The argument from marginal cases claims that you can't both think that humans matter morally and that animals don't, because no reasonable set of criteria for moral worth cleanly separates all humans from all animals. For example, perhaps someone says that suffering only matters when it happens to something that has some bundle of capabilities like linguistic ability, compassion, and/or abstract reasoning. If livestock don't have these capabilities, however, then some people such as very young children probably don't either.
This is a strong argument, and it avoids the noncentral fallacy. Any set of qualities you value are going to vary over people and animals, and if you make a continuum there's not going to be a place you can draw a line that will fall above all animals and below all people. So why do I treat humans as the only entities that count morally?
If you asked me how many chickens I would be willing to kill to save your life, the answer is effectively "all of them". [1] This pins down two points on the continuum that I'm clear on: you and chickens. While I'm uncertain where along there things start getting up to significant levels, I think it's probably somewhere that includes no or almost no animals but nearly all humans. Making this distinction among humans, however, would be incredibly socially destructive, especially given how unsure I am about where the line should go, and so I think we end up with a much better society if we treat all humans as morally equal. This means I end up saying things like "value all humans equally; don't value animals" when that's not my real distinction, just the closest schelling point.
[1] Chicken extinction would make life worse for many other people, so I wouldn't actually do that, but not because of the effect on the chickens.
I also posted this on my blog.
I would need to know (or have some prior for) which species, or animals of which species, are affected by policies A and B. I would give very different odds for monkeys, cats, mites, and moss.
Also, individuals (whether human or animal) are a natural sort of thing to assign moral value to. But "species" are not. Species are defined as "groups of individuals, all of whom are capable of interbreeding". (Even then there are exceptions like ring species, and also things like parthenogenic clans; it's not a definition that cuts reality at its joints.)
There is no particular reason for me (or, I think, most people) to care about an animal in inverse proportion to its number of potential mates (= size of breeding group = size of species). I do care about variety, but the 5500 or so known mammal species are far more diverse than many sets of 5500 different insect species, for instance. And the set of all rodents (almost 2300 species) is far less diverse than the relatively tiny set of (Chimpanzee, African elephant, Great white shark). Being separate species is incidental to the things we really care about.