Since reindeer in southwestern Greenland have no naturally occurring non-human predators, harvesting quotas are established to help regulate the number of reindeer in an area and prevent overgrazing and death from starvation. During the winter, the great effort of pawing down (known as "cratering") through the snow to their favorite food, a lichen known as reindeer moss, can cost them too many calories in expended energy, causing them to lose strength and die. Without human monitoring and regulation, mass starvation of reindeer would be a recurring problem.