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So I'm wondering if a human civilization or a group of civilizations that evolved on an Earth-like planet comprised exclusively of islands no bigger than 500,000 km^2 (most of them would be smaller, though) and covering no more than 5% of the planet's surface would have population limits different than those of Earth. Obviously, the population could never grow as big as that of Earth, since there's much less land (again, 5% of the total surface area), but would the fact that all of this land is islands inhibit the population growth even more, or would it not make a difference?

In short, would an all-island Earth-like planet impose any unique population barriers to humans? Again, assume the planet is Earth-like in basically every way, save the decrease in surface landmass.

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    $\begingroup$ How close together are the islands? Is it easy to get from one to another? $\endgroup$ – Monica Cellio Apr 5 '17 at 3:31
  • $\begingroup$ @Monica Cellio, generally between 250-500 km apart, so yes, fairly close (if you were sailing at around 6 knots, which was a good speed for ancient-world vessels, you could make the trip in roughly 24-48 hours, which was also sometimes the limit of how long a ship's rations could last, if you were sailing in a trireme or a similar ancient-world military vessel). Obviously, some islands will be much farther apart, and obviously, it all depends on the kind of vessel used, but I'm generally assuming that the vessels are comparable to those of antiquity and that they can resupply at closer isles. $\endgroup$ – C. S. Wright Apr 5 '17 at 17:45
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You've told us that islands are "no bigger than 500,000 km^2". I checked Wikipedia, and that's actually fairly large. California is 423,970 km^2. Hawaii (the state) has an area of 16,635 km^2.

The thing to think about is: most (though not all) of our major population centers are right on the coastline. Being on the water is good for trade, and it's a good source of food if you have fishing boats. The world you're building is practically all coastline, so I would think it would have a decent population density. Hawaii (the state) has a population density of 211 people-per-square-mile, compared to 84 people-per-square-mile for the United States overall.

There are things to think about, certainly. You don't get big rivers when you're on an island, which means you have to think more about fresh water. When you're relying on fish for most of your food supply, overfishing can be a hazard. And technology doesn't grow as quickly if you don't have good mineral mines. But overall I think the world can sustain a decent sized population.

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  • $\begingroup$ I'm aware of the need for mineral mines for technological progress. I've been doing some research on that as well. Good answer. $\endgroup$ – C. S. Wright Apr 6 '17 at 21:34
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Polynesia had ongoing population problems, they were solved by emigration, endemic warfare, blood feuds and in times of drought cannibalism which in some places was formalised. Warfare became a part of everyday life, all men were trained warriors and sudden death was a constant threat.

Genocide was not unknown although not normal, but whole districts were known to have been totally depopulated. Whole tribes became extinct in the larger Islands.

Some small Polynesian islands lost their whole population due to lack of resources when interisland trade collapsed due to warfare on other islands.

Micronesia there are tiny islands which could not survive population pressures without trade, and some practiced infanticide and/or killing mothers to prevent them having more kids (according to legends).

So population can be managed, but it's not pretty.

Polynesia still has population problems which are now solved by poor health care and bad food being dumped there, rampant obesity and emigration. But if for some reason they were once again isolated, it would in short order become a deadly struggle over resources again I would posit.

One other issue no one has touched is there is no continental land, this means that some resources are actually pretty scarce in most places in terms of minerals, metals and things like that, you can have lots of trees with none really suited to building on a particular Island, this is how it works on earth anyway. Most of Micronesia, Melanesia (not so much since some of that is ex-continental) and Polynesia have this problem which was partially solved by trade and by replacing the native flora with imports, most of their staple foods in terms of plants were imported. Very few are native to the Islands. Take away just a few like taro, yams and breadfruit, and humans can't survive in any great number. Coconuts can survive a sea journey on their own for about three months, but none of the others can. So you need an equivalent of those for a start.

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    $\begingroup$ So are natural resources and artificially imposed measures the primary limiters of the population, or are they just two factors among many? $\endgroup$ – C. S. Wright Apr 5 '17 at 17:52
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    $\begingroup$ They are the two main, not much else exists, no natural predators except crocs which are easy to take care of, no one would import dangerous animals, no disease unless imported etc,. Polynesia for example was paradise until a bunch of humans arrived there.... but whole islands populations were all but killed by measles when European explorers arrived. Large familes 15 to twenty kids was common, the bigger your extended family the stronger you were in war and defense. You practised population control by killing your neighbours rather than limiting yourselves. $\endgroup$ – Kilisi Apr 6 '17 at 1:22
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The carrying capacity of Earth is hard to guesstimate. Historically, before the development of artificial fertilizers and the green revolution which increased agricultural output severalfold, it was estimated at ridiculously low values (see Malthusianism); modern estimates may be as high as 40 billion people -- which will probably never be tested, as current projections indicate a maximum of about 10 billion people by 2075, followed by a steady slow decline.

About 1/3 to 1/2 of Earth's land is utterly unsuitable for agriculture; anyway, at present cultivated land represents about 12% of Earth's total land area. Your world has about 1/6 of the land area of Earth; depending on how fertile the land is, how much fresh water is available etc. that world may sustain a population comparable to ours, or it may not. I think that you can safely assume that 1 billion people is achievable.

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  • $\begingroup$ Alright, thanks. It's good to see some comparisons to Earth since the planet in question is Earth-like (save for land differences). $\endgroup$ – C. S. Wright Apr 4 '17 at 23:10
  • $\begingroup$ The Malthusian values aren't ridiculously low: what you're overlooking is the non-sustainability of those agricultural techniques; an unsustainability that's becoming pretty obvious to anyone who cares to look. $\endgroup$ – jamesqf Apr 5 '17 at 4:05
  • $\begingroup$ @jamesqf: Europe has sustained modern agriculture for about one hundred years without any degradation of productivity. It is not the modern agricultural technology itself which is unsustainable, it is short-sighted careless practices which are the problem. $\endgroup$ – AlexP Apr 5 '17 at 10:50
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    $\begingroup$ @AlexP: 1) A century isn't very long. 2) Europe imports quite a bit of food. $\endgroup$ – jamesqf Apr 5 '17 at 17:39
  • $\begingroup$ @jamesqf: Europe also exports quite a bit of food. It's called commerce. Let all countries prosper by specializing in what they do best. Anyway, since modern agriculture is at best about one century old, all we can do is hope that one century is enough to see whether something bad happens... $\endgroup$ – AlexP Apr 5 '17 at 21:44
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Polynesia did alright, once they figure out boats expect a population boom. Obviously quite likely followed by mass death as some expeditions will run out of food.

What they need is local food and a decent chance of getting from Island to Island. So no large storms, plenty of wood etc.

edit: on the subject of storms. Storms are destructive. Storms would wreck your boats and likely any wooden buildings. Perhaps this would force them to develop masonry sooner. Tropical storms are nasty business.

So I see that slowing down their progress as they go through cycles of destruction. Rebuilding is time wasted on not developing further. As your main goal seems rapid development. It could however foster a technological boom to survive the storms. But not without a cost.

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  • $\begingroup$ No problem with distance and natural resources; the islands are all pretty close together and have fertile soil and thus plenty of food, wood, etc. Sailing is also obviously an important part of these civilizations. Unfortunately, they do in fact have big storms. How much does this affect your answer, @Mormacil? $\endgroup$ – C. S. Wright Apr 4 '17 at 22:40
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    $\begingroup$ WRT Polynesia, consider Easter Island. $\endgroup$ – jamesqf Apr 5 '17 at 4:16
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Ok, I'm just going to list a few problems that come to mind for this scenario and how they could possibly be solved.

Note: This is referring to the point in time that the civilization has outgrown the islands.

Problem 1. Food I've heard of a thing called 'floating farms' https://practicalaction.org/floating-gardens which are basically crops grown on a floating platform on water... IF they figure out how to use that, especially over salt water, food would not be an overly large issue in population growth. Of course, you may need some defence against Storms & Waves - especially if you need to expand them further into the sea.

Problem 2. Living space This depends on a multitude of factors, such as weather and oceanic problems that may arise from island living (though, on reflection, when has that ever stopped us?) If there are regions of very shallow water, then stilt-houses are a possibility. If not, one only really needs a large enough boat to live in, as long as there are ways to get food & clean water.

Problem 3. Water Most humans get their water from freshwater sources, such as rivers, lakes, snow and springs. These will be limited to the islands. Unless some other way of easily getting water is available - maybe rainwater collection or evaporation & condensation- then it may be difficult for early civilizations to have enough to get off the ground.

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    $\begingroup$ Fresh water is one of the least important problems on sizable islands. Oceanic moisture tends to accumulate around the islands, providing steady rains which lead to multiple streams and abundant groundwater. $\endgroup$ – Alexander Apr 5 '17 at 0:46
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    $\begingroup$ Yes, but i'm talking about the situation if the Civilization has overgrown the islands, which probably wouldn't happen for a long time anyway, but i wanted to include it.. $\endgroup$ – Hannah Apr 5 '17 at 11:41
  • $\begingroup$ Good points, Alexander and Hannah. I am, in this case, assuming that some of these islands are quite big (so there would be plenty of fresh water, according to Alexander) and that few if any have overgrown their population limits (Hannah). $\endgroup$ – C. S. Wright Apr 5 '17 at 17:51
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The major limiting factor in this problem to me is the advancement of technology. Since our only comparison is Earth, you can see that population growth over our history was fairly linear and minimal until about the 17th - 18th century. The advancements in medicine, agriculture, transportation, etc all gave us the ability to increase the average rate of death.

Therefore, if you assume that the population of the 'Island Earth' would be able to make the same strides in tech, then populating many small islands would not be an issue.

I would think that many islands would eventually take on specific roles. For example, in one archipelago, you could have central island that the majority of the population lived. The surrounding islands could be used to support the center by being put into roles such as "agriculture islands", "mining islands", "manufacturing islands", etc. This would fit into some land use models that explain the separation of cities and towns.

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  • $\begingroup$ Welcome to WorldBuilding! Interesting start. If you have the time please take the tour and visit the help center to learn more about the site. Have fun! $\endgroup$ – Secespitus Apr 5 '17 at 19:52
  • $\begingroup$ @Matt, I like your idea about different islands fulfilling different economic roles quite a bit. This was something I already had in mind but it's cool to see someone else think of it, too. $\endgroup$ – C. S. Wright Apr 5 '17 at 21:08
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Some Polynesian islands have practiced zero(ish) population growth. On small islands it is easy to see that your finite resources are best shared with limited numbers of people. If your people recognize this they may institute cultural norms that limit growth.

However if you have larger islands who don't have to worry about it as much or you lose enough people to storms or wars there may be incentive to have more growth.

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