Peter your kinda making me crazy I did not know, that this was not understood. Its taught in basic biology. Its population dynamics.
It kinda makes my head hurt to think so far back for the right terms. So I will try most likely using the wrong terms, but hopefully you will get the idea.
First most here think in terms on one or two. That is, right or wrong, left or right up or down. This is reason why man has a hard time saving nature.
Nature is complicated. Nature is a balance that never stays balanced. Its like ripple or waves in the water. Plants insects and animals populate, another species benefit and populate(recruit), and on and on. Its topped off by the apex predator. You using humans is totally off as we are apex predators at this time. We are waiting for the big desease thing to readjust our populations.
Again the base energy source is consumed by a layer of life, and so on and so forth. As each consumes its food source, it not only allows it to recruit, but impacts the food source. In essense, no layer stays the same, they cannot, because their predators populate as much as they can to consume its energy source. So like humans, they all overpopulate. The food source is impacted and then there will be a die off of its predators and on up the chain.
With this in mind, all populations are in some stage of bloom then bust. While in the bloom stage, they go through range expansion.
NOW, all of this is dependant on enviornmental conditions. Droughts, fires, floods(katrina) sort of wipe the slate clean, so they are something that are easy understand. A real extreme. These areas get depleted of a normal balance. What comes back first is, plants and insects, then their predators. The ripple effect is in action. While one layer preys on the other, it also does not have a normal predator load. So that layer expands. It expands until its predator load is great enough to stop it. Or it uses up its resources, then in both cases the population adjusts. Its rarely ever constant.
I ask you, do you get warm winters, cold winters, dry winters, wet winters? How about all that with spring and all that with summer, and all that with fall? Weather variations have a direct effect of plant and insect loads. Which have a direct effect on each layer of predator, the ripple. Up to and including the apex predator.
Apex predators are effected by lack of resources and desease. Not by another predator.
The above is a very general idea of whats going on.
I mentioned nature being complicate. You know those ripples, they not only spread in a outward direction, but they also ripple up and down, and in. Which means many many things are effected. Kinda why man cannot get it right. But alas, nature is constantly re-adjusting, It has always re-balanced. But then something happens. As an example, many life forms are based on parasites, which are in snakes.
If you take snakes, its very common for them to overpopulate, They have a reliable energy source. So all you have to do is take away predator loads. Such as farms and feral fields. Or the spaces between agricultural fields. They often house huge populations of such snakes as kingsnakes and of course pits. The fields are grain, which equals mice, they are watered regularly, all equal dense snake populations. Then they plow that dang fields which causes population re-adjustment. hahahahaha Which leaves the remaining individuals with excess prey. So bloom they go.
All and all, I get the feeling you need to find some books on population dynamics. They explain this well. And yes, a population can stay balanced for very short periods. But its not likely to stay balanced over any extented period. Another example, such things as mice, voles, rats, have distint population cycles, which end or start with a bloom(dynamic overpopulation)
Now, if you can understand that. Then you may want to entertain this. Snake populations, at least natural ones, have an age factor. In the populations I have studied, colubrid populations last about 5 to 8 years in a given area. While one population ages, others start and mature. This occurs within populations. Sorta like family groups. They come and go. Which directly effects the movement of the whole population.
Then if you consider a ever larger picture. Hmmmmmmmmmmm this is long enough but please look up what ancestoral ranges mean. In general it was the area a species use to inhabit. In many cases, their ranges are shrinking, in other cases they are expanding. This has to mean the population is growing or shrinking. Not constant. Sorry for being so long but I still cannot believe your not aware of this. With any or all of this in mind, its very easy to say 90% die, but its really not accurate at a given place and a given time. Cheers