The Global Amphibian Decline:
Trends, Causes, and Challenges

Ruby poison frog
(Epipedobates parvulus)
photo by Latisha Robinette

Introduction

Global Trends
North America
Central and South America
Australia
Europe

Causes
Habitat Modification
Exotic Species
Acidification and Toxic Contiminants
Ultraviolet Radiation
Climate Change
Pathogens

Challenges

Bibliography

Links

Introduction

At the First World Congress of Herpetology in 1989, researchers began gathering largely anecdotal evidence that a global pattern of population decline and species losses were occurring among the world's amphibians. Herpetologists from several continents reported drastically dwindling and even completely extirpated amphibian populations that had been observed thriving only a few years earlier. Since then, increased evidence and analysis of population trends have suggested that massive amphibian declines are occurring at local scales in many regions around the world, including North and South America, Europe, and Australia. Moreover, analysis of hundreds of amphibian populations worldwide has indicated a global amphibian population decline that may have been occurring for several decades. There is now a consensus among scientists that alarming rates of amphibian declines have occurred.

Amphibians are integral components of many ecosystems, and in many habitats their combined biomass exceeds that of all other vertebrates combined. Due to their important role in the food web of many communities, declines in amphibian populations could have an important impact on other organisms. For instance, overharvesting of wild Indian bullfrogs (Rana tigrina) in India and Bangladesh for the frog leg trade led to an overabundance of insect pests that are the frogs' typical prey. Adult amphibians are thus important carnivores in many ecosystems, as well as prey species in others. Larval amphibians (tadpoles) can be important herbivores as well as prey.

Stauffer's treefrog
(Scinax staufferi)
photo by Jeff Chow

Rio Grande leopard frog
(Rana berlandieri)
photo by Jeff Chow

The most important question is whether these declines are the result of natural population fluctuations or the result of human-induced disturbances to the environment. Declines of amphibian populations due to habitat loss and introduced exotic predators have been documented and are obviously the result of human causes. However, more alarmingly, researchers have also documented drastic declines in relatively pristine areas such as nature reserves. Though a common cause among these unusual declines has yet to be identified, there is evidence that they are also the result of human perturbations. Because of their complex life histories and ecological interactions unique among vertebrates, amphibians may be good bioindicators of subtler forms of environmental degradation. Their life histories involve eggs and larvae in aquatic systems and adults in terrestrial systems, making them vulnerable to degradation of both types of habitats. The adults of many amphibian species spend their lives against or in a substrate such as mud, sand, leaf litter, and water, making them vulnerable to toxic contamination of that substrate. With skin that is permeable to water, electrolytes and gaseous molecules, individuals at all life-history stages are vulnerable to environmental contamination. Moreover, amphibian eggs are covered only by a layer of gelatinous material, resulting in relatively direct exposure to the environment and making recruitment of new individuals into a population very susceptible to environmental conditions. Vulnerability of amphibians to subtler forms of environmental degradation has led researchers to suggest that amphibian populations are declining due to local causes such as toxic contaminants and acid precipitation, as well as due to global phenomenon such as climate change and increased ultraviolet radiation from ozone depletion. Amphibians weakened by the above factors may have lower rates of successful reproduction or be more susceptible to pathogens, which have also ravaged amphibian populations.

Relatively little knowledge about the population dynamics of amphibian populations worldwide continue to challenge researchers. Understanding the extent of the problem and its nature requires an understanding of how local populations naturally fluctuate, and how local factors affect the dynamics of these populations.