The sheep started to increase slowly then rapidly went straight up, then is leveled out because it ran out of resources and it reached carrying capacity.
Density-Independent does not effect the sheep like some other animal. Due to there large wooled coat cold weather does not bother them. But sometime sheep get heat stress because they are less tolerable than most animals.
Some Density-dependent factors for sheep would be availability to food. Also decease in a flock could bring down the population of the sheep.