Organisms hunt for survival and have evolved different ways of hunting. In nature, the weak and the strong may eventually become prey. This is the law of the jungle.

Parasitism means that one benefits the other and the other suffers from it. As being highly efficient is one of the most successful ways of surviving, many species have adapted to live the parasite lifestyle.

"Lianli tree" are not really in love with each other, hermit crabs catch each other's shell. Butterflies climb the woodpecker to court, and egrets build their nests. Life seems to be avoiding that competition.

Nature is not necessarily ruthless. Some species originally lived together, and two species would bring benefits when they came together. Sometimes it is difficult to survive and reproduce without each other. This relationship is called symbiosis.