The Galapagos are desert islands. Along the shore, the rainfall rarely exceeds 20 inches per year the same as San Francisco, California. Hence plants adapted to dry, hot conditions thrive in the Galapagos, particularly cacti. In fact, most Galapagos cactus species are found nowhere else on Earth. Included among these are the world's largest cacti species the tree cacti, which belong to the prickly pear family.