The objection seems to me to suggest that as living organism we need other living beings for source of food for basic existence. So should we abstain from consuming animals or should the lion not eat the rabbit, starve and die for the sake of morality?
I anticipate that Singer's response to this objection will be of the following form: we as humans have reached a stage where we could become full vegetarians, get the nutirients we need and abstain from killing animals, and therefore, end their agony at our hands. Whereas before we did not have a choice, now we do. Hence, abstaining from eating animals is consistent with the claim that all beings capable of pleasure and pain are worthy of equal consideration.