“Frogs of the United States and Canada” (2013; Johns Hopkins University Press) by C. Kenneth Dodd Jr. arguably stands as the best modern reference book ever written on this important group of amphibians. The term “frogs” includes the common toads with which everyone in America is familiar, and the book covers pretty much everything scientists have learned about their biology. The comprehensive coverage of the ecology, reproduction, diet, and conservation of the 100 native U.S. frog species (which includes the 27 found in Canada) is excellent.
The two-volume, 982-page book has almost 400 color photos of frogs, toads, and treefrogs as well as the eggs, tadpoles, and habitats of many of them. Range maps show the distribution patterns of the naturally occurring species plus those of six “established nonnative species” that are now firmly entrenched in one or more states.
More than 6,800 living species of frogs are known today, so the paltry 1.7 percent of the total number of the world’s species that are found north of Mexico might seem insignificant. But as with many groups of animals, some of the most in-depth ecological research on frogs has been done by scientists in the United States and Canada. Consequently, a thorough summarization of this nature constitutes not only a major literary undertaking but also a significant contribution to the field of herpetology.
As Dodd says in the introduction of his book, “Frogs are now at greater peril worldwide than at any time in recent geologic history.” The same can be said about many other groups of free-living animals including turtles, sharks, and monarch butterflies. One progressive step in the conservation process is a synthesis of what is known about the life history and ecology of particular groups of organisms in prescribed places. The author is to be commended for doing an admirable job on behalf of the frogs and toads of the United States and Canada.
He addresses the status of six frog species that arrived in the United States without an invitation. For half of these frogs from other countries that have found a U.S. home, Hawaii is the only state where they have become established. Ironically, Hawaii is also the only state with no native frogs whatsoever (even Alaska has three native species). Other invasive species include the greenhouse frog, which is native to Cuba but found throughout most of Florida and parts of Georgia, Alabama, Mississippi, and Louisiana, as well as Hawaii. Cuban treefrogs, first reported in Florida in 1931, are now established in the Florida peninsula and have been sighted in Savannah, Houston, and other isolated cities where they might have arrived via horticultural shipments, motor homes, or boats on trailers. The African clawed frog, first introduced to U.S. laboratories in the 1930s for pregnancy tests and 40 years later in the pet trade, are totally aquatic. Populations are now established in California and Arizona. How these various nonnative species might negatively affect other frogs or, especially in Hawaii, even other animals is still being investigated.
Regarding conservation, the book has an important and sobering assessment in the last two pages, a section titled “Index of Potential Stressors.” References to scientific studies are given for a total of more than 100 natural and commercially produced chemical contaminants that have been suspected or documented to have negative impacts on frogs when they are introduced into natural habitats. The author states that frogs, “with their unprotected eggs, aquatic larval development, [and] permeable skins ... are being saturated by a host of lethal and sublethal toxic substances.” Some, such as DDT, lead and mercury, are not surprising to find on the list. Others are chemical names listed in various pesticide and herbicide products found in garden sheds. Not all have been proved to be detrimental to frogs but each bears supporting research to find out if we should be concerned. We may not have as many frogs as other places, but we certainly want to keep the ones we’ve got.
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Whit Gibbons is an ecologist and environmental educator with the University of Georgia’s Savannah River Ecology Laboratory.