by Whit Gibbons

July 4, 2004

The worldwide declines of wildlife continue to generate concerns among conservation biologists. Although some causes are obvious, others have been inexplicable. Research with amphibians has been insightful, underscoring that an endless variety of environmental and biological factors can cause negative impacts on animal populations. The studies emphasize the global complexity of environmental interactions and of interpreting the cause of the problems.

Recent warming trends have been implicated for some animal declines. Scientists used a global climate model to explain biological responses in the forests at Monteverde, Costa Rica. One of the most notable examples was the complete disappearance of the rare and beautiful golden toad, an animal that has not been seen since the late 1980s. The investigators made calculations based on climatic data and concluded that crashes in animal populations observed in several animal species in the region were linked to a reduction in the frequency of mists during the dry season. The increased dryness during what would normally be at least a moderately moist time of year was found to have a strong relationship to warm surface temperatures in the equatorial Pacific Ocean. The implications of the research are far-reaching, indicating that climatic conditions in one part of the world can have dramatic effects in regions far away. In recent years, memories of experiences with El Niño in response to warming of the southern Pacific are still vivid to people in some parts of the country.

Meanwhile, on the U.S. West Coast a combination of field observations and laboratory experiments were used to explain some cases of abnormal limb development in frogs. Mutations in frogs are not a new phenomenon, cases having been reported more than a century ago. But some scientists believe frog mutations are on the increase. Two independent research programs concluded that some mutations are caused by parasitic flatworms that invade the tadpole stage and modify limb development.

In one of these studies, scientists analyzed deformed frogs having more than four legs. They determined that the abnormalities were related to infestations with cysts of a flatworm parasite. Meanwhile, other investigators observed abnormal limb development and low survivorship in Pacific treefrogs that had been exposed experimentally to high concentrations of the flatworm parasites. The abnormal limb development was similar to that observed in frogs of the same species at field sites in California that harbored an aquatic snail, the parasite's primary host.

To understand the snail's role and truly appreciate how complex the natural world is consider the life cycle of the parasites. A predator, such as a garter snake or heron, must eat an infected frog. Inside the predator, the parasites become adults and lay eggs. When the predator passes the eggs out of its body, those that land in water develop into swimming larvae that infect pond snails. The snails release more larvae, which eventually infect tadpoles.

The parasites in abnormal frogs should not be taken as a natural explanation for why amphibians are declining. Although specific causes for declines can be identified in some instances, as with the parasites, the intensity of the effect may result from a lowered resistance because of other, human-caused environmental stressors. Increases in snail abundance and parasite infections have previously been shown to occur in response to some forms of pollution, further demonstrating how environmental factors can influence one another and how human impacts on one part of the world's environment can have effects in unexpected ways and in unexpected places.

Another example of a new and hitherto unsuspected amphibian problem was reported along a streamside in a mountainous site in Panama where mass mortality and population declines of amphibians were reported. Frogs of several species were abundant in 1993 to 1995; by 1997 few individuals of any species could be found. The researcher autopsied several frogs and discovered that all were infected by a chytrid fungus, an organism not previously known for being so menacing to any wildlife species.

We are losing much of the wildlife that makes our world an interesting place. Scientists who take the first step by uncovering the numerous and complex reasons for wildlife losses and declines serve an important role. For as with any other predicament, to achieve a solution we must first identify the problem.

If you have an environmental question or comment, email

(Back to Ecoviews)