begs a very important question as part of its central premise. Even given the idea that animals have moral weight, it does not follow that there exist a number of them that are of equal moral weight to a human (or another kind of animal, etc). There exist infinite sequences that converge to finite values.
It seems pretty clear to me that moral weight is not linearly additive. This is why we consider it worse when a species goes from 1,000 members to 0 members than when it goes from 1,001,000 members to 1,000,000 members, for instance.