The Global Amphibian Decline:
Trends, Causes, and Challenges

Natterjack toad (Bufo calamita)
photo by Arie van der Meijden

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

Actual and Potential Cause: Acidification and Toxic Contaminants

Throughout parts of Europe and North America, acid deposition, typically caused by sulfate and nitrate emissions from industrial processes, has reduced the biodiversity of many aquatic ecosystems. The acidity of aquatic habitats can have major impacts on amphibian distribution, reproduction, development, and mortality. Sensitivity to low pH varies within and among species. Acidity causes mortality in the relatively exposed embryonic and larval stages by hindering absorption of the yolk plug, stunting development, and deforming the larvae. Other sublethal effects of acidity include delayed or early hatching, reduced tadpole body size, aberrant swimming behavior, and slower growth rate due to reduced ability to capture prey. At the population level, effects are less well understood, though possible ones are decreased recruitment, lower densities, and less diverse communities, which often occur in more acidic breeding sites.

Besides habitat destruction, natterjack toad (Bufo calamita, above) populations in Britain have also suffered from long-term acidification of ponds. Reduced pH and metal contamination in an Appalachian stream has also been observed to cause long-term declines in the local salamander community.

However, despite these and other well-documented effects of low pH on amphibians, there are no rigorously documented cases where acidification of natural habitat has been identified as the cause of recent, drastic population declines. While acid deposition was proposed as an agent in the decline of tiger salamanders (Ambystoma tigrinum) in the Rocky Mountains, subsequent studies indicated that pond drying was an equally likely cause.

Similarly, the negative effects of insecticides and herbicides on larval amphibians has been extensively documented, but there is little data to suggest that they are the cause of drastic population declines. Like acidity, toxic chemicals and metals can kill amphibians directly or indirectly by impairing development and reproduction, or by increasing susceptibility to disease.

It has also been hypothesized that long-range contamination of pesticides in Costa Rica, combined with an unusually severe dry season, resulted in accumulation of contaminants typically present at sublethal background levels. Such a climate-linked concentration of toxins may have been responsible for the sudden extirpations of the golden toad (Bufo periglenes) and the harlequin frog (Atelopus varius).