@df
That piece on ethics of immigration was an interesting read. I was going to post some thoughts there, but they ended up long, so I'll post them here instead:
In a finite world, this gets inevitably tangled in the politics of reproduction and overpopulation. Some seek to tactically out-reproduce as a way of politically dominating, or to fear that others will, but even more basically, if we ignore such factions and stick to sheer numbers: We can be fruitful and multiply only to a point in a finite world. At some point, multiplying isn't fruitful but exhausts all fruit.
And so when people move around in a sparse world, there there may be more capacity to absorb them than in a dense one, regardless of how you define words like sparse and dense and capacity. And, of course, density is distributed unevenly. Certain standards of living or ways of living will not support more than a certain number of people. So questions arise as to whether it's the right of a people or a region to have any such standard or way of living.
Without a theory of why people would taper off reproduction as the world becomes crowded, the politics of immigration becomes tethered to the politics of reproduction. Birth itself is a kind of immigration, and sometimes people who want to taper immigration seem to nonetheless want to encourage birth, without seeing or acknowledging that each of these increases population density and strain on finite resources. In an "empty" world, the risk is of having too few people, and both birth and immigration seem obvious opportunities to mitigate risk. In a "full" world, the risk is of having too many people, and both birth and immigration seem points of concern and opportunities for risk to attach.
Of course, density is not distributed evenly in the world, so these concepts apply differently in different places, but it simplifies discussion to keep things simple. To have a coherent ethics on this, one must know where one is in the empty/full spectrum. And, of course, all gets even more complicated if one realizes that people will inevitably overlay race, religion, wealth, or nationality, but the essential problems are best seen if you hold those at bay long enough to understand it is not really possible to ignore overpopulation as a core driver of the stress. In some sense, those other factors just create ways of magnifying, partitioning, and sadly scapegoating that stress.
Discussions of both reproduction and immigration are intrinsically difficult because they get quickly into issues of how we may fairly distribute finite resources in an ever more crowded world as cultures with very different and deeply held practices and expectations come ever more inevitably into contact and, too often, conflict with one another. Achieving any sense of fairness can be a nontrivial task because each is starting from a different place and there is often no commonly agreed starting point for what can be expected to be fixed and what must be up for negotiation. It probably have to start with everyone agreeing there's a real problem and that we all need to be equitably involved in a solution, but I sense there's a lot of denial on both of these points.
Greg Cocks 
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An archaeological study of human settlement during the Final Palaeolithic revealed that populations in Europe did not decrease homogenously during the last cold phase of the Ice Age. Significant variation in regional population sizes indicate differentiated reactions nested in an overall shift of settlement areas towards the east
