We are familiar with the term "racism," defined as a prejudice or attitude of bias in favor of the interests of members of one's own race and against those of members of other races. "Sexism," too, is familiar - a prejudice or attitude of bias in favor of the interests of members of one's own sex and against those of members of the opposite sex. Less familiar to most, however, is the term "speciesism," which Peter Singer invented and which he describes as "a prejudice or attitude of bias in favor of the interests of members of one's own species and against those of members of other species." According to Singer, speciesism is a moral mistake akin to racism and sexism.
I think Singer is wrong. There are good philosophical grounds for thinking that human beings should be accorded a greater moral status than non-human animals. In terms of goodness, all human beings share a nature that is greater in goodness than the nature of non-human animals, such as dogs. Humans and non-human animals are alike in having existing, living, and sensing natures, but we know that only humans among all the animals also have a rational nature. Thus, human nature is greater in goodness than non-human animal nature, since it has everything of non-human animal nature and more. Because of their greater goodness in nature, human beings should be loved more than non-human animals. The fact that we should love human beings more is another way of saying that human beings have greater moral status than non-human animals.
Speciesism therefore differs radically from both sexism and racism. Since male and female human beings share equally in human nature, they are equally to be loved. In a similar way, the various races of human beings all share in the same human nature, so they too are to be equally loved. Thus, there is no inconsistency in defending speciesism but condemning sexism and racism. Equality in nature provides grounds for supporting the equal basic rights of all members of the human species, but denying such rights to dogs and cats. Equality in nature also provides grounds for condemning sexism and racism.
Nonetheless, non-human animals have a nature greater in dignity than plants or non-living things do. It is thus a moral error not to love non-human animals as we should, in a manner appropriate to their particular dignity. As Shelly Kagan pointed out in his recent critique of Singer's views on speciesism, virtually no one holds that only the interests of human beings count and that the interests of non-rational animals count not at all. No one thinks that arbitrarily setting fire to a cat is right or permissible. Nor does anyone think that trivial human interests outweigh any animal interests whatsoever. The human desire to find out what a cat on fire sounds like would never justify dousing Mittens with gasoline and lighting a match. Properly loving the goodness of animals - a lesser goodness than that of human beings, but a goodness nonetheless - is incompatible with inflicting pain on them without sufficient justification.
Does holding that all human beings have basic rights constitute a form of speciesism, a bias in favor of one's own species? Not at all. I believe that I am a human being, but let's say I am wrong. Suppose I merely look like I am human, but it turns out that I am actually of the same species as Clark Kent/Superman. Clark Kent/Superman looks, sounds, and usually acts like a human being, but is really a Kryptonian. Let's say that today I discover my superpowers and realize that I am not human, but rather a Kryptonian. If I realized I were not human, I would still hold that human beings have a higher moral status than all animals lacking rational nature. I would realize that there are also other animals having rational nature, namely Kryptonians. The fact that human beings are (or are not) my own species is irrelevant to defending the view that all human beings have intrinsic dignity and basic rights.
How are persons to be treated? Thomas Aquinas held that we should imitate God, and that God relates to persons (beings with rational nature) "as objects of care for their own sakes; while other creatures are subordinated, as it were, to the rational creatures." A Kantian might say, persons are beings that deserve to be treated as ends in themselves rather than used simply as means. A contemporary philosopher might say that the good of persons gives us ultimate reasons for action rather than merely instrumental reasons. Put colloquially, persons must be loved but non-persons may be used.
These ethical principles do not claim that only human beings are to be treated as persons: These principles are open to the possibility that other species should also be treated as ends in themselves and never used simply as means. People of faith explicitly reject the idea that human beings and human beings alone have moral status as persons. For example, Christians believe in Divine Persons namely the Father, Son, and Holy Spirit; choirs of angelic persons such as Gabriel and Raphael; and legions of demonic persons such as Lucifer and Beelzebub. It is not being human that is necessary for being a person with moral status, but being an individual substance of rational nature.
Maybe other species of material beings exist - such as Kryptonians, Martians, or Atlantians - who merit respect as persons, or maybe humanity is the only such animal species. These are open questions, since being a human being is a sufficient but not a necessary condition for being a person. To claim that all individual human beings deserve respect is not to deny that other species deserve respect. If someone says that all women deserve respect, that person is not (even implicitly) denying that men and children also deserve respect.
Inasmuch as moral status has something to do with what the individual is - his or her nature - and nature has something to do with species, there is nothing illegitimate in taking species into account in making moral judgments. Speciesism is, therefore, nothing like racism or sexism.