More than two thirds of the world's plant species are found in the tropical rainforests: plants that provide shelter and food for rainforest animals as well as taking part in the gas exchanges which provide much of the world's oxygen supply. Rainforest plants live in a warm humid environment that allows an enormous variation rare in more temperate climates: some like the orchids have beautiful flowers adapted to attract the profusion of forest insects. Competition at ground level for light and food has lead to evolution of plants which live on the branches of other plants, or even strangle large trees to fight for survival.