Environmental Science 15th Edition by Scott Spoolman,Tyler Miller

Exercise 23

SCIENTISTS ARE SEARCHING FOR THE CAUSES OF AMPHIBIAN DECLINES
H erpetologists, the scientists who study frogs and other amphibians, have identified a number of factors-both natural and human-caused-that threaten these species at various points in their life cycles. One of the natural causes is parasites such as flatworms that feed on certain amphibian eggs. Scientists think this has caused birth defects such as missing limbs or extra limbs in some amphibians.
Some herpetologists hypothesize that viral and fungal diseases, especially the chytrid fungus (Figure 4.B) that attacks the skin of frogs, are reducing the frogs' ability to ingest water through their skin. This leads to death from dehydration. Such diseases can spread fairly easily, because adults of many amphibian species congregate in large numbers to breed.
Another major threat to amphibians is habitat loss and fragmentation. This is mostly a human-caused problem resulting from the clearing of forests and the draining and filling of freshwater wetlands for farming and urban development. Another threat is prolonged drought, which can dry up breeding pools that frogs and other amphibians depend on for reproduction and survival through their early stages of life (Figure 4.C).
Another human-influenced problem is higher levels of UV radiation, which can harm embryos of amphibians in shallow ponds as well as adults basking in the sun for warmth. Historically, such radiation has been screened by ozone in the stratosphere, but during the past few decades, ozone-depleting chemicals released into the troposphere from human sources have drifted upward into the stratosphere and destroyed some of the protective ozone found there.
Pollution is another human-caused threat to amphibians. Frogs and other species are increasingly exposed to pesticides in ponds and in the bodies of insects that they eat. This can make them more vulnerable to bacterial, viral, and fungal diseases and to some parasites. Amphibian expert and National Geographic Explorer Tyrone Hayes, a professor of biology at University of California, Berkeley, is conducting research on how some pesticides can harm frogs and other animals by disrupting their endocrine systems.
Overhunting is another problem, especially in areas of Asia and Europe, where frogs are hunted for their leg meat. Yet another threat is the invasion of amphibian habitats by nonnative predators and competitors, such as certain fish species. Some of this immigration is natural, but humans accidentally or deliberately transport many species to amphibian habitats.
Most herpetologists believe that a combination of these factors, which vary from place to place, probably is responsible for most of the decline and disappearances among amphibian species.
Critical Thinking
Of the factors listed above, which three do you think could be most effectively controlled by human efforts?
FIGURE 4.B Frogs killed by the chytrid fungus at a high-elevation lake in California.


FIGURE 4.C This golden toad lived in Costa Rica's high-altitude Monteverde Cloud Forest Reserve. The species became extinct in 1989, apparently because its habitat dried up.