<P> There is considerable evidence that parasitic trematode platyhelminths (a type of fluke) have contributed to developmental abnormalities and population declines of amphibians in some regions . These trematodes of the genus Ribeiroia have a complex life cycle with three host species . The first host includes a number of species of aquatic snails . The early larval stages of the trematodes then are transmitted into aquatic tadpoles, where the metacercariae (larvae) encyst in developing limb buds . These encysted life stages produce developmental abnormalities in post-metamorphic frogs, including additional or missing limbs . These abnormalities increase frog predation by aquatic birds, the final host of the trematode . </P> <P> A study showed that high levels of nutrients used in farming and ranching activities fuel parasite infections that have caused frog deformities in ponds and lakes across North America . The study showed increased levels of nitrogen and phosphorus cause sharp hikes in the abundance of trematodes, and that the parasites subsequently form cysts in the developing limbs of tadpoles causing missing limbs, extra limbs and other severe malformations including five or six extra or even no limbs . </P> <P> In 1998, following large - scale frog deaths in Australia and Central America, research teams in both areas came up with identical results: a previously undescribed species of pathogenic fungus, Batrachochytrium dendrobatidis . It is now clear that many recent extinctions of amphibians in Australia and the Americas are linked to this fungus . This fungus belongs to a family of saprobes known as chytrids that are not generally pathogenic . </P> <P> The disease caused by Batrachochytrium dendrobatidis is called chytridiomycosis . Frogs infected by this disease generally show skin lesions and hyperkeratosis, and it is believed that death occurs because of interference with skin functions including maintenance of fluid balance, electrolyte homeostasis, respiration and role as a barrier to infections . The time from infection to death has been found to be 1--2 weeks in experimental tests, but infected animals can carry the fungus as long as 220 days . There are several hypotheses on the transmission and vectors of the fungus . </P>

Name three factors scientist attribute to the global decline of amphibians