STATUS OF AMPHIBIANS IN MINNESOTA
Report by John J. Moriarty, Scientific Advisor, 'A Thousand Friends of Frogs,' Natural Resources Specialist, Ramsey County, Maplewood, MN 55109
ABSTRACT: There has not been a well documented study on population changes in Minnesota's 20 species of amphibians. There have been good ecological studies on some of the Anurans (Frogs and Toads), but no long term monitoring. Declines in several species have been noted, including the possible extirpation of one species. A number of studies have been initiated and are ongoing to assess and monitor the changes in amphibian populations.
Introduction: Minnesota currently has 20 species of amphibians, including 14 anurans and six salamanders. One of the species, Northern Cricket Frog, may be extirpated (Oldfield and Moriarty 1994). Historical information on the abundance and distribution of amphibians is rather limited. There was no reliable information prior to Breckenridge's research (Breckenridge 1941, 1944). This work set a baseline for all future studies.
A number of studies were carried out in the 1960's and 1970's on various aspects of anuran natural history. Bellis (1957, 1961) and Fishbeck (1968) studied the ecology and movements of Wood Frogs. Breckenridge and Tester (1961) and Tester and Breckenridge (1964) studied the movements and hibernation of Canadian Toads. Their studies found large numbers (3,000+) of toads per hibernation site. The distribution and ecology of Green Frogs was studied by Fleming (1976). Hedeen (1970, 1971, 1972) looked at the ecology and natural history of Mink Frogs. Merrell (1965, 1970) conducted numerous studies on the ecological genetics of Northern Leopard Frogs in Minnesota. The Life History of the Leopard Frog, Rana pipens in Minnesota (Merrell 1977) provided a wealth of information on distribution and population sizes. These studies documented the population and distribution of their respective species at the time of the studies. There have not been any studies monitoring possible changes in these populations.
Current Research: A pilot anuran call survey based on the Wisconsin model (Mossman and Hines 1985) was started in 1993 (Moriarty 1996). The survey is currently limited to 34 routes in 22 counties. The Minnesota Department of Natural Resources is joining the survey project for the spring of 1996. This will help with data analysis and addition of new routes in the future.
The Minnesota anuran call survey has not shown any major declines during its limited sampling, but has added to the distributional information of Wood Frogs, Bullfrogs, and Gray Treefrogs.
The Minnesota County Biological Survey (MCBS) has been conducting amphibian surveys since 1988 (Moriarty 1988, Dorff 1996). These surveys are not designed for long term monitoring, but have led to a number of important distributional finds, including the Four-toed Salamander (Dorff 1995). The MCBS is expected to continue for the next 12 years (Dorff 1996).
The Minnesota Pollution Control Agency (MPCA) has been involved in the investigation of deformed Northern Leopard Frog populations at three sites in Central Minnesota (Helgen 1996). The populations discovered in the summer of 1995 consisted of a large number of juvenile frogs with multiple or missing appendages. The causes of these deformities are being investigated by the MPCA, the University of Minnesota and the United States Environmental Protection Agency (J. Helgen and R. McKinnell, Pers. Comm.). It is hoped that they can identify the causes by mid-1996.
Recent work on the Kandiyohi and Burnsi morphs of Northern Leopard Frogs (Hoppe and McKinnell 1991) has led to an ongoing study looking at the decline in numbers of "lunker" frogs (Hoppe and McKinnell 1996). The decreased survival of large adult frogs has raised concern with a number of frog researchers.
Current Status: Population information on other species is limited and somewhat conjecture. The current status of all Minnesota's species is as follows. These comments are summarized from Oldfield and Moriarty (1994) and Coffin and Pfannmuller (1988) unless otherwise cited. Salamanders Blue-spotted Salamanders (Ambystoma laterale) are widespread in Northeast Minnesota and appear stable. The isolated populations in the southern half of the state are restricted to remnant maple-basswood forests. These populations are threatened by additional habitat fragmentation.
Tiger Salamanders (Ambystoma tigrinum) are found throughout the state and are the state's most common salamander. Tiger Salamanders are used by the bait and biological supply trade (Konrad 1996), but there is no data on the numbers taken. The populations have declined from the past when people regularly found them in window wells of houses in suburban areas.
Mudpuppies (Necturus maculosus) are found in the larger rivers and associated lakes in Minnesota. They are absent from the Mississippi River Drainage north of Saint Anthony Falls (Minneapolis). The status is unknown, but large quantities are being taken in the Alexandria area for the biological supply trade (Konrad 1996).
Eastern Newts (Notophthalmus viridescens) are uncommon salamanders in Minnesota. There is no population information on this species. There is still an isolated population in the Twin Cities Metropolitan area (Lake Minnetonka), which was recently rediscovered (J. LeClere, Pers. Comm.)
Four-toed Salamanders (Hemidactylium scutatum) populations were located in 4 east-central Minnesota counties in the spring of 1999. These populations are limited to small wetlands in hardwood forests. These discoveries help connect the previosly known population with known populations to the east.
Redback Salamanders (Plethodon cinereus) are found in the northeastern portion of Minnesota. They appear to be widespread and stable in that area, but there are no specific population studies for the state.
Anurans: American Toads (Bufo americanus) are found throughout the state and are one of the most common anurans encountered. Their populations seem stable. Large choruses are still found in agricultural and suburban areas.
Canadian Toads (Bufo hemiophrys) are restricted to the western third of the state and are more common in the northwest region of the state. There has been no published data on this species in the last 20 years, but the sites of Breckenridge and Tester (1964) are still marked and should be resurveyed.
Great Plains Toads (Bufo cognatus) are found in the western tier of counties in Minnesota. There has been little work done on this species in the last 20 years. The status of their populations are unknown.
Northern Cricket Frogs (Acris crepitans) was rediscovered in the summer of 1998 in Hennepin County, Minnesota. This population was a single, isolated population along the Minnesota River in Bloomington. This site is over 100 miles north of previous records. The population had up to 20 calling males. Surveys conducted in June of 1999 could not find any frogs. Baker (1996) proposes elevating the status of this species from special concern to endangered in the state.
Cope's Gray Treefrogs (Hyla chrysoscelis) are widespread in the state. The true range is still confused with H. versicolor. They are still abundant on the anuran call surveys.
Gray Treefrogs (Hyla versicolor) are widespread in the state. They are not as common in the prairie region of the state. They are common and appear to be stable.
Spring Peepers (Pseudacris crucifer) are restricted to the forested regions of the state. Populations in the Twin Cities Metropolitan area have gone through severe declines. They seem to be stable in less developed parts of the state.
Western Chorus Frogs (Pseudacris triseriata) are widespread and common in the state. They are still the most common frog in most areas. Wisconsin's Frog and Toad survey has
Bullfrogs (Rana catesbeiana) have been expanding their range in Minnesota through introductions and range expansion of introduced populations in Iowa and Wisconsin. A proposed change in the State Threatened and Endangered Species list would delist the species (Baker 1996). To date, the Bullfrog has not been a threat to other anurans, but that may change in the future (Lannoo 1996).
Green Frogs (Rana clamitans) are found throughout the eastern half of Minnesota. There have not been any in-depth surveys in 20 years, but they are still found at many sites in the Twin Cities metropolitan region. It appears that the populations are stable.
Pickerel Frogs (Rana palustris) are restricted to the southeast corner of Minnesota. Recent surveys have found it in a number of new localities (C. Dorff, Pers. Comm). Pickerel Frogs are currently listed as a Species of Special Concern, but are recommended for delisting since they are stable in the southeast (Baker 1996).
Northern Leopard Frogs (Rana pipiens) are widespread throughout the state and are the best known frog in the state. The are heavily collected for fish bait and for the biological supply trade (Konrad 1996). There is little regulation on the collection of Northern Leopard Frogs in Minnesota. Populations suffered a major crash in 1973 which halted the commercial collection for other than bait from 1974 until 1987. Recent harvest reports have only been in the 1,000 to 2,000 pounds per year. Reports in the early 1970's were in the 100,000 pound range (Tester 1995). Northern Leopard Frog populations have declined significantly from the past and are probably still declining.
Mink Frogs (Rana septentrionalis) are found in northeastern Minnesota. There has been no information on this species in the last decade, but they seem stable.
Wood Frogs (Rana sylvatica) populations appear stable. The range appears to be expanding west into the prairie region (D. Hoppe, Pers. Comm.). The biggest threat is forest fragmentation in developing areas.
Future Needs: The current research needs to continue into the future. The Anuran Call Survey should be increased to 100 routes statewide and modified to adjust for seasonal differences going from south to north in Minnesota.
A state funded project to develop biological assessment criteria for wetland health is being initiated at the University of Minnesota in 1996. This study will be using larval amphibian communities as one of the criteria (S. Galatowitsch and J. Tester, Pers. Comm.).
Additional surveys and field work should be conducted on the rare species in Minnesota. The Northern Cricket Frog localities need to be resurveyed, especially in southwest Minnesota. A recent MCBS survey of this species in southeast Minnesota did not locate any calling males (C. Dorff, Pers. Comm.). The distribution and habitat of the recently discovered Four-toed Salamander needs to be studied and documented.