Causes of Amphibian Declines

A striking feature of amphibian population declines, and what sets them apart from the general biodiversity crisis, is that they have occurred in some of the best-protected natural areas. For example, Sierra Nevada Yellow-legged Frogs have disappeared from wilderness areas high in the United States mountain range known as the Sierra Nevada. While remote locations such as these have escaped obvious habitat alteration and destruction, they are still being affected by human activities. Scientists have investigated five possible causes for the mysterious rapid amphibian declines that have occurred in such remote locations: (1) introduced nonnative species, (2) increased ultraviolet radiation, (3) disease, (4) climate change. and (5) toxic contaminants such as pesticides.

People introduce nonnative species for a variety of reasons, including for recreational fishing, for human food, and for biological control of pests. For example, as early as the 1800s, fish stocking of high-elevation lakes in the Sierra Nevada was done by horseback. Today, trout are raised in hatcheries and dropped by airplane into remote sites in mountain ranges throughout the West. Trout are heavy predators and will eat both tadpoles and adult frogs, so they could easily be the cause behind some declines. While introduced species such as trout have significant negative impacts on certain amphibian species, scientists are concluding that they are not the predominant cause of rapid declines.

Increased ultraviolet radiation was long thought to play a role in declines, and it has received much scientific attention. The release of ozone-destroying chemicals such as chlorofluorocarbons (CFCs), formerly used in refrigerators, has resulted in increased ultraviolet radiation reaching Earth’s surface. Ultraviolet radiation can cause DNA damage and affect animal immune systems. However. the largest number of rapid declines has occurred in tropical mountain-slope forests where amphibians are sheltered from sunlight by the forest canopy. Thus, ultraviolet radiation is unlikely to be a primary cause of these declines.

In 1998 scientists discovered a previously unknown fungus that was associated with frog die-off in both Australia and Panama. The fungus, Batrachochytrium dendrobatidis, is in the chytrid family of fungi and is the only member known to be a pathogen (cause of disease) to vertebrates. The discovery of the chytrid fungus proved to be a turning point, and now disease, particularly that caused by the chytrid fungus, has emerged as the leading explanation for rapid amphibian-population declines. Chytrid fungus has now been found in more than 400 amphibian species and has been associated with population die-offs in North, Central, and South America, as well as in Africa, Europe, Australia, and New Zealand. While it is clear that chytrid fungus is the immediate cause of many rapid amphibian-population declines, there is still much debate about the disease. Is chytrid fungus a newly emerging disease, spreading and attacking defenseless amphibian hosts? Or have recent environmental changes facilitated disease outbreaks by a pathogen that has long been widespread?

Global climate change, especially warming trends, could be contributing to chytrid fungus outbreaks. Unusually hot spells may stress amphibians, which would weaken their immune systems and make them more susceptible to disease. Another possibility is that temperature shifts caused by climate change may create environments more favorable for the growth of chytrid fungus. In addition, climate change may cause drier conditions, forcing amphibians to congregate in smaller bodies of water, facilitating the transmission of disease. All these factors point to a possible connection between climate change and disease, but to date the necessary research has not been done to determine if any of these inks are actually operating.

Pesticides and other contaminants may facilitate disease also. Such chemicals can be carried long distances from where they are applied and often wind up in remote ecosystems. Studies have shown that contaminants can suppress immune systems in many organisms, including amphibians, which may lead to disease outbreaks. In California, researchers have found a strong association between the geographic pattern of population declines of a number of frog species and the pattern of pesticide applications. However, as with climate change, few studies have been conducted to establish a clear link among contaminants, disease, and amphibian declines. More research is clearly needed.

1.A striking feature of amphibian population declines, and what sets them apart from the general biodiversity crisis, is that they have occurred in some of the best-protected natural areas. For example, Sierra Nevada Yellow-legged Frogs have disappeared from wilderness areas high in the United States mountain range known as the Sierra Nevada. While remote locations such as these have escaped obvious habitat alteration and destruction, they are still being affected by human activities. Scientists have investigated five possible causes for the mysterious rapid amphibian declines that have occurred in such remote locations: (1) introduced nonnative species, (2) increased ultraviolet radiation, (3) disease, (4) climate change. and (5) toxic contaminants such as pesticides.