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There are a lot of questions on this site about historical population levels and densities, and they have good answers about taking into how hard it was to survive in the past, and how inefficient many practices were.

But I’m not asking about medieval Europe or China, this society has a lot more knowledge and resources at its disposal than pretty much anywhere had prior to the industrial revolution, but it still doesn’t have even the dream of a steam engine.

Geographic Background:

•Land area is roughly the size of the Iberian Peninsula and at the 40th parallel

•The region is very well watered with predictable rains and several navigable rivers coming from a major mountain range to the west.

•Soil is highly productive due to ancient volcanism.

Agriculture:

•This region has a super crop that is extremely productive and nutritious, allowing for a well nourished populace.

•Crop rotation is a well known science to the population, with a program comparable to the Four Course System as opposed to the much more inefficient three field rotation. Biochar is also used occasionally

Megapodes are raised for meat and eggs rather than chickens, leading to much greater quantities of meat and eggs compared to chickens.

A large rodent speciesis raised as another source of meat. It reproduces much faster than cows or pigs.

•Seed drills, the horse collar, and mouldboard plows are used.

Infrastructure:

•A network of roads comparable to those of the Roman Empire connects all of the major population centers.

•The centralized government maintains storehouses similar to those used by the Inca in order to soften the blow of crop failures.

•The major rivers are all connected with canals.

•Aqueducts serve clean water to the cities

•Warfare has been avoided for several generations due to a powerful but isolationist government.

Scientific Knowledge:

•The germ theory of disease is widely known and accepted due to an enigmatic prophet in their religion. Strong alcohol is used for hand washing rituals.

•Sanitation is likewise common practice, with sewage and trash disposed of carefully and water is often boiled.

•Healers here laugh at the theory of the Four Humors, and actually wash their hands before delivering babies and stitching wounds.

So with these factors in mind, how many millions of people could live in this region, or if that’s too broad would it be feasible for such a population to exceed 50 million?

Edit: How about 20 million instead?

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    $\begingroup$ You do of course know that the population of the Iberian peninsula is 57 million (47 million in Spain and 10 million in Portugal). And we live in the 21st century, with excellent and abundant food, great healthcare, no wars, very efficient transportation networks, fully functional social security, perfectly safe water etc. $\endgroup$ – AlexP Feb 17 at 12:39
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    $\begingroup$ @AlexP Yes, but that’s with industrial transportation and agriculture, I’m essentially asking if these factors can compensate for a lack of industrialization $\endgroup$ – NixonCranium Feb 17 at 15:50
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    $\begingroup$ You cannot possibly get anywhere near the productivity level of modern agriculture with pre-modern technology. You'd be lucky to come within an order of magnitude. And pre-modern transport is even worse; the vast majority of stuff which is transported today across significant distances would be utterly uneconomical in pre-modern times. Steam engines and fertilizers really really made a difference. The gist of my comment is that the population of the Iberian peninsula is barely above 50 million people with modern agriculture and chemistry and mechanization and transportation. $\endgroup$ – AlexP Feb 17 at 16:05
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    $\begingroup$ Parts of India and China in 1800 had areas close to the population density you propose. It's possible, in a few small areas, and for a few generations until famine or pestilence intervene. You have done a good job removing many common vectors of pestilence...but there are more. $\endgroup$ – user535733 Feb 17 at 17:42
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    $\begingroup$ Re: Transport. Could some method of producing cheap steel in bulk (like Bessemer) have been developed before steam engines? Steel wheels on steel rails cut rolling resistance a lot, & so would make land transport of goods cheaper even if draft animals are the only motive force. This would increase the possible population density though not up to 21st century levels $\endgroup$ – Jim Baerg Feb 17 at 20:57
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Industrialization has not allowed us to grow significantly more food per acre. It lets us grow more food per farmer.

Even if you exclude the landmass of Portugal, the population of Spain is already about 50 million; so, I'd say your civilization could very easily hit your 50 million population mark. In the present day, Spain is about 50% farmland and and exports about half of what it grows. That means Spain is already growing enough to support about 100 million people, but could probably be optimized to support about 200 million people using thier current crops if the whole landmass was usable farmland.

When you add in the fact your population is focused on super producers and not worrying about all those inefficient crops that hold Spain back, you are really looking at a landmass that could probably support closer to 500-1000 million people.

Now for the caveats:

Your civilization will have a much higher percentage of farmers than modern society; so, even if your total population is 10x as big as modern Spain, your cities will not necessarily be bigger. In preindustrial Europe, about 90% of the population had to be farmers to support the remaining 10% of the Urban population. In the modern world, it's more like 10% can support the 90%. Also, with lack of trucks and trains, supplying large cities becomes much harder; so, you will probably see a larger number of smaller cities.

Also, your setup does not have pesticides, refrigeration, or preservatives. That means that even though your land could ideally support a billion people, you are much more vulnerable to unexpected food shortages. Also, by only growing the most efficient crops instead of more diverse crops, you are more vulnerable to blights or whether conditions that might wipe out all of your food production in one fell swoop (See the Great Potato Famine). So, if you want a largest "sustainable" population, then I'd suggest you diversify your portfolio and produce enough excess to survive the unexpected. This would put your population realistically back at the 100-200 million mark.

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  • $\begingroup$ Vertical farms has a word to say $\endgroup$ – user76358 Jun 9 at 19:38
  • $\begingroup$ Seaweed farms too.... $\endgroup$ – user76358 Jun 9 at 19:39
  • $\begingroup$ And hydroponics allow 100% of the population to be a producer with as little space as possible. $\endgroup$ – user76358 Jun 9 at 19:43
  • $\begingroup$ Seaweed farms have existed since ancient times; so, I think it's fair to say industrialization is not a prerequisite. Hydroponics and vertical farming combined make up less than 3% of food grown in the US. Most other nations use them even less to the point researchable statistics are hard to find. While these technologies have the potential to grow more per acre, in practice, they don't have any significant impact on how much food per acre modern nations are producing. $\endgroup$ – Nosajimiki Jun 9 at 20:50
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So, let's pull out our good old medieval demographics made easy.

http://web.archive.org/web/20180416114529/http://www222.pair.com/sjohn/blueroom/demog.htm

Now, while this world is not fully medieval any more, this is still a good starting point. Given your circumstances, you're looking at the upper end of the population density scale: 120 per square mile. With Iberia being 230400 square miles large, this gives you a population of 27.6 million.

Now, what do those influences you have listed contribute here? The geographic and infrastructure factors, as well as the knowledge of crop rotation, seed drills, the horse collar, and mouldboard ploughs, are all possible for a well-organised late medieval society. These factors all contribute to me putting the population density at 120/sq mile.

Then your animals. Meat as food doesn't typically increase the number of people you can have in an area. Quite to the contrary, because an animal eats more than you get out of it as food, it normally decreases the amount of people you can have. Now, what it will do is make your population have a more varied and nutritious diet, which will improve health and lifespans, but that by itself has no impact on the number of people that can live in an area.

Your super crop has a similar effect, it's a great part of the diet, and improves the health of your people. Now, because it has such exceptional yields, it also does something else: one farmer can bring in much more food than they need. This means not everybody needs to be a farmer, making them available for things that aren't agriculture. This, in turn, will likely lead to greater urbanisation.

The scientific knowledge, again, helps improve general health and longevity. It also makes larger cities more liveable.

But none of these things lead me to think that your total population density should be above 120.

One key thing to understand here is that improving longevity doesn't actually increase the number of people you can have in an area. The number of people is completely tied to the amount your land can feed. Assuming there have been no plagues, wars, or other major disasters in the past century or so (and it looks like there aren't), your population will be near the maximum. With a population growth of 3% (which is around the current rate for several sub-Saharan countries), your population increases 19-fold in a century.

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