Fantasy novels are well known for skimping on details of ecology and economics; one often ends up wondering "what do these underground monsters eat?" or "what does this town in the middle of nowhere do?" (Actually, I often wonder that about real towns.) Tolkien is a mixed bag when it comes to this. On the down side, there are the underground orcs and goblins (The Hobbit makes it clear they raid a lot, but enough to feed their armies all the time?) and the millennia without sunlight in the prehistory of the Silmarillion, which Elves and trees somehow survived anyway, and the also-underground dwarves, and the lack of obvious Elvish farmers even after the Sun was created. On the up side, when, in The Lord of the Rings, Sauron is blotting out the Sun over Mordor and Gondor, and Sam and Frodo wonder how anything can live in Mordor's wastes, there's mention of the sunny fields around the lake of Nurnen, in southern Mordor, where masses of slaves toil to raise to food to feed the armies, which is more of a nod to ecological concerns than usually found.
But a related question is what people trade, and where people live. My thoughts along these lines start with Boromir, who in Fellowship seems to have made a heroic and to him unprecedented solo journey from Gondor to Rivendell, on information a thousand years out of date. A few lines in the Unfinished Tales suggest that Chris Tolkien thinks Boromir's physical courage and prowess was not fully emphasized in the passage. Yet Gandalf and Aragorn have made similar or harder journeys multiple times. Of course, they're even more extraordinary than Boromir. Elves such as Celebrian have gone from Lorien to Rivendell, but those are High Elves, and Celebrian did get captured and abused by orcs, proving that even Noldor aren't risk-free. But then there was Saruman's secret pipeweed trade with the Shire, and worst of all, the men who'd come up from the South to Bree when Frodo and his companions first arrive. One of them was a half-orc, presumably Saruman's, but presuming that the whole band wasn't actually Saruman's caravan then were did the rest of them come from? The books don't mention any population between Bree and the Shire in the North and Rohan in the South. The great middle expanse along the coast feels completely desolate, and the Fellowship doesn't seem to be making any effort to avoid people, there being no people to avoid.
So: Saruman and mysterious Southerners aside, Gondor hasn't contacted the North in forever (though it's been secretly contacted by Aragorn and Gandalf), nor has it dealt with Lorien or the populations attested to in The Hobbit up the Anduin, between the Misty Mountains and Mirkwood. Nor has there been contact via ship between Dol Amroth and the Grey Havens, though this would seem particularly easy. Gondor is claustrophobically isolated, though in itself bigger and richer than any other realm.
Up North, the situation is a bit different, despite the lack of documented settlements. Even in Bilbo's time dwarven trade (trading iron for food?) between the Blue Mountains and Bree/the Shire seems unextraordinary, and we see the dwarves again in Fellowship. Over in the East, there's regular trade between the Elvenking in Mirkwood and the Men of the Long Lake, and mention of men coming up the Running River from the South to pick up wine barrels. The river runs due south into the eastern edge of Mirkwood, then curves off to the great inland sea of Rhun. We have no idea if the men come from settlements near the Lake, or all the way from Rhun; I'll refer to their origin as "Rhun", to be suggestive but uncertain. It's not clear if the dwarves of the Iron Hills are part of this network, but even so, at a time when Gondor and Rohan haven't seen elves from Lorien in, like, forever, "lesser" men of the Lake and "Rhun" have been hobnobbing with elves on a regular basis.
Then, in Frodo's time, Bilbo is able to send to the Lonely Mountain and Dale for lots of birthday presents, and have them return. Then he himself goes and visits that area, and returns without incident to Rivendell. My inference is that there is now regular trade from the Blue Mountains to the Lonely Mountain, dwarves liking to keep in touch when they can. Which, when we put the pieces together, gives us a continuous trade network from the Blue Mountains in the West to the Long Lake and "Rhun" in the East, connecting four races -- dwarves, hobbits, elves, and humans -- mostly on the backs of dwarves.
I find the contrast with the feel of Gondor to be immense. The North has trade over a huge distance, through goblin-infested mountains, bridgeless Great River, and Evil Forest (though maybe the regular traffic went around the north of Mirkwood, like Bilbo and Gandalf on his first journey home) while the South (Gondor) can't manage to visit neighbors like Lorien, or cross the open land to visit the North.
I've learned some things since I wrote this. I think Dorwinion gets mentioned as part of the Long Lake trade network, northwest of Rhun, and the source of wine. Tolkien actually went to some work to justify the desolateness, like multiple (Sauron-related?) plagues, and a flood wiping out Tharbad no more than a century before Boromir's trip. Barbaric fisher-folk along the coast of otherwise empty Enedwaith and Minhiriath get mentioned in notes or a letter; also the Numenoreans having completely deforested those regions in their ship-building. At some point I recalled the Corsairs of Umbar, who might make marine contact between Gondor and the Havens more risky -- though "none at all" seems like an improbably thorough blockade. Though I don't know how much naval capability Gondor had after losing Umbar. I've read that Tolkien mentioned Noldor like Gildor making regular trips to the western-facing palantir in the Westmarch, to look to Tol Eressea or get glimpses of Elbereth. I also noted that in The Hobbit, when Bilbo goes to Rivendell it says he loved elves but rarely saw them, implying that he *had* seen them before, which again is more contact than anyone in Gondor or Rohan seems to have had.
Also, I seem to have failed to raise a key question: how was Thranduil paying for all that wine and apples from Dorwinion? The barrels go back empty... There's also mention of trade with their "southern cousins", which calls to mind Lorien, but with a long and dangerous land route, that'd be a crappy connection. I like to think it means more wood elves to the south east, somewhere along the Running River -- maybe in Dorwinion!