nanochromis. The dead fish are in full colour, and dead. It's also a softwater Micropoecilia problem - picta, parae and branneri. The common thread is large numbers in what the population explosion has turned into a small tank. Is it oxygen? Ammonia? Bacteria? Good question. I suspect it's respiratory problems from chronic ammonia colliding with low oxygen levels.
Form two is the apparent and obvious bacterial infection. It rips through a tank but with symptoms - red abdomens and gills in Corydoras, open mouthed corpses in cichlids, etc. Again, it is a breeder problem, when you have allowed things to get to the point where water changes and filters can't keep up with wastes, and the life support system you run for the fish crashes under stress.
You can have the slow outbreak from overpopulation - Ich, velvet, fungus - the stress of low water quality kills the fish over a week or two, usually in a bacterial big bang at the end.
Fry tank crashes are often velvet (oodinium sp) parasites, in my opinion, as they flourish with uneaten artemia and can't be seen on something as small as a fry. They tend to hit velvet susceptible species.

I refer to it as a crash as while you can see it coming logically, it happens very quickly and very thoroughly. You don't learn it by being smart - it's the opposite.

Right now, with my Xiphophorus montezumae, I am thinning the herd. A fish like that can easily reach 500 individuals in a 40 gallon, if you permit that to happen. Through predation, every montie under 2 cm is now gone, and I am going to have to cull some damaged adults. I'd like to get the tank down to 25-30 individuals - the Heterandria ate around 300 fry or juvies over the past 3 months. I hate to see it, but it is the only mechanism I know of. Stores want adults, and I would need 500 gallons to raise that many in good shape and colour.

On one level, you would expect an overcrowded tank to lose a few fish daily until a reasonable number is attained, but my experience is that fish deaths happen in waves, and an overcrowded tank that looks to be filled with beautiful, healthy fish in the morning can be a mess when you get home from work.

Again, if you buy that many fish, you deserve to lose your tank. But it is more a problem for people who breed prolific fish.

It is also something I have seen working with wholesalers.

By the way, the lemmings in that famous film were being chased, and the famous film of them leaping is now known to have been a fraud.
http://www.snopes.com/disney/films/lemmings.asp

Thank you! My question was mostly about the non-sickness related crashes as you described with the livebearers. So, basically you have not noticed that these fish have a mechanism for 'regulating' their own population.

regarding the lemmings: I never saw the movie, I learned it in school way back.... So, you are saying that that mechanism doesn't exist?

Apparently, that mechanism doesn't exist. I was convinced of it too.

The crashes have to be sickness related, but they are catastrophic results of overpopulation.