How do the Black brothers farm the valley in the story, The King of the Golden River?

The Black brothers "farm" the beautiful Treasure Valley by killing "everything that did not pay for its eating." So fearful are they that their profits will be diminished by sharing what they grow with the animals that they "shot the blackbirds, because they pecked the fruit; and killed the hedgehogs, lest they should suck the cows; they poisoned the crickets for eating the crumbs in the kitchen; and smothered the cicadas, which used to sing all summer in the lime trees."