Low-stakes question

Mike314

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Hopefully this is the right forum for this question. Apologies if it isn't.

So: imagine we have 500 vampires, and a human population of 8,000,000,000. Every week, each vampire snacks on a human. Whenever a human is bitten, they turn into a vampire.
Under these simple circumstances, it takes 24 weeks (or slightly under) for the entire human population to become vampires.

I can do that much on my own. What I'm struggling with is increasing the complexity of the calculation.
For example, we assume the humans instantly turn into vampires, but what if it takes a day or two?
Are the human birth/death rates negligible over the estimated period, or would they affect the final result?
How do we account for vampires taking different lengths of time between feedings? (For example, if they get peckish after a week, but they NEED to eat at least once a month.)
How do we map the changes in human behaviour? (500 vampires would have no problem feeding, but once half the human population has been converted, the surviving humans are going to be significantly less agreeable.)
Given the last two factors, would there even be a risk of vampiric starvation before the humans all die out?
These are the factors I just can't get my head around. I haven't done serious maths in nearly a decade.
(For the sake of the scenario, assume animal blood is not a viable source - it has to be humans. And yes, I'm aware that means vampiric extinction eventually, but I'm concentrating on the human aspect.)

For context: I'm a short-story writer, and I'm trying to write a bit on the struggles of having a large vampire population.
But for that, I'd like something a bit more complex than the simplest formula possible: "[starting_population] * 2 ^ x", getting the exact week of extinction. That's not narratively satisfying.
So I'm not after a perfect answer (which is likely impossible, given the fictional nature of the question), just a much more intelligent estimate, even one with a slight range to the time of extinction.
If you know how to account for the factors I've provided, or even come up with any of your own, I welcome any help!
And again, sorry that this question is a bit vague. As I said, I'm mainly after an answer with some mathematical depth.
 
I recommend that you look at a "rabbit and fox population model" (search on the internet). I don't know why these two animals always seem to be the ones chosen in predator-prey modelling. However, this model wouldn't completely suit a vampire take-over scenario since...
- isn't it true that vampires don't starve?
- at some point would the remaining humans (maybe those in nuclear subs) decide to nuke the vampires?
 
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I recommend that you look at a "rabbit and fox population model" (search on the internet). I don't know why these two animals always seem to be the ones chosen in predator-prey modelling. However, this model wouldn't completely suit a vampire take-over scenario since...
- isn't it true that vampires don't starve?
- at some point would the remaining humans (maybe those in nuclear subs) decide to nuke the vampires?
To add to what Cubist said, if vampires were to turn all humans into vampires, then they'd be out of food and their own population would also start to decline. So it's only logical for them to find an equilibrium where humans can reproduce to feed their own kind.
 
Honestly, you're not far off the story premise I was aiming for.
It's logical for vampires to find a stable equilibrium, rather than wiping out all their food, in the same way it's logical for our human population to stop polluting and heating up our own planet. But ... you know ... not everyone listens.
So instead, I was going to have a couple of vampires desperately trying to convince the rest "sure, we've got plenty of humans NOW. But by our calculations, we'll hit an irreversible tipping point in just X weeks! Does nobody here understand exponential population growth?!"

The rabbit-fox model looks pretty helpful, though, so thanks for that. Now I just have to remember how to solve differential equations...
 
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