Frog Prince & His Pesticides

February 21st, 2007

By Jacqueline L. Scahill
The Frog Prince may still be waiting for that special, life-transforming kiss, however, with global amphibian populations on the decline, his chances of living until and reproducing before the princess comes are decreasing.

Recent studies, such as one conducted by Marc Brodkin, a professor and researcher at Widener University, are pointing toward the role of environmental pollutants in amphibian population declines. Much evidence shows that some environmental contaminants disrupt the hormones of various animal species. These chemicals, known as endocrine disrupting compounds or EDCs, are “external agents that interfere in some way with the role of natural hormones in the body,” according to the Environmental Protection Agency (EPA). In two frog species, EDCs can cause abnormalities in reproductive organs, thereby reducing their ability to reproduce.

Brodkin states the population decline has been on the radar of scientists for over 25 years.

“There are lots of factors, including environmental degradation and over-utilization that are contributing to these declines,” says Brodkin. “Another is enigmatic factors, which are factors that have not yet been thoroughly defined.”

Although it is likely that the decline in amphibian populations are a result of a combination of many factors, scientists often look at one aspect of that compound.

“Our study and many other studies, look at one variable,” Brodkin said. “However, when scientists make a cocktail of chemicals that amphibians are exposed to, their study is much more actively representing the actual environment than by just looking at one factor.”

Widener’s study focuses on the compound, atrazine, which currently is the most widely used herbicide in the United States being used in chemicals like Round-Up. The study, which was recently published, considered the effects of atrazine on the immune response itself. It determined that concentrations of atrazine that cause endocrine disruption also cause immune disruption.

While studies using a concentration of 0.1 parts per billion (ppb) of atrazine in water found that this could cause endocrine disruption and Brodkin’s research found to cause immune disruption, the EPA’s maximum concentration for atrazine in drinking water is thirty times higher.

Atrazine has effects on frogs at levels lower than the three ppb allowed in drinking water, and yet, in streams of agricultural runoff and pools to which they lead, atrazine can be found at concentrations up to 250 ppb.

The immune response is a complex series of steps that work to attack organisms and substances that infect the body. White blood cells are part of this defense system – there are two types: phagocytes, like Pac-man, engulf invading organisms; lymphocytes, help the body to remember former invading organisms in case of future attacks. Widener’s findings suggest that the mechanisms that underlie recruiting white blood cells to the area of infection are more sensitive to atrazine than the mechanism that regulates the amount that phagocytes chew invaders.

Also, organisms have two types of immunity. The first is innate immunity, with which individuals are born. This includes skin, hair, mucous, tears, saliva, and coughing and sneezing, which expel pathogens from the body. The second is an acquired immunity, which includes cells that attack foreign bodies. The study further indicates that low levels of atrazine can act as a suppressor of the innate response, similar to a person with HIV being more susceptible to an array of diseases as a result of their immunodeficiency. Recent documented amphibian extinctions in Australia, Britain, North America, and Central America, are associated with pathogens, so, immunosuppresion may play a role in global amphibian declines.

In addition, Brodkin’s studies found that a pH of 5.5, which reflects that of many ponds in the northeast (due to acid rain), results in mortality of leopard frogs as a result of a suppressed immune system, and thereby increased infections.

“Carbon emissions are not only involved in global warming, but they are also involved with the acidification of bodies of water, including oceans,” said Brodkin.

What is so important about amphibian population declines, anyway? A person who is not a true lover of these cold-blooded creatures may wonder what their significance is in the world. Amphibians can prove a measure of the health of the environment, for one. Diminished atmospheric ozone layers have resulted from many human activities; pollutants are accumulating in the natural systems on which all organisms depend. Amphibian declines may be a response to these gradual, but fundamental changes in the ecosystem. In addition, amphibians are an integral part of balancing many habitats and populations. Many amphibians may also have biomedicinal properties – their skins often yield substances that can be useful in medicine, and many still have yet to be discovered. Populations of amphibians may go extinct without humans ever knowing that a chemical they produce may have significant medicinal value, whether it is a potential cure for cancer or for a particular disease. Finally, there are many amphibians that have much intrinsic and aesthetic value. By losing populations of amphibians, we are losing a beautiful part of the ecosystem.

The global decline in amphibian populations is an important reason to study the effects of environmental pollutants on individuals. However, studying the effects of a chemical such as atrazine on frogs’ immune systems may also indicate potential effects on humans. With further studies and implementations of change, the decline of amphibian populations may slow. Hopefully the Frog Prince will catch a kiss before he catches a cold.

Jacueline L. Scahill is a senior biology major who has kissed many many frogs in search of many many princes. Email her at
jscahil1[at]ithaca.edu.

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