Cactus can obtain the required amount of water from the desert ground unlike other plants. Therefore they are able to stay alive in that given ecosystem.
Camels drink lots of water and then can go without days with it. They feed on the desert grasses and cacti that live in the desert. The cacti and grass can survive, because of a waxy mixture on them,...
All types can live on cactus, its only prickly on the outside, but lovely and juicy and waxy on the inside. Wild boars, rabbits, I guess cows would, and a lot of other desert animals could feast on...