By your reductionist logic, the word "natural" would mean practically nothing. The Moon and Lunar Excursion Module are both equally "natural." Even abstractions like "knowledge" or "security" are just human inventions and thus products of the ecosystem of which we are part.

Humans are NOT nature, though at least partly a product of it. We regard creations of human intellect, creativity and mechanical inclination as something other than purely natural. We therefore distinguish products of artifice from those of nature. Whether processed foods, the factories where we make them or the stores in which we house them are artificial is indisputable. A better question might be whether beaver dams or anthills are natural. For my part, I would say they are simply because neither require tools. Either way though, that is still far more debatable than whether something created in a plant rather than grown on one is natural.

natural

Pronunciation: /ˈnatʃ(ə)r(ə)l/
adjective
1 existing in or derived from nature; not made or caused by humankind:
carrots contain a natural antiseptic
natural disasters such as earthquakes
having had a minimum of processing or preservative treatment:
natural food
our nutritional products are completely natural
(of fabric) having a colour characteristic of the unbleached and undyed state; off-white.

http://oxforddictionaries.com/definition/natural

artificial

Pronunciation: /ɑːtɪˈfɪʃ(ə)l/
adjective
1 made or produced by human beings rather than occurring naturally, especially as a copy of something natural:
her skin glowed in the artificial light
an artificial limb
artificial flowers
(of a situation or concept) not existing naturally; contrived or false:
the artificial division of people into age groups

http://oxforddictionaries.com/definition/artificial

Red highlighting is mine. Anything man made is not natural, because natural does not mean simply "part of an ecosystem," but specifically "not man made."