Here is a pretty geeky article about the average appearance rate of the extraplanar gates and their influence on Thalassa's demographics. Summarizing: the extraplanar gates are necessary to replenish the population of Thalassa (and of the whole mundane plane) because the natural death rate is bigger than the natural birth rate (the interbreeding of people coming from very different places of the multiverse is not so easy even if they belong to the same species). Thalassa's population is fairly constant throughout the ages.
Now, if you like to read the longer version here is the article:
Every day throughout Thalassa (on sea, land and atmosphere) there are about 400 magical storms; every of them will open a small extraplanar gate to a random location in the multiverse, carrying in some suitable intelligent creatures. A "standard" magical storm of this size would carry in between 1 and 1000 creatures, but the probability is not evenly distributed: a storm carrying many beings is far less probable than one carrying just a few of them. On average, such an extraplanar gate carries just 7 people. Assuming gates are evenly distributed across Thalassa's surface there is a single gate opening event per year in a 30km radius' circle.
During an average year the number of beings carried in Thalassa from other planes of existence is about 940.000: such a contribution is of primary importance to keep Thalassa's population stable. Without this contribution, in fact, the natural birth/death ratio would be negative: for every race there is an average of about 1.95 children per woman, slightly below the replacement threshold. For humans (assuming a 75 years average lifespan) it would mean a net loss of about 1 person every 3000 people per year. For elves (assuming a 600 years average lifespan) it would mean a net loss of 1 person every 24000 people per year.
The negative birth/death ratio is due to the fact that people belonging to the same race (say "the humans" or "the elves") on Thalassa are in truth a melting pot of individuals coming from a myriad of slightly different races scattered in the multiverse. For this reason interbreeding is not always successful and fertility rates are pretty low in many circumstances.
There is a fairly constant proportion in the number of beings collected by the extraplanar gates, which depends on their lifespan: the longer the life expectation the shorter the number of collected beings. Approximately 39% of the collected people has the same lifespan of humans (say 75 years), 40% has the lifespan of humanoids or shorter (say 35 years or less), 15% has the lifespan of halflings (say 150 years), 5% the lifespan of dwarves (say 300 years) and 1% has the lifespan of elves or longer (say 600 years or more).
Of the whole collected creatures, about 35% live underground, another 35% live underwater, 25% live aboveground and 5% live in the atmosphere.
As intelligent life on Thalassa is extremely ancient a demographic equilibrium between population losses and new contributions from extraplanar gates has been reached since long. Weighting for the average lifespan of various lifeforms it results a total intelligent population on Thalassa of about 3.2 billions people, of which about 800 millions live on the continents (and a total human population which is far less than 310 millions, if we take into account that not all intelligent beings with a human lifespan and living on the continents are actually humans). Considering that Thalassa is bigger than Earth, and its surface is about 50% larger, the average population density on the continents is pretty similar to Earth's levels around 1500 B.C. (about 2.1 people per square kilometer, or 5.4 people per square mile)
The large number of extraplanar gates which open every 3600 years during a full planetary alignment (partial planetary alignments do not increase the number of extraplanar gates) represents a net increase in Thalassa's population which bypasses the demographic equilibrium described above. During a planetary alignment the large number of major gates opening across the surface brings between 10.000 and 10.000.000 new people on Thalassa (the average is 70.000 people, mostly belonging to the same race). This would bring a slow increase in Thalassa's population after many millennia.
We left our old worlds, each of us. This is it. We can't go any farther. This is The Edge.
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