Evidence For Population Declines and Malformations

Proving that amphibian populations are truly dwindling has been challenging for scientists. A major problem with documenting amphibian declines is that, in most regions and for most species, there are no reliable data on past population sizes to serve as a basis for comparison with recent observations. Because populations fluctuate naturally--due to such factors as drought, variations in the availability of food, and changing numbers of predators--an apparently worrisome decline could be just a short-term phenomenon. In addition, it is difficult to confirm the suspected extinction of a species. Just because a species is not found by a group of researchers at a certain time and in a certain place does not necessarily mean that the species no longer exists there.

Because of these difficulties, scientists participating in the DAPTF focus their studies on areas where historical data on amphibians are available. And they make observations over several years under a variety of conditions. A number of such studies have found unmistakable declines in amphibian populations. For example, in the mid-1990's, my research team (affiliated with Iowa Lakeside Laboratory near Milford) found a hundredfold to a thousandfold decline in amphibian numbers in the Prairie Pothole Region of northwest Iowa, compared with the early 1900's. Of the seven native species of amphibians in the region, we believe that one, Blanchard's cricket frog, has become extinct. A second species, a type of salamander called the mudpuppy, has not been seen in the region in about 30 years. It's hard to say if this species is extinct however, because its silent, nocturnal (active at night) nature makes it difficult to track down.

Other research teams have made similar findings. A group at the University of Alberta in Canada discovered that the northern leopard frog, once common in Alberta, had virtually disappeared since 1979. In the Canadian province of Quebec, researchers documented a dramatic decline in the number of chorus frogs and chorus-frog habitats throughout the St. Lawrence River Valley since 1988. And Australian scientists found that 14 species of frogs in the rain forests of eastern Australia had either disappeared or greatly declined in number since the late 1970's. These are just a few examples among many.