Hungry chickens will devour tender greens and will over time denude their yard [what is the density of chickens necessary to accomplish this in different ecoregions?], particularly in proximity to their coop.
Other creatures like to harvest chickens too: Dogs, foxes, coyote, raccoons and raptors. Predator protection is necessary depending on the nature of the predator and how hungry for chickens they are. Protection can be incorporated into either the yard or the coop, or both.