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.
Ultimately, it seems as hard to come up with a ratio of X:Y:Z as it would be to come up with a personal valuation ratio of Apples:Oranges:Education:747s:Laptops.
You are taking morality, which is some inborn urges you have when confronted with certain types of information, urges which started evolving in you long before your ancestors had anything approaching a modern neocortex, and which absolutely evolved in you without any kind of reference to the moral problem you are looking at in this comment. And you are trying to come up with a fixed-in-time, transitive, quantitative description of it.
In the case of Apples:Oranges, the COST of these to you in a store may be close to constant, but their VALUE to you are all over the map: sometimes the Apple wins, sometimes the Orange wins, often the Laptop wins, and when the 747 wins it wins big time.
It seems likely enough that your moral urges would be all over the map, variable in time. And that your effort to summarize them completely with fixed static numbers makes less sense than describing nature using the four elements of earth, air, fire and water.