It's a Genesis thingy, every species is specialized for it's role in maintaining the equilibrium of the biosphere.
"Then God said: “Let the earth bring forth living creatures according to their kinds, domestic animals and creeping animals and wild animals of the earth according to their kinds.” And it was so. And God went on to make the wild animals of the earth according to their kinds and the domestic animals according to their kinds and all the creeping animals of the ground according to their kinds. And God saw that it was good.
26Then God said: “Let us make man in our image, according to our likeness, and let them have in subjection the fish of the sea and the flying creatures of the heavens and the domestic animals and all the earth and every creeping animal that is moving on the earth.” And God went on to create the man in his image, in God’s image he created him; male and female he created them. Further, God blessed them, and God said to them: “Be fruitful and become many, fill the earth and subdue it, and have in subjection the fish of the sea and the flying creatures of the heavens and every living creature that is moving on the earth.” Ge1:24