So I'm claiming that a bias in favor of human beings is analogous to a bias in favor of individuals based on their race or sex. The label of "speciesism" is meant to bring to analogy to the forefront, in a way that "anthropocentrism" does not. Central to the accusation of speciesism is that this bias is a deep-seated disposition that permeates many other beliefs, just as racism and sexism do. Thus it is unhelpful to the anthropocentrist's case to argue from intuitions about certain cases, because part of the speciesist charge is that we should expect our intuitions to be biased. What the defender of speciesism as a legitimate viewpoint needs to explain is how a difference in species could be structurally significant to the assignment of moral categories. This task, I believe, is insurmountable.
These debates have been hashed out pretty thoroughly over the past forty years, but I will give a brief summary of how they tend to go. The anthropocentrist proposes a certain quality or set of qualities that humans possess (such as capacity for moral deliberation, or have complex relationships, or the ability to make plans) that animals do not, and argue that this gives them higher moral status. The animal rights theorist reply is two-fold. First, there's no prima facie reason to believe that such qualities give rise to superior moral status. Second, and more definitively, whatever qualities the anthropocentrist can name, not all humans possess these qualities, and some animals might possess them. Some humans are quite young and have not developed yet, and some have impairments that prevent them from having certain abilities. Yet we still allow that all human beings are entitled to fundamental moral rights.
Some defenders of anthropocentrism try a very ad hoc move. They argue that it's not about possessing those certain characteristics, it's about having some kind of "species-potential," that makes you a member of the rights-bearing group. It's hard for me to see how this can have any plausibility at all, but some people do hold such a view. There is, however, no reason to suppose that anything like "species-potential for certain traits" could have significant moral implications. No one supposes such a thing until they've been backed into the corner by the argument in the previous paragraph (which is why it is ad hoc.) The reason it may sound plausible to some is that it backs up their intuition of human superiority, but I've already explained why that is inadmissible.
There is another direction to go, and if someone were to try to take this tack with me in personal conversation, I would probably walk away. One could stick by the claim that it is certain qualities that give human beings their moral rights, and that those humans who don't possess those qualities don't have the fundamental moral rights that other humans have. They will then likely argue that we feel sentimental towards those who are intellectually disabled, or babies, or whomever, and so we should treat them well because of that. But that is completely wrong. We, quite obviously, have strong moral duties towards such individuals, dependent on their intrinsic features, not on our particular emotional whims. To suggest that it is some act of generosity that we consider them members of the moral community, or that they are candidates for charity rather than justice, is deeply offensive. Thankfully, this is not a view that is often defended.
Other claims have been made, all on weak ground. Some claim that we are part of a human community, and we only have (strong) obligations to members in our community. This begs the question. We're trying to answer, "Who should be among the individuals we consider in the moral community?" It's no argument to simply assert that only humans belong in that category. Similarly, in an essay called "The Human Prejudice" Bernard Williams writes of animal rights theorists,
"They suppose that we are in effect saying, when we exercise these distinctions between human beings and other creatures, that human beings are more important, period, than those other creatures. That objection is simply a mistake . . . These actions and attitudes need express no more than the fact that human beings are more important to us, a fact which is hardly surprising."
Such an claim could just as easily be made in favor of giving significantly less weight to the interests of intellectually disabled humans, and most of us would rightly reject it. This is just the affirmation of the speciesist bias, not a defense of it.
I'm not assuming here that the speciesist position is incorrect. There might be a rational basis for the prejudice that Williams is describing. However, because there is reason to be deeply suspicious of prejudices, Williams needs to provide a strong argument for the moral importance of species membership, that could not apply to racism, sexism, ableism etc.
It may appear as if I'm picking and choosing weak arguments. But I am not. Consider an analogous moral question, which I imagine most of my readers share my view on, gay marriage. The arguments against gay marriage are surprising parallel to the arguments against animal rights. Opponents to gay marriage often say that there is a unique quality that heterosexual couples possess, such as the ability to have children, which give them the right to get married. Proponents of gay marriage say that it's morally irrelevant quality with regards to state protection of romantic unions, and further that many heterosexual couples choose not to or are unable to have children (yet obviously should be allowed to get married.) Opponents then make a weak and unpersuasive argument about "potential," when clearly it's not potential that matters. These days, opponents to gay marriage often are just defending their right to hold such a view (a right that has never been challenged). That's because there's really no further step in the argument to make. Once you start claiming that there's some kind of special moral significance to the "potential" to have some quality, it's clear that you've run out of substantive moral ground.
Why is "potential" as irrelevant as I claim? In part, it's because potential is an extraordinarily vague concept. There's a sense in which I have the "potential" to become a pianist, and also a sense in which I have the potential to become the first man on Pluto, or the first walrus-human hybrid. It's not clear what kind of potentials are supposed to matter for moral argument, other than the ones stipulated by those trying to defend prejudiced views. In fact, what matters are the actual qualities I have. I'm a citizen of the US, who is of age, so I should be allowed to vote here. I go to work and provide services for a company, so I'm entitled to my paycheck. Most importantly, I'm a sentient being for whom things can go better or worse, and so I'm entitled to recognition of my moral rights.
Species itself is a vague concept. Speciation comes about gradually, and is only rather loosely defined. When thought about this way, it's hard to see how it could have such central moral significance. Rather, I believe, it's the intrinsic features of individuals that should determine whether or not that are considered members of the moral community, and have rights. The right to be eligible for a driver's license might require some pretty sophisticated capacities. But to be entitled to the right not to be unjustly killed or used, all that one has to be is sentient.
Speciesism is difficult to defend, because it is structurally identical to other irrational prejudices. In fact, most of the arguments I've discussed are not actually arguments in favor of speciesism, because once you start naming other qualities that you think justify prejudice against non-humans, it's those qualities that are important, rather than species membership. People make these arguments because, despite the deep-rooted nature of our prejudice, it's not hard to see that it is irrational.