Salon has an interview with Animal Liberation author Peter Singer.

So the argument is that this is also an arbitrary stopping place; it's also a form of discrimination, which I call “speciesism,” that has parallels with racism. I am not saying it's identical, but in both cases you have this group that has power over the outsiders, and develops an ideology that says, Those outside our circle don't matter, and therefore we can make use of them for our own convenience.

That is what we have done, and still do, with other species. They're effectively things; they're property that we can own, buy and sell. We use them as is convenient and we keep them in ways that suit us best, producing products we want at the cheapest prices. So my argument is simply that this is wrong, this is not justifiable if we want to defend the idea of human equality against those who have a narrower definition. I don't think we can say that somehow we, as humans, are the sole repository of all moral value, and that all beings beyond our species don't matter. I think they do matter, and we need to expand our moral consideration to take that into account.