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So most zombie apocalypse stories involve wiping out 99.9% of the human race and having a small minority survive to try and keep living.

But, let's say I want society as we know it to collapse, but I still want a substantial number of people around to keep some of modern life going. However, if there are too many people they would eventually be able to clear out the zombies...

Also, we need to have enough people so that we can keep repopulating both the human population and the zombie population...

So what is the minimum number of people needed to keep a small city running? For the sake of argument, let's say it started off with 250k and I want to cull at least 80%.

What are the essentials that could be covered? (Power, water, sanitation) And how many people would be needed to cover them?

How many people would be needed to handle farming and livestock?

Obviously, need to figure out how many people would be necessary for security, which would probably need the most employees.

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The problem you have is that this is an unstable equilibrium.

In a conventional predator-prey dynamic you have a feedback loop that keeps things balanced. If the number of predators grows too high then the amount of prey reduces and the predator numbers crash, allowing the prey populations to recover.

With zombies and humans though this would not happen. If the number of zombies increases then the humans will die, likewise if the number of humans increases the zombies will die.

The options that give you what you want are quite limited. One would be humans living on small islands or fortified towns with the surroundings infested with zombies. That would be stable for a while at least, but eventually the humans would clear out the zombies.

To get to the meat of your question though - human populations and tribes can range from as few as a few hundred people although you really want a few thousands to be sustainable. The other questions linked to by Caleb cover most of those points. If you wanted the people to be long-term sustainable then they would need substantial amount of farmland and people working that land. This would be hard to wall and protect although you could theoretically do so. Again they would probably be better off moving to an island assuming the zombies can't cross the water.

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    $\begingroup$ What if we got the upper hand over the zombies, decimize their number and then a bunch of hippies declare them an endangered species? $\endgroup$
    – overactor
    Commented Oct 14, 2014 at 6:25
  • $\begingroup$ @overactor - the theme of your question is somewhat an element of my story, that is, as the "zombie infection" spread, people were unwilling to act against them as they were still seen as people (technically they were)... this inaction helped spread the disease... $\endgroup$
    – MadPink
    Commented Oct 14, 2014 at 15:48
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    $\begingroup$ To introduce the Predator/Prey relationship, the zombies could just be too strong for a human to reliably kill. They would also have to die from starvation. Alternatively, the humans have found a way to prevent being zombified and could exterminate them - but it requires ongoing injections or similar with stuff that only zombies produce, and then they would outbreak themselves if they don't continue to take it. If zombies can starve, that could result in some harsh scenarios. "Either we find a new zombie, we feed someone to the dying zombie, or we use the first person who turns." $\endgroup$
    – DoubleDouble
    Commented Jul 31, 2015 at 20:16
  • $\begingroup$ "Since I feel myself turning already, I elect Kevin to be this month's sacrifice." $\endgroup$
    – DoubleDouble
    Commented Jul 31, 2015 at 20:26
  • $\begingroup$ "I hear that the anti-zombie vaccine causes autism. My child won't get the biteys because I feed them a healthy all natural diet. If you are so sure the vaccine works, why are you worried about my child chewing your child's face off?" Just because the humans could eradicate the zombies doesn't mean they would. Once the risk of zombie outbreaks drops to a reasonable level, humans are likely to go back to politics as usual. $\endgroup$
    – gmatht
    Commented Jun 6, 2017 at 13:00
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Ok so lets break this down.

  1. So what is the minimum number of people needed to keep a small city running? For the sake of argument, let's say it started off with 250k and I want to cull at least 80%.

  2. What are the essentials that could be covered? (Power, water, sanitation) And how many people would be needed to cover them?

  3. How many people would be needed to handle farming and livestock?

  4. Obviously, need to figure out how many people would be necessary for security, which would probably need the most employees.

  5. The whole balance/repopulating thing.

  1. So you are looking at roughly 50,000 survivors. Nice and easy.

  2. For power water and sanitation for a population of 50,000 lets look at an example. Woodland, CA. Population 2010: 55468

    • According to their 2014 - 2015 Budget Proposal they have 277 City FTE's
    • Going through the list just to necessary support, i.e. utilities, fire/rescue/police we can pare that down to around 225. This does not take into account contractors and you can expect a major spike in employees needed for building stuff...for example walls if we are trying to keep zombies out.
    • This also doesn't include county/state/federal infrastructure and support that may be provided to a town.
    • An additional important note is if you cull 80% do you have the special skills and training available to manage all this? I have the population but no civil engineers for example.

  3. Well according to this document it takes about one acre to feed a person for a year. So we are looking at farming at least 50,000+ acres, and its good to have a reserve for drought etc so lets go with 75,000 Acres. The data I found on workers per acre is a little old (1990) but it puts the number at 1 worker for every 740 acres. So our simple math works out to about 101 farmers.

    • This is assuming the same level of infrastructure and tech (big tractors, grain silos, etc etc etc)
    • I doubt this number accounts for seasonal workers for planting and harvest, which when you reduce the tech level would increase significantly.

  4. Security is a bit of a challenge and without a bit more information its hard to say.

    • How large is the city? (Land size)
    • How many/how aggressive are zombies
    • What kind of weaponry is available.
    • Is the entire city walled off? Can zombies scale the walls?
    • My suggestion here is more of a citizen militia. EVERYONE gets trained, and called as needed with a core of lets say 1000 full time soldiers/police. These would man the walls and run patrols in the (Im assuming) un-walled farmlands. This 1000 number is arbitrary and subject to the questions listed above.

  5. I am not sure how this would work. Humans have done a pretty good job of taking themselves out of the predator prey relationship. And if we are going with stupid walk into obvious death zombies I don't see a good way to keep a zombie/human population balanced...knowing how your zombies work would help, do they die without human flesh? Are they smart enough to plan things out, like ambushes for example?

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    $\begingroup$ You've also neglected any of the non-city services, like power companies (utilities), water company (which may be sewage too), etc, etc. And that doesn't include manufacturing and other stuff. Self-sufficiency will require you to get (or provide transportation for) mining, and all types of other stuff. $\endgroup$
    – user3082
    Commented Jan 18, 2015 at 17:23
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One possible way to evade the instability mentioned by Tim B would be if some humans, for unknown reasons, repel zombies. That has to be a non-inherited trait (because otherwise soon immune humans would dominate, and you'd end up with humans and Zombies just living separately).

Those who repel zombies would themselves be safe (and being in their vicinity would also make others safe), but they also cannot hunt zombies. But since that trait is not inherited, there will always be both humans who are safe and humans who aren't.

The main problem is, then, to determine the probability for a human to be born anti-zombie. If that probability is too low, there would not be a sustainable population. If that probability is too high, the effect would mostly me as if all of then were protected, because people could simply always keep in groups, where the probability of some anti-zombie then is very high.

The second question is how far the effect reaches. Of course the effect has to be strong enough that the zombie doesn't get in a range where it can be killed (or otherwise the anti-zombies would be very efficient zombie killers). On the other hand, you don't want the range to be too large, because otherwise any settlement could be easily protected by just having a few anti-zombies living at strategic places; if the border of a city is sufficiently stuffed with anti-zombies, the inner part should be protected as well, since the zombies cannot pass the border.

One way to reduce this effect would be if zombies can feel being seen by an anti-zombie and that is when they are repelled, so an anti-zombie cannot hide and hunt zombies, nor can he give absolute protection (for example, while he sleeps, he's not protected, nor are others; also if the zombie is behind him or hidden behind a wall, he will not repel that zombie).

So let's assume that anti-zombies have the ability to repel zombies by just looking at them, but are unprotected against zombies they don't see. That way, the anti-zombies can only give effective protection as long as they are awake and looking in the right direction. Of course when zombified, an anti-zombie loses his special ability (as a special twist, a zombified anti-zombie might be able to resist the repelling effect of being seen by an anti-zombie for a certain time)

It is likely that towns would be guarded by anti-zombie guards watching their borders. So the number of anti-zombies must be low enough that they are not sufficient to prevent all zombie attacks.

Another way to avoid the instability is if a zombie attack is not the only way a human can be zombified; just like with the plague, zombification could be additionally brought by animals (rats, mice, whatever), so zombies might pop up occasionally in apparently safe regions. Then even if humans at some time managed to kill all zombies, suddenly a new zombie outbreak could be caused by a zombie-rat.

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  • $\begingroup$ Anti-zombie is an interesting idea but it would be hard to explain it in a way that isn't either inherited or teachable. I do like the idea of plague carriers bringing the zombies in - that could solve the whole problem. $\endgroup$
    – Tim B
    Commented Oct 14, 2014 at 8:10
  • $\begingroup$ I always hate the idea of zombie animals. Assuming they can fly, a flock of zombie birds seem to make things almost impossible to defend against. And then if you include zombie insects... mosquitoes are hard enough to prevent already - that being said maybe mosquitoes still attempt to feed from zombies and then infect people without becoming zombies themselves. $\endgroup$
    – DoubleDouble
    Commented Jul 31, 2015 at 20:08

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