Most of the time, I feel that my task is to demand that the speaker think more critically about
1. why we should prefer a natural to an unnatural state since, for example, a world without smallpox seems to be a better world overall;
2. whether we can really ever identify a “natural” state, after millennia of human disturbance in most tropical and temperate regions and with the propagated effects of climate change, pollution, and transported species reaching even into apparent wilderness regions.
But to be fair, the shorthand of preferring natural conditions to unnatural ones is very often legitimate. Whether our concern is for human well-being or stability in ecosystems, the natural state is one that has been tested and proved, and the unnatural one has not. The proper justification, then, for this preference for what is natural is empirical evidence, not metaphysics.
Critics of the popular desire for political action on global warming like to point out that there are plenty of people who stand to be better off should their climate grow a little warmer. It will be possible to grow more wheat in Canada, for instance, and the Northwest Passage will become a viable alternative to shipping goods overland or through the Panama Canal. While it’s true that human changes may improve the world for some human goals, the fear is that they will disturb delicate systems with uncertain, and probably unwelcome, results.
A recent report in Science (Araki, Cooper, and Blouin, “Genetic Effects of Captive Breeding Cause a Rapid, Cumulative Fitness Decline in the Wild”, Science 318: 100-103) adds another case to the roster of well-intended interventions with negative long-term implications. It seems that captive breeding and wild release of fish stock may lead to lower reproductive fitness in only a few generations.
The report concludes
The evolutionary mechanism causing the fitness decline remains unknown. We suspect that unintentional domestication selection and relaxation of natural selection, due to artificially modified and well-protected rearing environments for hatchery fish, are probably occurring…To supplement declining wild populations, therefore, repeat use of captive-reared organisms for reproduction of captive-reared progenies should be carefully reconsidered.
This indicates that the study species--steelhead trout--are being domesticated. The alternative to stocking wild populations is to do more (much more) to prevent decline in the first place. This means reducing how many are taken and/or preventing habitat destruction. Neither is easy or popular.