frog survey Feature-eNotes

Peepers and Creekers and Snorers, Oh My!

By Michele Arduengo, Ph.D., Promega Corporation

The Franklin Street Bridge proved a disappointment, no frogs or toads tonight. A few waterfowl cried in the distance, but I did not hear a single anuran call. My search for frogs and toads would be soggy and chilly, but I continued to the next listening site, a low spot on HiLo Road in Kettle Morraine State Forest.

Thousands of volunteers around the world participate listening surveys to track frog and toad populations. But why frogs and toads? What is it about these animals that piques our curiosity? And what can we learn from monitoring frogs and toads?

Figure 1. One of the first frogs heard in the spring in Wisconsin is the Chorus Frog. The call of this frog sounds like the sound produced when you run your finger across the teeth of a fine-toothed comb. [Picture by Lisa Hemesath, Iowa State University; kindly provided by and used with permission of Eugenia Farrar Iowa State University.]

An Historical Perspective on Frogs and Toads

According to FrogWatch Canada, Frogs and toads (anurans) have been around for 150 million years, and are the first land animals with vocal chords. Frogs and toads have captured the imagination and respect of humans throughout history. In ancient Egypt, frogs were associated with resurrection and creation, and they were often mummified along with the dead. Ancient Christian and Hebrew stories describe a plague of frogs as one of the ten plagues of Egypt. A Burmese/Indo-Chinese legend states that the frog was responsible for the lunar eclipse because it swallowed the moon (1).

Today, perhaps, we are less inclined to attribute celestial events to the vocalizing anuran, but frogs and toads continue to get our attention. In 1995 a group of school children on a field trip to a Minnesota farm made national headlines with their findings. While they were learning about the normal animals and plants in the area, they found a large number of frogs with extra limbs, missing limbs and other deformities (2). This immediately launched a series of dire warnings about our environment from sources both reliable and bogus and led to predictions of a world without frog and toad calls to fill the summer evenings.

Figure 2. In June and July, you can hear the Green Frog call. The call of the Green Frog sounds like a plucked banjo string.

The Wisconsin Frog and Toad Survey: An Example to Follow

To learn from our anuran friends, we need long-term, reliable data that can describe population and behavioral trends. The Wisconsin Frog and Toad Survey (WFTS), begun in 1981 by Ruth Hine of the Wisconsin Department of Natural Resources (WDNR), is perhaps the longest running anuran survey in existence, and its methods form the basis of the international North American Amphibian Monitoring Program (NAAMP). The WFTS was started in response to known and perceived declines in anuran populations. Although field scientists and local people thought something was changing with frog populations, they did not have carefully collected data to back up their assumptions. The WFTS has two primary goals: 1) to determine the status and distribution of the 12 Wisconsin anuran species and 2) to educate the general population about anurans and their contribution to ecosystems (3).

The WFTS is now coordinated by Robert Hay, Ph.D. at the WDNR and is conducted by volunteers who learn to recognize the breeding calls of the eleven species of frogs and one species of toad found in Wisconsin. Each volunteer's listening route comprises ten roadside amphibian habitats. The habitats range from ephemeral wetlands, to marshes, meadows, creeks, ponds, lakes and wooded swamps. During three specified periods each summer, the volunteers go out into the nighttime world and visit their ten sites, listening for the calls of the frogs and toads.

There are anywhere from 85–105 volunteers listening to anurans as part of the WFTS. The goal of the program is to establish two listening routes in every county; in 2003 only 80 survey routes were run. Even though coverage does not yet meet the DNR goal for the program, the news is good with “more positive than negative trends based on the data collected so far” according to Hay. In 2003, five of the 12 anuran species in Wisconsin showed an increase in percent occurrence from 2002.

So far the data analysis has been limited to documenting the percent occurrence and relative abundance of each of the twelve species. Soon however, Hay says that the WDNR will be adding new tools to look at the data. The goal is to develop “animated maps” that allow scientists to manipulate the data in a variety of ways. For instance, they can ask how an increase in one species, such as the bullfrog, affects other species in the area, like leopard frogs. They can look at the overall trends for a suite of species or look at environmental data. The new way of looking at the data will enable scientists to ask new questions and perhaps improve their survey methods.

A special case: The Blanchard’s Cricket Frog

Wisconsin’s only endangered anuran species is the Blanchard’s Cricket Frog, which is found in three Wisconsin counties: Iowa, Lafayette and Grant. The WDNR is making a concerted effort to learn more about this frog. “We’re discovering how much we don’t know about this frog,” says Hay. The frog is too small to track, so scientists have been unable to follow the adults into hibernation at the end of the summer. This frog has no freeze tolerance, and scientists are baffled at how it survives the harsh Wisconsin winters. Some studies in Ohio suggest that the frogs may hibernate in the chimneys of subterranean crayfish burrows, where the temperature is above freezing, but it is unclear if the Wisconsin populations could use that strategy.

Figure 3. The Blanchard's Cricket Frog is on the endangered species list in Wisconsin. Scientists are trying to understand how this frog survives the tough Wisconsin winters. The call of this small frog sounds like metal balls being clicked together. (Picture by Aubrey Heupel, kindly provided by and used with permission of Eugenia Farrar, Iowa State University.)

These frogs are quick to move into new places. If a pond is created near an area where these frogs are breeding, they will quickly take up residence in the new pond. The WDNR is looking for sites where the frogs have not been described before, and they are also measuring land use and water quality parameters in addition to tracking the population changes. There seems to be a negative correlation between the turbidity (murkiness) of the water and the population. However, these measures of water quality are in constant flux, so more research needs to be done before scientists can determine if characteristics of water influence the success or failure of the Blanchard’s Cricket Frog populations.

What can we learn from a frog?

Frogs and toads are excellent bioindicators, species that can be monitored to determine the environmental health of an area (4). They have a complex lifecycle in which major developmental events occur in both land and water. Their skin is permeable, and they are very sensitive to water chemistry during their egg and larval stages.

A frog or toad population decline can result from many types of environmental stresses such as water pollution, increase in UV radiation from changes in precipitation and climate, or disease (5,6). Local population declines might result from changes in land use, destruction of habitat, or introduction of a new predator into an area. One study in California was able to establish a relationship between trematode infection and high rates of developmental deformities in Pacific treefrogs (Hyla regilla; 7), and other studies have described an fungus that is killing frogs (Pounds et al. 2001).

Deformities in frogs might be due to the presence of trace amounts of chemicals that are interfering with normal development. Since humans share many biochemical and genetic pathways with frogs and toads, learning how the environment influences frog and toad development may shed light on human development. Also, changes in the timing of anuran breeding seasons can provide clues to changes in climate. For instance, trends toward earlier breeding seasons could indicate global warming.

Following the ups and downs of frog populations and looking for trends over a long time period can teach us a lot about what is happening in the environment. However, we need more data and we need a better understanding of the biology of anurans before we can draw any meaningful conclusions about how changes in anuran populations can serve as predictors for environmental and human health.

Works Cited

Canadian Nature Federation (2001) Frog Log, Volume 1. http://www.cnf.ca/froglog/folk.html (accessed on September 29, 2004)
Mossman, M.J. et al. 1998. The Wisconsin Frog and Toad Survey home page. [http://www.mbr-pwrc.usgs.gov/wifrog/frog.htm] Wisconsin Department of Natural Resources (Madison) and USGS Patuxent Wildlife Research Center, Laurel, MD.
Mossman, M.J. et al. (1988) Monitoring long-term trends in Wisconsin Frog and Toad Populations. In: Status and Conservation of Midwestern Amphibians. Lannoo, M.J., ed. University of Iowa Press, Iowa City. 169–198.
Pounds, J. A. (2001) Climate and amphibian declines. Nature 410, 639–40.
Kiesecker, J.M., Blaustein, A.R. and Belden, L. (2001) Complex causes of amphibian population declines. Nature 410, 681–684.
Johnson, P.T. et al. (1999) The effect of trematode infection on amphibian limb development and survivorship. Science 284, 802–4.
Carpenter, H.M. (1999) Frog deformities: Searching for answers. Minnesota Medicine. 82,14–19.