Addendum: Smith appears not to understand the animal-rights movement. He writes:
The animal rights movement grew out of the same poisonous soil. Animal rights ideology holds that moral worth comes with sentience or the ability to suffer. Thus, since both animals and humans feel pain, animal rights advocates believe that what is done to an animal should be judged morally as if it were done to a human being. Some ideologues even compare the Nazi death camps to normal practices of animal husbandry. For example, Charles Patterson wrote in Eternal Treblinka—a book specifically endorsed by People for the Ethical Treatment of Animals—that "the road to Auschwitz begins at the slaughterhouse."Animal-rights advocates do not believe "that what is done to an animal should be judged morally as if it were done to a human being." What they believe is that animals matter, morally. Animals have weight on the moral scale. Morally speaking, animals are something, not nothing. Inflicting pain on animals must be justified. This is not to say that it can't be justified, only that it must be.
Addendum 2: Smith wants the circle of moral concern to be the same as the circle of biological humanity. In his view, neither animals nor plants have rights. He seems to think that if we expand the circle to include animals, we will have to expand it, on pain of inconsistency, to include plants. This is false, for there is, as I say, a morally relevant difference between animals and plants that justifies drawing a line between them. The circle of moral concern should include all sentient beings, not all living organisms.
Addendum 3: When I was a law student at Wayne State University in the early 1980s, I took a graduate philosophy course in ethics with Bruce Russell. He allowed me to write a term paper entitled "Do Plants Have Rights?" Little did I know that I'd be coming back to that topic a quarter of a century later!
Addendum 4: Smith should grapple with the biocentric arguments of Paul W. Taylor in Respect for Nature: A Theory of Environmental Ethics (Princeton: Princeton University Press, 1986). It is one of the best books I've read. It sounds to me as though Europeans are taking Taylor's theory seriously.