The population and size of a post-apocalyptic desert fortress

One of my story/series concepts is set in a post-apocalyptic desert fortress with a town, a farm and some animals inside, and a Legion of Lost Souls-style military that patrols the walls and has made limited-range expeditions outside, bringing in more people and animals. The soldiers have some horses, donkeys, and mules for transport and some people have hunting birds or canines.

The fortress is very crowded, with tiny homes layered on top of and right next to each other in tight lines kind of like asian slums except more coherently and more solid, mostly made if rock, wood, and sandstone. Each line's residents cook and eat communally in a room at the line's end and washing areas and bathrooms are also shared by the lines to conserve room and because they don't have the resources to fix the blueprint and not much room left to expand into within the fortress.

The bathroom system mostly works and they have running water they pump from an undergound river and the original builders managed to build the bathrooms and washing areas so that they wouldn't contaminate the drinking water or the farmland, though outbreaks occassionally happen due to the cramped living conditions and the fact that many people have to walk a long way to wash their hands, which results in some people trying to avoid it.

The questions i need answers to:

• How many people and animals of each species would the fortress need to maintain a healthy/stable population indefinitely or at least several decades^note  preferrably without having to force everybody to have sex with everybody or forcing every woman to be pregnant? The numbers i get through internet search vary rather wildly.
• How large farmlands would they need for keeping that amount of people and animals sufficiently alive? What would be the best things to farm in this scenario?
• How many different things would you need to farm to keep the people from getting ill from too little dietary diversity?^note  when i try to search "dietary diversity human", i only get stuff about small children and pregnant women and unclear stuff.

Edited by Nukeli on Mar 24th 2021 at 8:21:34 PM

> The numbers i get through internet search vary rather wildly.

That's because we do not have a solid answer. The best guess we have is that humanity was cut down to a couple thousand at some point in the past and it survived. With ideal genetic picking you might get away with a couple of hundred. But realistically a self-sustaining civilization would take thousands, likely.

That number likely goes down the less strict the situation becomes. Like suppose you have 10 families made of 10 people, you could probably go several generations before incest becomes a problem. So if they had last contact like 50 years ago, that could be sustainable until next contact.

Yeah, there's been precious little research on that issue, so the estimates are extremely crude. I once did some research on sustainable space habitats, and IIRC one square kilometer of food production can support 100 people. That was assuming hydroponics, so it still applies to your desert outpost, if they use indoor greenhouses (also, you can stack hydroponics so a double tier of plants each two meters high cuts the surface area in half). This is plants only, I have no idea what the energy inputs for domestic animals are, except that they're higher than those for plants. That's a problem in space, but here on Earth animals can digest things humans cant and convert that into meat, so it's still an efficient source of food. As far as genetics are concerned—it depends on the loss rate due to genetic inbreeding. As long as your survivors have enough children to match the population level even after subtracting the loss rate, they're good. The problem is that the smaller the population, the greater the risk due to unpredictable catastrophes: 1000 people can more easily be wiped out by a random disease than 10,000 can. Since you, the author, control the rate and severity of unpredictable catastrophes, it's really up to you.

Some kind of a solid conclusion has to be reached, because literally everything else in the setting (animals, farmland, the minimum size of the area) depends on the population number.

Nothing big actually happens after the original end of the world, so incest would propably be the biggest threat.

Edited by Nukeli on Mar 25th 2021 at 2:55:56 PM

You could just copy Kowloon and it's 33,000 population. Maybe with better organization and less crime.

Edited by Belisaurius on Mar 25th 2021 at 9:12:32 AM

You're supposed to base decisions like that on the plot, and hand wave whatever size the story requires. If it's a well-told story, very few readers will care.

You could go the long way, calculate things like how much food the land produces and how many animals it can sustain etc.

But the simpler method is simply eyeballing a number. It depends on how long the fortress has to survive on it's own. For a couple of decades you can probably get away with hundreds. For centuries, thousands. For indefinite, probably tens of thousands. Then look up whether you can find isolated nations or even historical nations and what kind of situation they had. Like, japan had a period of isolation, how did it manage.

Most people will not hugely care about precise numbers. As long as it sounds plausible, it for all intents and purposes is correct. A decently sized town of a couple of thousand people is a matter of googling some towns like it, and you have an overall size. Consider the food you get from the land and you have a rough idea of the area you need. Fudge it a bit if the numbers sound weird. Animals like pigs and chickens can eat loads of things that humans can't, so there's no additional food pressure. Animals like dogs and cats primarily eat meat, but can eat scraps.

All that's left to do then is to consider some scaling factors. is your town very dense, or your agriculture really advanced? Is there a nearby river or do you need wells. and so forth.

Edited by devak on Mar 25th 2021 at 6:11:19 PM

You may have already found this but it hasn't been mentioned yet, so...

Minimum viable population has been suggested to follow the "50/500 rule": 50 reproduction-capable individuals to avoid inbreeding, 500 to avoid genetic drift. You won't need to worry about the latter if it's only been a couple of generations. This number has been contested (it may not be universally applicable and some argue it is too few) but it's a benchmark guideline.

But that said, I do think audiences will be more willing to give Willing Suspension of Disbelief for a post-apocalyptic setting; I agree with others that the number can just be eyeballed.

I don't know if you've seen Mad Max: Fury Road, but they had hydroponic agriculture◊ in their post-apocalyptic desert outpost.

Edited by Synchronicity on Apr 7th 2021 at 9:43:32 AM

How much agricultural space (and/or animals) would be needed to keep A) 500 B) 1000 people fed? How about dietary diversity?

That depends heavily on the soil, the climate, the available technology, cultural standards of work, available plants, available water, and probably more.

the animals also depend on what's available, but sheep, pigs and chickens would be desireable. Goats, mules, or camels would have roles.

If you want a solid answer that's true in most cases, a common historical family unit consisted of eight individuals with about 5 acres of land. more here. You could round it and say half an acre per person. So 500 people needs 250 acres, 1000 people need 500 acres.

As to animals, a pig, a handful of chickens and some sheep per household would be viable. Pigs eat just about anything, so you can turn waste into viable food. Chickens are scavengers and find their own food. Sheep produce milk, wool, dung and meat.

That estimate wont work because family farms were heavily dependent upon imports (ie, no family farm produced all the resources needed to autonomously guarantee survivial, which I assume a post-apocalyptic settlement would). The problem with coming up with any estimate is that at no point during at least the last 10,000 years has any human settlement been entirely cut off from a network of other settlements. Even hunter gatherers are connected to a much larger ecosystem (within which extensive energy transfers are taking place) than the territory their individual tribe controls. It's literally never happened, and so we don't know what the minimum amount of cultivated land would have to be to support X population (or what the minimum self-sustaining human population is).

Realistically, a single remaining human fortress isolated in the desert probably couldn't survive, regardless of size. We just aren't evolved to live under such conditions.

Which means you can handwave anything that the audience will accept as plausible. As a matter of curiosity, how does the audience of your work find out how much agricultural land there is? Is it an important plot point?

Yeah, if it's not a big part of the story I would not worry too much about it.

An extreme example: The 100 had a handful of people survive in space for six years on a diet of algae and water. But it was brushed over as part of the six year timeskip, so it kind of felt like an "okay, sure" type of detail.

I think you overestimate the need for trade. A group of subsistence farmers would produce all the food, clothes and tools they'd need. Trade did happen, but this was more to deal with either excesses (you produced more food than needed) or for luxuries (relatively speaking, but things like different dyed threads). A good example is the UK-EU fishing agreement: the fish caught near the UK, the brits don't eat and the fish caught in EU waters, europeans don't eat. But they do eat each other's type.

The one about the environment is a fair one, since it's hard to say how many people exactly could be sustained by a certain patch of desert (or rather, how much desert you'd need to terraform to be 100% self-sufficient).

Edited by devak on May 26th 2021 at 1:07:34 PM

I modeled the area in Minecraft, assuming that 1 block = 1 meter.

The area is currently 100 meters X 100 meters, and one wall is used as the starting point of said house lines (12 lines with 40 mini "apartments" each, is 480 apartments). Adding another 12 lines on the opposite wall, i'd have 960 apartments for the intended population of 1000^note  (and no idea where to put that one extra building), and between them would be the space of 32 X 100 meters for the farmland and animals^note plus the soldiers' barracks and stables. If the people would be able to utilize some farming stuff that survived the apocalypse, could that room be enough?

That sounds very small, especially for the farms, unless you somehow have a many-story design. By my previous estimate, the farms would need roughly 500 acres, which is around 1500 by 1500 meters. You could reduce this number by having highly advanced vertical farms, but it's hard to see how that would be sustainable in a post-apocalyptic world -let alone a desert one. You'd run out of complicated parts pretty quickly.

"A group of subsistence farmers would produce all the food, clothes and tools they'd need."

Taken literally, that's patently impossible. I realize that the OP didnt explicitly mention the environment, but all living things on Earth are interconnected by calorie exchanges and other inter-dependencies. One implication of this scenario is that if the apocalypse damages the ecosystem, all previous estimates for human habitation self-sufficiency are going to be wrong. But even without taking environmental damage into account, various things can disturb the surrounding ecosystemic network, thereby interfering with the ability of the local population to continue to exploit resources as they normally would—this means a species maximizes its' probability of survival when it occupies more than one biome at a time. Humans take this approach to another level—humans in one biome might suffer from an ecological disturbance, but make up for this by engaging in trade with settlements in a different one. For example, your estimates for minimum size of a self-sufficient settlement might work for a while—until a new disease blights the main crop, and then the entire settlement faces the possibility of collective starvation. Larger population size spread out over many decentralized population areas simply improves the chances of long term species survival. Over large timescales, a single desert settlement, regardless of it's own size, would be a very precarious basis of human survival.

I didn't get the impression that this settlement was meant to last forever, just long enough. I think i've mentioned this numerous times. A desert settlement would be extremely vulnerable to distaster, yes, but that seems to be the point?

From the OP: "How many people and animals of each species would the fortress need to maintain a healthy/stable population indefinitely or at least several decades?"

"or at least several decades". Pretty clear that establishes the actual intended timeframe. We established early on in this thread that indefinite survival would require huge populations and the responses indicate the idea is more of a small Mad Max type civilization that lives just decades After The End. So my overall response still stands, nothing in this thread gives me the impression we were going for indefinite.

Edited by devak on Jun 7th 2021 at 12:39:28 PM