What is there less of at the top of a mountain?

Most mountains, the higher you go the colder it gets, the thinner air, less food and water sources, so its hard for any life to exist on the tops

Much of the life forms on Earth cannot stand the dry, cold conditions in high altitudes. humans, for example, can't stay up there for long because of the decrease in oxygen. plus, there are no suitable grounds for agriculture in higher altitudes. Another reason would be there is much less UV protection. Acute Montain Sickness caused by hopoxia-a lack of oxygen in the blood) affects over half of lowlanders who spend more than a few hours above 3,500 metres (11,480 ft).