We must conduct more research to determine which, if any, of these factors are responsible for declining amphibian populations in relatively pristine habitats. One approach would be to compare undisturbed sites where amphibian populations are healthy to similar habitats where the populations are in serious decline. One such grouping exists in the Andes mountains in Ecuador, Colombia, and Venezuela. While amphibians continue to thrive in high-elevation habitats in Colombia, they have disappeared from virtually identical habitats in Ecuador and Venezuela. Might something as straightforward as introducing predators such as trout into the waters of Ecuador and Venezuela, but not Colombia, be responsible? Or might atmospheric transport of agricultural chemicals applied in lowland regions of Ecuador and Venezuela be causing problems? An elegant set of comparative studies and experiments could be designed to address such questions at these and other promising groups of undisturbed sites in lowland and cloud forest habitats of Africa, South America, southeast Asia, and Madagascar.
Another approach would include studies aimed at rejecting regional or global factors as causes of amphibian declines. Most research has tried to verify the link between reduced frog populations and factors such as high UV-B concentrations. But some studies suggest that UV-B, as a single factor, is not responsible for all amphibian declines, since several species, such as the golden toad of Costa Rica, are never exposed to the sun’s ultraviolet rays. In fact, golden toads lived underground all year long, except for a few days at the end of the dry season when they emerged to breed. But even then they were protected under the canopy of Monteverde’s elfin forest, which (even though short by tropical lowland standards) effectively filters out the ultraviolet radiation. Moreover, because females chose to lay their eggs in well-shaded pools, the now-extinct golden toads were never exposed to UV-B even as eggs or larvae.
Such an analysis doesn’t mean that rising UV-B levels are not killing off amphibians elsewhere. In fact, studies of amphibians exposed to such radiation are under way in the mountains of Chile and Argentina. It does, however, suggest that no single factor may be responsible for all declines. Perhaps more significant, the analysis also raises the possibility that more than one factor may be at play at each location. For example, if an amphibian population is subject to sublethal stresses from habitat fragmentation and acid rain, might it be more likely to succumb to an additional stress from some regional or global factor such as climate change or estrogen mimics?
Some research shows that such scenarios are possible. A study of the western toad Bufo boreas, common to the Elk and West Elk Mountains of Colorado, serves as one example. Cynthia Carey, a biologist at the University of Colorado, who began studying these toads in 1974, discovered that they had contracted “red leg” disease, a normally nonfatal illness caused by Aeromonas hydrophila, a naturally occurring bacterium. Over the next eight years, Carey found that the toads, once common in the mountains, had almost completely disappeared. Her conclusion was that some environmental factor, or the synergistic effects of several factors, may have caused the toads to secrete elevated levels of hormones that compromised their immune system and led to their infection and eventual death.
Studies such as these demonstrate that the underlying causes of amphibian declines may be far more complex than anyone originally imagined. Thus, studies that examine possible synergistic effects and help us tease out the relative contribution of each must be among our research priorities.
Though much research lies ahead, we can take some practical steps immediately to halt the decline of amphibian populations. Perhaps the most obvious is to preserve remaining amphibian habitats. One novel approach would be to consider the health of amphibians in environmental impact assessments. In fact, this practice proved highly successful at a highway-construction site in British Columbia recently. Typically, whenever highways are built in the forested Canadian province, workers create roadside ditches and scour them of all vegetation. But in this case, thanks to a herpetologist included on the environmental-impact study team, the road builders added parts of fallen trees to the ditches, enabling native amphibians to use them as breeding sites.
Another simple but valuable step would be to consider amphibians in environmental assessment programs as bioindicators of overall ecosystem health. Because the eggs of many amphibians lack a protective covering and are laid at or near the surface of a body of water, they are very sensitive to both air- and water-borne pollutants. Also, because the climatic factors typically determine the onset, duration, and intensity of amphibian mating activity, careful monitoring of breeding populations can provide an extremely sensitive assay of climate change.
Finally, the latest findings regarding causes of amphibian declines need to be communicated both to international policymakers, who are in a position to set research priorities and fund additional studies, and to the public at large, which can influence their decisions. Americans are now much more aware of issues concerning amphibians than they were even a decade ago, thanks in large part to a number of excellent television documentaries that have focused on dwindling amphibian populations. But scientists and the media must continue to spread the word to convince people around the world that these precious creatures are worth their concern.