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Amphibians of the Northern Grasslands

What Do We Know?

No cry of alarm has been sounded over the fate of amphibian populations in the northern grasslands of North America, yet huge percentages of prairie wetland habitat have been lost, and the destruction continues. Scarcely 30% of the original mixed-grass prairie remains in Nebraska, South Dakota, and North Dakota (See Table 1 in this chapter). If amphibian populations haven't declined, why haven't they? Or, have we simply failed to notice?

Amphibians in the northern grasslands evolved in a boom-or-bust environment: species that were unable to survive droughts lasting for years died out long before humans were around to count them. Species we find today are expert at seizing the rare, wet moment to rebuild their populations in preparation for the next dry season. When numbers can change so rapidly, who can say if a species is rare or common? A lot depends on when you look.

Some changes brought upon the northern Great Plains by human enterprise mimic this climatically induced variability. Frogs, toads, and salamanders that find themselves in a rare remaining wetland will thrive. Progeny will issue forth from the wetland in comfortingly large numbers, and those that return to breed the next year will be rewarded with a reasonable likelihood of success. Those that strike out for new breeding territory, as some percentage must, will likely be less fortunate but also less conspicuous: who searches wheat fields for the frog that didn't make it?

Other changes have no precedent. Aquaculture—which involves modifying the water regime in a previously semipermanent wetland so that it can support stocked fish populations—brings vulnerable larval amphibians into contact with predators against which they have no defense. In addition, to supporting stocked fish, these modified wetlands provide new habitat for predatory bullfrog larvae that require two years of stable water to mature. As predatory species take hold, native amphibians die out (Hayes and Jennings 1986).

One way to decide if amphibian populations are changing is to conduct surveys in areas that were studied long ago. Often past data were collected as an offshoot of a detailed study of a particular species. The danger here is that the past data were probably collected at the very best sites investigators could find; populations at these sites are far more likely to decline than to increase, simply as a result of natural fluctuations (Johnson and Larson 1994). Nonetheless, it is possible to find past surveys (as opposed to ancillary species lists), and comparisons between current and former occurrences are, at the least, instructive.

Another method of assessing the stability of amphibian populations is to enlist volunteers in broad-scale monitoring programs, similar to the well-known U.S. Geological Survey Breeding Bird Survey. The North American Amphibian Monitoring Program, administered through the U.S. Geological Survey Biological Resources Division's Inventory and Monitoring Program, aims to do just that. Volunteers are being mobilized to conduct surveys of calling frogs and toads on carefully selected routes that will stand up to statistical scrutiny. Such a program could prove especially valuable for the northern Great Plains, where relatively few species are silent (tiger salamanders, skinks, and mudpuppies) and thus uncountable, After a reasonable number of years, such surveys will yield valuable information to help us distinguish between natural year-to-year fluctuation and real changes in population sizes.

Two Case Studies

Amphibians at the Cottonwood Lake Study Area

The Cottonwood Lake Study Area is a 49-hectare complex of 17 wetlands in the mixed-grass prairie of central North Dakota. We have monitored adult and larval amphibians there by using drift fences, pitfall traps, and aquatic funnel traps since the spring of 1992. Several noteworthy changes have occurred.

We have observed only four amphibian species at Cottonwood Lake: the gray tiger salamander, the striped chorus frog, the wood frog, and the northern leopard frog. Only tiger salamanders and chorus frogs have been common. Toads have been noticeably absent in our study. Although four species of toads are known to occur in the region (the American toad, the Canadian toad, the Great Plains toad, and Woodhouse's toad), we have not captured or observed a single individual of any toad species since monitoring began in 1992.

The dynamic nature of the Great Plains is well illustrated at Cottonwood Lake. In 1992, water levels were the lowest since record-keeping began in 1967, then record precipitation in 1993 and 1994 resulted in the highest water levels ever recorded at the study area. Tiger salamanders have responded accordingly: in 1992, we captured only 32 individuals; with the progressive return of water to wetlands, the number of captures jumped to 567 in 1993, 1,270 in 1994, and 2,862 in 1995. As water levels in wetlands have become more stable, tiger salamander larvae have begun to successfully overwinter. Throughout all of 1992, 1993, and 1994, only two overwintered salamander larvae were captured during our spring sampling (both in 1994); in 1995, we captured 50.

Striped chorus frog numbers also increased at Cottonwood Lake, but not until 1994. We captured only eight chorus frogs in our funnel traps in 1993. Captures increased to more than 300 in both 1994 and 1995.

Although leopard frogs are reported as abundant throughout the prairie pothole region, we have captured only 13 in our funnel traps (3 in 1993 and 10 in 1995). All 13 were adults, and all were captured in late summer, suggesting that they were newly metamorphosed individuals dispersing into the study area from surrounding wetlands. To date, we have observed no evidence of the reestablishment of a resident leopard frog population at Cottonwood Lake.

Wood frogs have been, and continue to be, captured in very small numbers at Cottonwood Lake. We captured three individuals in our funnel traps in 1992, two in 1993, three in 1994, and only one in 1995. The absence of an increase in wood frog numbers during this period of increasing water levels suggests that drought is not a key factor limiting wood frog populations.

Prairie Pothole Amphibians: Changes Since the 1920's

In the summer of 1920, Frank Blanchard visited the Iowa Lakeside Laboratory in Dickinson County in northwestern Iowa, where he conducted what may have been the earliest study of prairie pothole amphibian populations. His expressed purpose was to provide baseline data for future herpetological surveys: "It is highly important that faunistic studies be undertaken here, and throughout our country, at as early a date as possible if we are to have any record of the composition and distribution of our native fauna, and if we are to deal intelligently with its preservation" (Blanchard 1923).

We repeated Blanchard's survey (Lannoo et al. 1994) and recorded the current amphibian diversity and relative abundance in Dickinson County. In addition to Blanchard's results, we relied on two locally written natural history accounts to determine the changes in amphibian populations. The first account (Anonymous 1907) estimated the number of northern leopard frogs taken from the region by commercial hunters, The second account (Barrett 1964) is a reminiscence about this early 1900's "frogging" industry in Dickinson County. These two accounts, while anecdotal, provide independent observations of the same hunting events and corroborate each other.

Five species reported by Blanchard persist: eastern tiger salamander, American toad, striped chorus frog, gray treefrog, and northern leopard frog. Two species reported by Blanchard were not found: mudpuppy and Blanchard's cricket frog. We collected two species not found by Blanchard: Great Plains toad and bullfrog, Great Plains toads may have migrated into Dickinson County from the west. Bailey and Bailey (1941) found Great Plains toads only west of Dickinson County; by 1984, Reeves (1984) found them east of Dickinson County. Together, these results suggest that between the early 1940's and the early 1980's, the Great Plains toad may have expanded its range eastward and entered Dickinson County. The bullfrog was introduced by state fisheries biologists.

Several changes have occurred in the relative abundances of amphibian species since 1920. American toads and striped chorus frogs now rank higher in relative abundance; tiger salamanders now rank lower, Blanchard (1923) stated that nearly every wetland he sampled had tiger salamander larvae. Today, only 13 out of 32 wetlands contain tiger salamanders. One cause for concern over this decline is that Dickinson County tiger salamanders, unlike any other known population of the eastern tiger salamander, have larvae that are polymorphic (that is, they exist in different forms), exhibiting typical, cannibal, and intermediate physical forms.

From descriptions of the commercial frogging industry in Dickinson County at the turn of the century, we estimate that the number of leopard frogs has declined by at least two—and probably three—orders of magnitude. This decline may be due more to the loss of wetland habitat than to past hunting pressure. In our opinion, the most immediate threat to the existing populations of native amphibians in northwestern Iowa comes from the introduced bullfrog, although aquacultural practices have been important in eliminating and isolating amphibian habitat in other portions of the eastern prairie pothole region.

This resource is based on the following source (Northern Prairie Publication 1058):
Larson, Diane L., Ned Euliss, Michael J. Lannoo, and David M. Mushet.  1998.
     Highlight Box:  Amphibians of the Northern Grasslands. Pages 450-451 in 
     M. J. Mac, P. A. Opler, C. E. Puckett Haecker, and P. D. Doran, eds.  
     Status and Trends of the Nation's Biological Resources, Vol. 2.  
     U.S. Department of the Interior, U.S. Geological Survey.  

This resource should be cited as:

Larson, Diane L., Ned Euliss, Michael J. Lannoo, and David M. Mushet.  1998.
     Highlight Box: Amphibians of the Northern Grasslands. Pages 450-451 in 
     M. J. Mac, P. A. Opler, C. E. Puckett Haecker, and P. D. Doran, eds.  
     Status and Trends of the Nation's Biological Resources, Vol. 2.  
     Jamestown, ND: Northern Prairie Wildlife Research Center Online. 
     (Version 21JAN2000).

See Literature Cited for references


Diane L. Larson
Ned Euliss
U.S. Geological Survey
Biological Resources Division
Northern Prairie Science Center
Jamestown, North Dakota 58401-7317
Michael J. Lannoo
Muncie Center for Medical Education
Ball State University
Muncie, Indiana 47306-0230
David M. Mushet
U.S. Geological Survey
Biological Resources Division
Northern Prairie Science Center
Jamestown, North Dakota 58401-7317

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