Speciesism is structurally the same error as racism or sexism

Singer's foundational move is to define speciesism as an unjustified bias favoring the interests of one's own species over the interests of members of other species, and to argue this follows the exact logical structure of other forms of discrimination society has already rejected. Just as it was once assumed obvious that race or sex justified unequal moral consideration, he argues it is now assumed obvious that species membership justifies it — and that assumption deserves the same scrutiny.

He's careful to note he isn't claiming all animals are equal to humans in every capacity; the argument is about equal consideration of interests, not identical treatment. A being's interest in not suffering counts morally regardless of species, even if other interests (like an interest in voting or higher education) may not apply to nonhuman animals at all.

Takeaway: ask whether a practice would be acceptable if the same suffering were inflicted on a human — if not, species alone isn't a sufficient justification for the difference.