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Farm ponds as critical habitats for native amphibians
A Field Guide to Amphibian Larvae and Eggs of Minnesota, Wisconsin, and Iowa
Field guide contents

Jeffrey R. Parmelee
Simpson College
Department of Biology
701 North C Street
Indianola, Iowa 50125

Melinda G. Knutson and James E. Lyon
U.S. Geological Survey
Upper Midwest Environmental Sciences Center
2630 Fanta Reed Road
La Crosse, Wisconsin 54603

About the authors

Apparent worldwide declines in amphibian populations (Pechmann and Wake 1997) have stimulated interest in amphibians as bioindicators of the health of ecosystems. Because we have little information on the population status of many species, there is interest by public and private land management agencies in monitoring amphibian populations. Amphibian egg and larval surveys are established methods of surveying pond-breeding amphibians. Adults may be widely dispersed across the landscape, but eggs and larvae are confined to the breeding site during a specific season of the year. Also, observations of late-stage larvae or metamorphs are evidence of successful reproduction, which is an important indicator of the viability of the population. The goal of this guide is to help students, natural resources personnel, and biologists identify eggs and larval stages of amphibians in the field without the aid of a microscope.

Anyone undertaking field identification of amphibians has a responsibility to avoid harming the amphibians or their habitats. Persons planning to sample amphibians should work in cooperation with state or federal wildlife professionals. Lack of knowledge about sensitive habitats or populations could result in the spread of diseases, damage to breeding habitats, or local reproductive failure of amphibian populations. State and federal laws protect amphibians from exploitation. Collection permits are required from the appropriate state or federal authorities before capturing, handling, or collecting amphibians. Permission for sampling should also be obtained from the landowner.

Research for this publication began with a study of farm ponds in southeastern Minnesota. We quickly recognized the need for a way to identify eggs and larvae of the amphibians encountered. We used our field knowledge of most of the species included here to produce this field guide. Additional information came from published keys for Wisconsin amphibians (Vogt 1981; Watermolen 1995; Watermolen and Gilbertson 1996) and descriptions from a variety of sources (e.g., Johnson 2000; Petranka 1998; Harding 1997; Russell and Bauer 1993; Stebbins 1985; Wright and Wright 1949).

There is no simple way to morphologically distinguish between many amphibian species during the egg and tadpole stage. This might be expected in closely related species such as the gray treefrogs Hyla chrysoscelis and Hyla versicolor, which vary in chromosome number and in subtle morphological traits. It is surprising that, to our knowledge, no one has reported a way to morphologically distinguish between larvae of several common frog species such as the Crawfish Frog and Southern Leopard Frog, which bear little resemblance to one another as adults. Even with the technical keys available to herpetologists, amphibian eggs and larvae are often difficult to identify to species. An excellent key to all species in the United States (Altig et al. 1998) is available on the internet (http: // The Altig key (along with earlier versions such as Altig [1970] for tadpoles and Altig and Ireland [1984] for salamander larvae) requires detailed knowledge of the anatomy of amphibian larvae. Altig et al. (1998) requires examination of tadpole mouthparts, which are only observable with dead specimens and the aid of a microscope. Tadpole biology is summarized by McDiarmid and Altig (1999). Additional research is needed to better describe and understand larval amphibians.

Three major factors make amphibian larvae, especially tadpoles, difficult to identify. First, many species are conservative in their anatomy and members of the true frog (Ranidae) and toad (Bufonidae) families are very similar in larval form. Second, morphology among tadpoles of a single species can vary geographically and with developmental stages. Third, larvae reveal differing anatomies depending on the physical and biotic environment where they are found. The proportions, coloration, and anatomy of larvae change as they grow from hatchlings to metamorphs. Larvae in early stages frequently do not show the characteristics necessary for their identification.

In one description, we summarize closely related species that cannot be distinguished without the aid of a microscope. We believe that the weakness of specific identification is balanced by ease of use in the field. In many instances, identification to species is not necessary to meet land managers' goals. However, professional herpetologists working for universities, museums, and state and federal agencies can assist with identifying larvae to species.

No key by itself can help identify every specimen; there will always be variant individuals. In identifying anurans, for example, knowledge of which species were calling at the pond may help sort out identification questions. It also is helpful to note the geographic distribution of species; you can reasonably rule out Canadian Toads if the site is in southern Iowa. Ruling out a species by geography is not easy or reliable, however, if your survey site is near the edge of a species' range. You could also return several times to the breeding site to observe late-stage metamorphs. Amphibians often are easier to identify once they develop mature traits. If a specimen cannot be identified using this key, preserve the specimen and attempt identification using a more detailed key such as Altig et al. (1998). If still not confident in the identification, contact the senior author of this key or another herpetologist in your area for assistance. Another approach, if adequate time and facilities are available, is to raise a few eggs or larvae to metamorphosis and identify the late-stage metamorphs or adults from their mature characteristics. We anticipate that rapid molecular DNA techniques suitable for field identification will be available within the next few decades. Until then, keys will be needed.

About the authors

Jeff Parmelee is an assistant professor of biology at Simpson College in Indianola, Iowa, where he teaches herpetology, anatomy, and general biology. He obtained his Ph.D. from the University of Kansas in 1998 and his research interests include tropical herpetology and rattlesnake ecology (

Melinda Knutson was a wildlife biologist at the USGS in La Crosse, Wisconsin, where she studied amphibians and birds and their habitats. She obtained her Ph.D. from Iowa State University in 1995 and her research interests include conservation biology and landscape ecology. She now works for the U.S. Fish and Wildlife Service. ( ;

James Lyon is a biologist and naturalist with special interests in amphibians, reptiles, birds, and native plants. Pearl Podgorniak created the drawings of larvae and eggs for this publication as an undergraduate research assistant with Dr. Parmelee, supported by Simpson College.

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