by humans and also have many cultural roles in literature, symbolism and religion. They are also seen as environmental bellwethers, with declines in frog populations often viewed as early warning signs of environmental damage. Frog populations have declined significantly since the 1950s. More than one third of species are considered to be threatened with extinction and over 120 are believed to have become extinct since the 1980s. The number of malformations among frogs is on the rise and an emerging fungal disease, chytridiomycosis, has spread around the world. Conservation biologists are working to understand the causes of these problems and to resolve them. Etymology and taxonomy The use of the common names frog and toad has no taxonomic justification. From a classification perspective, all members of the order Anura are frogs, but only members of the family Bufonidae are considered "true toads". The use of the term frog in common names usually refers to species that are aquatic or semi-aquatic and have smooth, moist skins; the term toad generally refers to species that are terrestrial with dry, warty skins. There are numerous exceptions to this rule. The European fire-bellied toad (Bombina bombina) has a slightly warty skin and prefers a watery habitat whereas the Panamanian golden frog (Atelopus zeteki) is in the toad family Bufonidae and has a smooth skin. Etymology The origin of the order name Anura—and its original spelling Anoures—is the Ancient Greek "alpha privative" prefix ( from before a vowel) 'without', and () 'animal tail'. meaning "tailless". It refers to the tailless character of these amphibians. The origins of the word frog are uncertain and debated. The word is first attested in Old English as , but the usual Old English word for the frog was (with variants such as and ), and it is agreed that the word frog is somehow related to this. Old English remained in dialectal use in English as frosh and frosk into the nineteenth century, and is paralleled widely in other Germanic languages, with examples in the modern languages including German , Norwegian , Icelandic , and Dutch . These words allow reconstruction of a Common Germanic ancestor . The third edition of the Oxford English Dictionary finds that the etymology of is uncertain, but agrees with arguments that it could plausibly derive from a Proto-Indo-European base along the lines of , meaning 'jump'. How Old English gave rise to is,