I never said we had a need for borders and I didn't say that nations were natural, I said they developed naturally. Actually you could argue that they are natural since various species mark their own territory that they then defend from others of their own species. A nation could just be a more advanced form of that.
I already said that your conception of the ontology of nations is ideologically loaded - its a form of social grouping sure, but it's not natural because animals do not conceive of legal systems to designate who gets to stay in the forest based on their legality as an entity, and if you think competition over resources is one such reason then I implore you to investigate symbiotic and mutually beneficial social structures in animals, such as with crows. A nation is not the natural outcome of a set of social formations, it is an arbitary grouping that developed historically based on contingencies. The Germans penned it, as did the Dutch for the concept of a free-city - another social grouping that works on the opposite end of free entry to anyone provided that they seek to trade and obey the rules. Both are equally as arbitary. For a third example we also have medieval commons, which worked based on cooperative collective ownership and were relatively open to newcomers, not bound by kinship relations. Again, another social formation that developed along a social grouping, again, arbitrarily when there other possibilities available.
None of these were prescribed in the first interactions of humans, and all of these developed based on the material conditions that they found themselves beholden to. by extension we don't know what sort of formation animals would take given sufficient development, least of all due to essential differences in cognition or whatever you think separates us from animals. A nation-state is arbitrary in this manner, since it's set up to benefit a particular grouping over others within it under the justification that it's for everyone else's good when the material circumstances can suggest other groupings. The concept thereof developed during the industrial revolution where it became quickly evident that there exists enough food and housing to take care of everyone, it just needs to be built and cultivated efficiently. Yet at the same time, resources were determined over the distribution and control of capital, so a necessary balance of material access was required by the structure (this is seen on the level of law, and how burghers sought to develop it on the basis of mercantile law). My beef is that in your formulation, it came as a natural development from a non-nomadic social modality, that immediately came with the caveat that its resources will be exploited unless fenced off, and that this mentality is what led to the nation state. Development is not a straight line, and it isn't predictable. That we arrived at nations is a historical consequence, but its development was not natural, but specifically constructed and developed over a long period of time. I refuse its natural ontology because I think its artificiality is cause to imagine and propose alternative modes of social organisation, away from such notions. If we hold ourselves to believe that it's a natural outcome, then we bind ourselves to be limited by the organisation it bestows upon us. History reveals the opposite, of always a multiple possible formations, yet the ones we have right now were determined arbitarily rather than necessarily. As for the need of borders, it is entailed if the origin of a nation-state is predicated on a fear of exploitation by the 'other',- defending your group as a necessity from others of the same group - since I remind you that this started over a discussion about migration and the ethics of nation-states. If you wanted it as a separate point, then you should have outlined it as such, since I can't take it in any other way than saying that borders are the natural outcome of states and that states developed naturally from the formations of the earliest human groupings.