I'm not sure I see what is so hard to understand about the Rabbit/Fox scenario. If they both were intelligent creatures, it seems pretty clear that there would be no moral justification in eating the rabbit, and the fox would be obligated to seek out another source of food. If you were to stipulate that the rabbit is the only source of nourishment available to the fox, this still in no way justifies murder. The fox would have a moral obligation to starve to death. The only remaining problem would be whether the fox has an obligation to his species to survive and procreate, but that is a claim that Eliezer already explicitly rejected.

Of course this reasoning only works with species largely similar to our own. I'm still not sure if it would be applicable to species which exhibit no sense of individualism.