Rainforests are characterized by a closed and continuous tree canopy, moisture-dependent vegetation, the presence of epiphytes and lianas, and the absence of wildfire. Rainforest can be classified as tropical rainforest or temperate rainforest, but other types have been described. Estimates vary from 40% to 75% of all biotic species are indigenous to the rainforests. There may be many millions of species of plants, insects, and microorganisms still undiscovered in tropical rainforests. Tropical rainforests have been called the "jewels of the Earth" and the "world's largest pharmacy" because over one-quarter of natural medicines have been discovered there. Rainforests, as well as endemic rainforest species, are rapidly disappearing due to deforestation, the resulting habitat loss, and pollution of the atmosphere.