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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.]
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.
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.
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.
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.
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