Appeal to nature is a simplified type of naturalistic fallacy in argument form.
An appeal to nature fallacy consists of a claim that something is good or right
because it is natural, or that something is bad or wrong because it is unnatural.
The Appeal to Nature is a common fallacy in political arguments. One version consists of drawing an analogy between a particular conclusion, and some aspect of the natural world -- and then stating that the conclusion is inevitable, because the natural world is similar:
"The natural world is characterized by competition; animals struggle against each other for ownership of limited natural resources. Capitalism, the competitive struggle for ownership of capital, is simply an inevitable part of human nature. It's how the natural world works."
Another form of appeal to nature is to argue that because human beings are
products of the natural world, we must mimic behavior seen in the natural world,
and that to do otherwise is 'unnatural':
"Of course homosexuality is unnatural. When's the last time you saw two animals of the same sex mating?"
A recent example of "Appeal to Nature" taken to extremes is The Unabomber Manifesto.
Animals kill animals, why shouldn't we kill animals?