[Man-of-the-Wold]

The demographics of Eriador is of endless fascination, and in the earlier half of the Third Age, it probably had a more notable population base than we see in Frodo's time, but even that reflects somewhat recent declines in southeast Eriador and North-South interchange, with the ultimate end of Tharbad and displacement of people, as witnessed by the recent refugees in Bree. These I attribute to the policies of Saruman over a generation or so, in building up a sort of power base east and north of the Isen, bolstering the strength of the less civilized human elements, with feelers all the way to the Shire. A parallel being the hill-men of northeast Eriador, under the tutelage of Angmar, but ultimately wiped out with it.

Ultimately, the Kingdom of Arnor never enjoyed the population base, dynamics and resilience of Gondor, which had seemingly much heavier Numenorean settlement and denser indigenous peoples, so that from the beginning, it had the ability to support a number of cities and continuous areas of settlement, and to fight and grow. With these Southmen and later influxes of Northmen, the Dunedain of the South did intermingle, but reflective of Faramir's commentary, this was both inevitable and a blessing, however, mixed.

At some time or another Elves and Numenoreans can enjoy population spurts, such that by the WotR, mostly non-eldarin Elves are a rather dominant population group in Rhovanian. Ironically, long life seems sooner or later to work against sustained population growth, and in many ways, Tolkien's underlying and very Christian point is how the longer lifespans and ennobling of the Numenorean beyond the norm of men was a mistake, however, marvelous the achievements and potential for wisdom could be.

So, while intermingling is clearly portrayed as a saving grace for Gondor, I would not say that it is so in purely population terms. Human populations can bounce back from civil wars and such very easily if the conditions are right. What intermingling did for Gondor more than anything was to introduce fresh blood and clean out tendencies for decadence and arrogance.

In the North, the Dunedain may have represented a larger share of the population, but both they and the other men of the region were much smaller in number—say, less than one-tenth that of Gondor—especially after the War of the Last Alliance and the Disaster of the Gladden Fields.

Besides Breelanders and such, the only other people or allies that we know of were of Edainic, probably Beorian-related, decent. Of those remaining in Arthedain and Cardolan in the Third Age, I would submit that they did intermingle and were assimilated among the Dunedain of the North: although this evidently did not really include the royal line, which allowed for a truly remarkable person like Aragorn to arise.

But the people of Arnor in general were always simply just too thin on the ground to not decline in the face of Angmar and pestilence. Intermingling, had there been sufficient numbers with which to interbreed, might have helped, and early on, the Dunedain there were prone to the same excesses and self-inflicted sources of decline that occurred with the Kingdom of Gondor, but it was a ultimately a story of strategic vulnerabilities, bad fortune, and the numbers game working against Arnor.