(2) "Thousand-mile levee breaks down in an ant nest" means that a small ant nest can make a thousand-mile levee break down, indicating the impact of organisms on the environment.
(3) The desert is dry and short of water, with great transpiration. In order to adapt to the fleshy stems in the desert, cactus can store more water, and its leaves become thorns to reduce transpiration and water loss, so cactus can adapt to the dry and water-deficient environment in the desert.
So the answers are as follows: ① Environment affects organisms;
(2) the influence of organisms on the environment;
③ Biological adaptation to the environment