The issue I see with that is that it defines "natural" in a way that is both useless, because it literally applies to anything in existence, and doesn't fit the way people generally use it, that being something more like "how things are when they have not been significantly altered by people". Under a more typical usage, human activity isn't natural by definition, not because humans are special in some way but just because the term natural has been arbitrarily created to describe everything except that activity.