Can anything be unnatural? Often, we describe human activities as unnatural; I do it myself. But does that make sense? The real distinction, if one can be drawn, must come between the ways that animals and humans adapt their environments. Are these just different instances of the same principle - say, quantitative differences - or are they better understood as separate phenomena altogether? All creatures adapt their environments. Trees absorb carbon dioxide and release oxygen. Small furry animals eat leaves and bugs and then pass feces. But the behaviour of humans certainly seems to fall into a different category on account of the sheer magnitude of changes we wreak.
The analogy of beavers may be particularly revealing here. Beavers chop down young, healthy trees so that they can create an environment that is conducive to their safety. By building dams, beavers 1) raise the water level so that their adaptation to swimming (including their ability to stay underwater up to 15 minutes) can be put into practice, and 2) create lodges inside which they can escape from predators. Building these dams disrupts the flow of water in a stream, creating a large pool upstream from the dam. It is clear that the niche created by these pools is filled by other creatures, but it is equally clear that this is not why the beavers started building the dams. Certainly the scale of disruption of beavers building dams is miniscule compared to the human practice of farming, but are they qualitatively or fundamentally different?
However disruptive our activities are to other animals, it certainly seems that we are animals ourselves, and products of the natural world. If we farm, then farming is natural, just as building dams is natural for beavers. If farming is more destructive than dam building, it is simply a function of our more elaborate tool-making abilities. But the reason that a micro-ecosystem thrives upstream from a beaver dam is that the beavers built dams, the dams created a niche, and the niche was occupied by enterprising organisms.
Taking the long view, which is the only appropriate view from an evolutionary perspective, the behaviours of an individual species seem to balance themselves out. A species that consumes faster than its environment can produce will run out of food and perish or at least diminish in numbers. This has happened before, and it will happen again. Sometimes, overconsuming species bring other species down with them. However, the rest of the world goes on its merry way. In terms of a cataclysm, on more than one occasion in the past, 90% of life on earth has been obliterated. The rest of the organisms fall over themselves to fill the available niches, and the earth continues to go on its merry way.
Please allow me to be abundantly clear on what I'm saying here: I'm not arguing that destructive farming practises are somehow absolved through their being "natural," nor am I arguing that the evolutionary perspective is the only or even the most appropriate way of approaching these kinds of issues. The big difference between humans and animals is that our actions have an analytical and an ethical component that the actions of animals do not appear to have. As a result, we can decide that the needless destruction of species due to human self-interest is a tragedy that should be avoided at all costs. Regrettably, we often do not use of our ethical and analytical resources in looking at the consequences of our actions. That is, we stumble around blindly, no better than the mythological lemmings that march en masse into the lake every few years.