Separating things into categories so that proper labels can be applied may be the mark of a subtle and careful thinker, but at the same time, it’s unclear to me what value such distinctions may actually grant. Yet it was with some chagrin that I witnessed in the comments at a blog I follow someone yet again trotting out a familiar canard asserting that because mankind (or humanity) is both product and part of nature, everything we do on the planet is no less “natural” than that of, say, an ant colony, troop of monkeys, or other animal social group. Under such as view, the entirety of activity on Earth is a “natural” outgrowth of processes already underway for eons. I haven’t researched what others have to say about the issue, which I’m certain has been addressed before, but at the risk of reinforcing valueless categories, I would suggest that to collapse everything into one category — natural — is to make a mistake of metonymy, where nothing can ever be “unnatural.”
In a relatively mild example, we distinguish between natural and artificial light readily and uncontroversially. There are numerous light-emitting organisms, but to my knowledge, most possess bioluminescence to attract prey and for rudimentary communication rather than for illumination the way mankind deploys artificial light through synthetic means. Composite pictures of the Earth at night (from space) depict “unnatural” light in the respect that it arises not out of biology but from human artifice, which is several steps removed from the machinations of Nature (capital N). Similarly, numerous other features visible from space result from human interventions and impacts, such as the oil slick in the Gulf of Mexico from the Deepwater Horizon disaster, soil erosion off the coast of Madagascar, or more humorously, the Univ. of Michigan football stadium. (That the Great Wall of China is visible from space is only an urban legend.) There are also many examples of synthetic plants (and some animals) created through scientific (read: man-made) intervention, not out of blind, evolutionary processes.
A worthwhile debate might ensue if the argument were simply about the degree of impact a species has on its environment. A keystone species is recognized for being an architect or engineer of its own environment, typically through its patterns of consumption and predation or its passive contributions to the persistence and disappearance of others species in an ecosystem. Examples include sea urchins, mussels, weevils, sharks, prairie dogs, mule deer, beavers, grizzly bears, and elephants. Humans are notably absent from the Wikipedia entry on keystone species, but there can be little doubt that from among the panoply of known species, the degree of our impact on the ecosystem as top predator and worst polluter/destroyer is well beyond that of any other. Also, to say that abandoned radioactive zones around Chernobyl and now Fukushima are natural would be an unnecessary stretch. Curiously, wildlife around Chernobyl is teeming with the top predator gone. The significance of plants, insects, and microorganisms lie beyond my expertise, but I find their omission from the Wikipedia entry similarly problematical. For instance, marine algae and plankton (algae blooms also being visible from space) are both the basis of the food chain and produce 70 to 80% of atmospheric oxygen.
Where the waters get really murky, however, is consideration of human behaviors and cultures. The debate is whether they arise out of nature (human nature or more generally Nature) or are purely abstract — products of human ideation. And it is here that useful categorical distinctions may break down. We are both shapers of and shaped by our environment. Teasing out distinctions between purely mental phenomena (e.g., fiat money and artistic creation) and behaviors observable throughout the animal kingdom (e.g., predation and survival instinct) doesn’t exempt humans from nature but also doesn’t account for the complexity of human institutions as something fundamentally apart from the wildness of nature as we typically understand it.