The observed declines in populations of freshwater species is particularly alarming as they indicate the extent of deterioration in the quality of the world's rivers, lakes and other wetlands. Freshwater amphibians are hard hit. The disappearance of the golden toad and other amphibians in Costa Rica has been attributed to climatic changes. Many losses have been recorded in national parks and nature reserves, indicating pervasive threats even in protected areas. In Australia, Panama and the US, about 20 frog species have been decimated by a previously unknown fungus.
Deformities are also widespread, caused by pollutants such as pesticides and other factors.
To get a picture of how rapidly species extinction is accelerating, the Canadian researchers compared current extinction rates with those from the fossil record. They calculate that the background rate of extinction for freshwater fish species is about one species every three million years. The modern extinction rate in North America, the study says, is about one extinction every 2600 years - about 1,000 times higher than the background rate. Ricciardi and Rasmussen predict that many species considered at risk will disappear within the next century. Non-native species pose a serious threat to indigenous freshwater animals.