[Brainwalter]

Humans are biological creatures, and as such our only biological imperative is to ensure our own survival and that of our progeny.

It also appears that humans are the only self-aware beings in existence. This enables only humans to act as moral agents. Discussing morality as it relates to livestock or nature is irrelevant, as animals and plants cannot return moral judgment, and quite possibly have no awareness when they are "wronged".

In a similar vein, only humans act purposefully based on reason, therefore only humans can be engage in economic activity.