AdoptAPond

Why Are Amphibian Populations Declining

What is being done to help Amphibians

Canada Species At Risk

Responding to an Amphibian Extinction Crisis

WHY ARE AMPHIBIAN POPULATIONS DECLINING?

Although amphibians undergo natural population cycles, the current decline is too fast and widespread to be considered a natural occurrence. There are several hypothesized causes for recent amphibian declines, including habitat loss, environmental contaminants, climate change and infectious diseases. Further, in many cases there is no single cause for any one population decline. Synergistic effects of 2 or more of the above causes (and others not listed) may threaten amphibians.

Habitat Loss
The greatest threat to the survival of amphibians is the destruction or fragmentation of amphibian habitats. Wetlands are destroyed by filling or drainage for development and agriculture. In Canada's prairies, approximately 70% of historic wetlands have been turned into farmland, and in southern Ontario 80% of wetlands have been drained. In tropical rainforests, amphibian habitats are being destroyed before the amphibians can even be identified. Amphibian habitat is not just confined to wetlands. A large number of species are terrestrial for parts of their life or are terrestrial for their whole life. This diversity in habitat requirements emphasizes that we can not take a simple approach to conservation. Even when habitat remains, it is often fragmented. Many amphibians rely on the movement between water-land habitats but when habitats are destroyed, populations become isolated.

Declines in amphibian populations in protected habitats, such as national parks and nature reserves, have led to the suggestion that there may be one or more factors other than habitat loss that are adversely affecting amphibians.

Environmental Contaminants
High levels of biodiversity and high human density often converge geographically. Indeed, the most biologically diverse area in Canada, the Carolinian Zone of southern Ontario, is also the most densely populated by humans. Urban and urbanizing areas, including agricultural landscapes, often have higher concentrations of toxic chemicals than peripheral areas. Unfortunately, as stated above, there is a higher degree of biodiversity, including amphibian fauna, in areas with high levels of pesticides, herbicides, and insecticides. These and other chemical contaminants may have adverse effects on amphibians because amphibian skin is very permeable to the environment - they breath through their skin!

Tyrone Hayes, professor of integrative biology at UC Berkeley (http://ib.berkeley.edu), is conducting exciting yet depressing research about the causes of amphibian abnormalities. Although there are counter-arguments from other scientists, Dr. Hayes and others are illustrating how chemical pollutants in everything from plastic water bottles to mosquito larval spray to herbicides have negative effects on the individual fitness of amphibians.

Amphibian abnormalities are not uncommon, but the correlation between high levels of chemical pollutants and high levels of abnormalities within a population can no longer be ignored © US Fish and Wildlife Service

Climate change
Climate change is more than just global warming; it is less predictable weather patterns: temperature change, drought in some areas, flooding in others and more severe storms. Amphibians are very dependant on the weather and dramatic climate changes can alter habitat availability, breeding, drought tolerance, immune function and population numbers. The extinction of the Golden toad and Harlequin frog and the decline of other amphibians are believed to be due to warmer temperatures drying out the moist environments that they depend on. In eastern Australia, a correlation was found between drought and massive declines of stream-dwelling rainforest amphibians. Here, in North America, the extirpation of Northern leopard frog (Rana pipiens) populations has also been associated with drought. Amphibian embryos may be exposed to more ultraviolet radiation (reducing hatching success), and creating more favourable conditions for the growth of pathogens.


Bob Johnson, Toronto Zoo's Curator of Amphibians and Reptiles, took these photos of Golden Toads and Harlequin Frogs in 1987. By 1989 these species were extinct and will never be seen again. © Bob Johnson

The Golden toad (Bufo periglenes) used to live in Costa Rica's Monteverde Cloud Forest Reserve. The disappearance of the Golden toad is of significance since its habitat is in a protected national preserve. It is speculated that the extinction of the Golden toad is due to climate change and/or fungal disease. © Bob Johnson

Disease
Infectious diseases have long been observed in amphibian populations, but the pathogenic chytrid fungus (Batrachochytrium dendrobatidis) has been identified as having severe impacts upon amphibian populations around the world. Chytrid, a deadly fungal disease, has caused the extinction of many frog species and is predicted to continue to spread in Central America causing a "death wave". Although the disease was only reported recently, it appears that it may have been responsible for amphibian declines dating back to the 1970s. The fungus invades the surface layer of an amphibian's skin, causing the skin to become resistant to oxygen and water, preventing the amphibian from being able to breathe or drink. Chytrid fungus is probably transferred by direct contact between amphibians or through exposure to infected water. There is no known cure or control for chytrid fungal infections.

Golden frogs (Atelopus zeteki), revered in Panama, are beautifully coloured and possess a unique communication strategy - they supplement their 'peeps' with a system of waving their legs to signal other frogs. Unfortunately, the Golden frog is listed as Critically Endangered because of a dramatic decline in population, ~80% over the last ten years, seemingly due to the chytrid fungus. © Pete Johantgen

Synergistic Effects
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