Labelled rainforest frogs. Dead frogs laid out in a herpetologist's tray, ready to be fixed in formaldehyde (a preservative) for research. The greatest diversity of frogs are in tropical regions, but frog populations worldwide have dramatically declined since the 1950s. More than a third of the world's species are believed to be threatened with extinction, and 120 have already died out since the 1980s. The biggest cause of this is habitat loss through pollution, climate change, deforestation, and introduced competitors or predators. Some rainforest frogs are important in medicine. For example, epibatidine, a painkiller 200 times more potent than morphine, is made by certain poison dart frogs. Photographed in Rio Blanco Valley, Zamora-Chinchipe, Ecuador.