The argument that humans have any sort of ethical duty at all assumes that humans and animals are fundamentally different. Animals don't have any ethical duty to prevent their food from suffering.
EDIT: Furthermore, human suffering is categorically different from animal suffering because human suffering can be inter-generational and have very long-term consequences. Animals don't transfer their suffering and their morality between generations, and that fact does make human suffering much worse than animal suffering, because human suffering can lead to arbitrarily catastrophic consequences. A tiger biting a human can lead directly to the complete extinction of the tiger. The emotional content of the suffering my be similar, but the consequences are not.