Parasites
Researcher Joseph Kiesecker focused research on a rural area of Pennsylvania where trematodes were present in ponds. His research indicated that frogs sheltered from the infiltration of parasites developed normally, while those that were exposed developed deformities. (He also monitored the pesticide levels in the same ponds and found a magnification in the rate of parasitism in the polluted ponds suggesting that the presence of the pollutants reduces the frog’s immune response to parasitism. ("Pesticides Mess with Immunity" J. Pickrell Science News. July 13, 2003 p. 20)
Chytrid fungus
A parasitic fungus belonging to the Chytrid family has been killing frogs in Australia, Central and North America. The time from infection to death has been found to be 1-2 weeks in experimental tests.
In 1998, following large-scale frog deaths in Australia and Panama,
research teams in both countries came up with identical results— a previously undescribed species of pathogenic fungus, Batrachochytrium dendrobatidis. Subsequent research has established that the fungus has been present in Australia since at least 1978. It now appears that some recent extinctions of Australian frog species were caused by this fungus.
The fungus may have been exported from Africa, perhaps traveling with the African Clawed Frog, Xenopus laevis, which is sold in pet stores and used in laboratories around the world.
Exotic species
Non-native predators and competitors are also affecting the viability of frogs in their habitats. The mountain yellow-legged frog which typically inhabits the Sierra Nevada lakes have seen a decline in numbers due to stocking of non-native fish (trout) for recreational fishing. The developing tadpoles and froglets fall prey to the fish in large numbers. This interference in the frog’s three year metamorphoses is causing a decline that is manifesting throughout their ecosystem. ("On the Rebound" Elizabeth Daerr, National Parks Magazine p. 49)
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