In everyday common sense, as I've always experienced it, a beaver dam or hut, a bird's nest or a termite mound are generally considered natural, while a human house is considered artificial. Given that beaver dams and beaver huts involve quite a bit of logging and engineering, termite mounds involve digging and using termite-produced chemicals to solidify the material, and bird nests can involve a bit of either technique, what is it that makes a human dwelling, such as a simple log cabin, more "artificial" than these animal-built structures? Where does "natural" end and "artificial" begin?

Good question! Usually we label as "artificial" that which is an artifact of intentional, purposive activity. I am inclined to think that beavers and birds are purposive and they seem to have desires but perhaps we should be reluctant to attribute to them the full blown power of deliberation and intentionality. In any case, it may (as you suggest) seem arbitrary to see the beaver dam as natural and a simple log cabin as artificial even if the latter (unlike the dam) is the result of deliberate planning and creative intelligence. But perhaps there might be some point to arguing that some human artifacts are more natural or, using a related concept, ecological than others. Insofar as I build a log cabin that does not involve laying waste an entire forest, diverting streams, destroying the habitat of significant animals, and so on, perhaps we might see that as more natural insofar as it is more in keeping with the ecology of the region than if I destroy the forest and put up a parking lot that (let us...