Back to Top

Natural History Writing
by Bob Thomas

If you love nature, it is difficult to find a cuter animal than a Red-Eyed Frog from Central America. What child doesn’t know about tadpoles and “hop toads”? Frogs can be found from near the Arctic Circle to the tips of South America and Africa. They have been with us since the heyday of such dinosaurs as Allosaurus, Apatosaurus, Brachyosaurus, about 175,000,000 million years ago (mid-Jurassic). Being found in habitats ranging from deserts to salt water, frogs fill many niches and range in size from two species that are less than one half inch long (Brazil and Cuba) to the large Goliath Frog of western equatorial Africa (one meter from tip of toes to the end of the nose). Of the 3,400 species of frogs that occur worldwide, 28 live in Louisiana.

In the late 1980’s, herpetologists (scientists who study snakes, frogs, and their kin) became concerned that amphibian (especially frog) populations, on a world-wide basis, might be in decline. This information surfaced at an international meeting in Canterbury, England. We think of all things scientific as being arcane and mystical, but the idea that frogs were declining surfaced during conversations among scientists from different continents. They were reminiscing about days gone by and how plentiful frogs were in their youths. Suddenly, they realized that a pattern was emerging in their discussion - all over the world, in very different habitats, at different elevations, there are simply not as many frogs as there were in the past. It was believed that changes in habitats (from natural to cities, suburbs, farms, highways, etc.) may be involved, but it was noted that certain remote, montane species (such as Australia’s Gastric Brooding Frog, Rheobatrachus silus, and Costa Rica’s Golden Toad, Bufo periglenes) were disappearing for no apparent reason. Since most amphibians spend a portion of their life history immersed in water, it is believed that they will be particularly sensitive to changes in water chemistry caused by water and air pollution and, possibly, such phenomena as increased ultraviolet light caused by thinning of the ozone layer. Specifically, the following concerns were discussed:

  • -acid rain
  • -heavy metal
  • -pesticide release
  • -deforestation
  • -hydrological modifications
  • -the many changes in land use
  • -extension of agriculture to wetlands and their overall loss
  • -release of irrigation waters
  • -stocking waterways with amphibian predators
  • -the introduction of species not native to the habitat (such as the Marine Toad, Bufo marinus, and the Bullfrog, Rana catesbeiana)
  • -Cajun recipes (just teasing)
  • The following special properties of amphibians make them particularly susceptible to anthropogenic (human caused) factors:
  • -the time they spend in the water (as mentioned above);
  • -the high degree of permeability of their skin to gases and liquids;
  • -they have very complex stages in their life histories (involving life on land and in the water) with many chemical cues that render them particularly susceptible to environmental change;
  • -anything that affects insects, crustaceans, and/or aquatic plants will affect them at some stage;
  • -they may have very scattered distributions (living as small, relatively isolated populations);
  • -they have inconsistent breeding patterns (some populations skip years of reproduction while others may breed several times a year).

A 13 year research project by Dr. Joseph Pechmann at a pond in the southeastern U.S. concluded that it is virtually impossible to separate human intervention from natural fluctuations, especially since each species in the study tended to have fluctuations that were not necessarily in phase with the other species.

Studies of western U.S. montane species in by Oregon State University herpetologist Dr. Andrew Blaustein have revealed interesting information about amphibian adaptation as it relates to population decline. He and his students found that some species were doing well (Pacific Tree Frogs, Pseudacris regilla), while others were disappearing (Western Toads, Bufo boreas, and Cascade Frogs, Rana cascadae). It was known that UV-B (the dangerous middle-range of ultraviolet light that we like to avoid when sunbathing) is dangerous to amphibian embryos. As UV-B enters the frog’s cells, it is absorbed by DNA, protein, and the like. With DNA, this can have devastating effects by changing the connections between atoms and molecules enough to cause the chemical elements to malfunction, resulting in anything from inefficiency to death. It was also known that an enzyme, photolyase, can protect cells from the harmful effects of UV-B. Since it was believed that the amount of photolyase varies from species to species, it was theorized that those species that were disappearing the quickest may have the least amount of photolyase (thus, they experience the most damage). Further research proved their theory. Of the frogs laying their eggs in open water, the eggs of the Pacific Tree Frog had three times the amount of photolyase as did Cascade Frogs, and six times that of Western Toads. This made ecological sense! Then they found that the least amount of photolyase was in the eggs of Dunn’s Salamanders (Plethodon dunni), which were plentiful. However, these salamanders lay their eggs under logs, thus never being exposed to sunlight - so it still makes good ecological sense! Though UV-B is not necessarily the only culprit, it certainly appears that it is the villain for some species.

In a still very perplexing study, Dr. Gary M. Fellers, Natural Resources Division of the U.S. Geological Survey, has documented the drastic collapse of a variety of amphibian populations in Yosemite National Park since the beginning of this century. Habitats have remained healthy, chemicals have not been widely used, UV-B doesn’t seem to be a problem, there is plenty of water, and the region has changed very little.
Herpetologists have noted that there are problems in some geographic areas, but not in others. Hot-spots of decline seem to center in southeastern Australia, the western United States, and central America. Also, they found that where there is trouble, it does not exist for all species of amphibians. What has everyone very concerned is wondering if these sensitive critters are an early warning system for problems in the ecosystem that are sneaking up on us!

The main thing that has come from this discussion is the realization of how little we know about amphibians. It is recommended that we initiate coordinated long-termed studies that focus on amphibian population biology and their role in the ecosystem. For example, we know that they are important in helping control insects, they provide meals for many other animals in nature, and we also know that in the U.S. amphibians represent the largest quantity (pound for pound) of all vertebrate groups, but we simply do not know what the world will be like without them. Such a simple question, such a surprising lack of information!