I think I see your point , and I think it depends at which level we are talking. At the level of molecules, I would say it is still possible to make the difference between living and non-living things. If you go deeper into the matter, then it becomes irrelevant, where everything is a question of energy.
At the level of dense matter, although I agree that ressources are all natural (nature is all there is to life), we can still make a difference between living and non living.
Biological or mineral molecules are different but they are all part of the natural cycle of: birth- growth/expansion-death/decay/metamorphosis.
But from them human have created synthetic molecules which seem to be "rejected" from that cycle. One big example is plastic, it loses its structure and eventually turns into microscopic elements, but no natural entity (living or not) seem to be capable of using it.
Concerning robots, if human is not there, I doubt that nature would see in those creatures something else than a bunch of natural elements plus "other non-usable things".