I don’t think anybody is saying there is well defined distinction. Technically people don’t need much more than the minimum amounts of food, water, and oxygen required to sustain life. Housing, clothing, and sanitation (among the other basic necessities) go a long way to improve the quality, but most animals go without. A more relevant example perhaps is what people want to eat versus what they need. I may want steak, but do I really need it? Maybe I could subsist off rice and tofu, but my desire for meat overrides any compunctions I have (which are basically nonexistent) about implicitly supporting the slaughter of beef cattle. There are those who would claim this is unethical, that I am benefiting from the destruction of another creature, but I have no problem with that–it’s the natural order of things from my perspective.
Continuing from that line of thought, that I need food and I choose to raise cattle or grow corn, I would see no ethical problem with killing wild animals that try to eat my livelihood, whether it be wolves or crows. If, however, I were to hunt wolves just to leave them rot in the woods because I thought it was fun, someone could easily draw an ethical line between those two actions. Thus the conclusion that as we retreat further from killing for biological necessity or mercy into killing for pleasure, greed, etc., we usually find ourselves in areas of grey or dark terrain.
Do bees or ants have ethics? 
Actually I think how ethics are derived is really the point here. (Pjen: “I find it difficult to construct a rational and socially acceptable and practical framework that is internally consistent.”)
Are ethics beneficial? Of course–they are society’s lubricant. Are they a necessity for individual human survival? I’d argue no, not at all. What ethics does a man living alone in the woods need?
It all depends upon your definition of “need”. We need them for society to operate efficiently and the general welfare of humanity; we do not need them for survival. In that way, they are a luxury, admittedly more so when we drift into situations that do not involve the death of one of the parties.
Are you sure about that? ISTR duelling being tacitly accepted at some point as a means of resolving disputes. Or was it always illegal in every culture?

Well, if humans shared a hive-mind, there would be no reason for laws or a social contract. Individual hives do wage war on each other all the time, though.