I remember hearing on the news in 1995 that schoolchildren in Minnesota had found a great deal of seemingly mutated frogs whose hind legs were either completely missing, missing extremities, split or who had up to five back legs. I remember listening to the report, looking at the videos of the captured leopard frogs and saying to Zak, “I bet it is heavy metal poisoning or pesticide exposure causing those mutations.”
It was not an unreasonable assumption. Both methyl mercury poisoning and pesticide exposure had been implicated in birth defects in human infants; it is not too far of a stretch of the intellect to assume that groundwater contaminated with these substances could easily penetrate the very permeable skin of amphibians such as frogs and cause similar mutations and birth defects.
I found this discovery of mutated frogs, in conjunction with the news that frog populations are declining worldwide disturbing, not only because it points to a loss of species diversity, but also because frogs are considered “indicator species,” by environmental biologists. Because of their permeable skin through which gases and liquids easily pass, any change in these animals morphology or population is an early indicator that something has changed in the environment. What I assumed that these mutated and disappearing frogs were telling us was that on a wide scale, our groundwater was contaminated with industrial and agricultural chemicals which are affecting these animals. And of course, while it may only affect frogs presently, toxic groundwater is eventually a great threat as the toxins gradually build up, climbing the food chain–the food chain which eventually ends not just with humans, but with our breast-fed infants. (Any and all environmental toxins present in the mother’s body are passed on to her child via her milk. For an in-depth look at this issue, read Having Faith by environmental scientist Sandra Steingraber. She frames her investigation into the human consequences of environmental industrial and agricultural toxins with the story of her own pregnancy and her experiences raising her daughter, Faith. I cannot recommend this book enough–not only is it educationally valuable, it is a very personal look at a global problem–in other words, instead of inundating the reader with statistics and figures–she shows us the human face of our collective disregard for the environment.)
Since the schoolchildrens’ discovery twelve years ago, malformed frogs have been reported in forty-four states; the problem is obviously more widespread than was first thought.
Environmental scientists started studying the problem, and at first, the most popular hypothesis was that it was exposure to ultraviolet radiation (caused by the thinning protective ozone layer) causing the malformed, missing and extra limbs, with agricultural or industrial chemical poisoning coming in as a close second favorite explanation.
One of the first possible culprits studied was UV radiation; it was shown that exposure to UV caused similar limb deformities in both salamanders and frogs, as well as causing eye damage in adult frogs and killing amphibian embryos and larvae. EPA scientists were quick to point out that while UV radiation could be part of the problem, in no experiments had it managed to cause a frog to grow extra limbs such as the ones in the frogs found in Minnesota, and now, all over the country.
Research into pesticide and herbicide contamination at first seemed promising, but later research found that most of these chemicals broke down quickly in the environment and were not likely to have directly caused the malformations. (Herbicides such as Atrazine were found to interfere with sexual development of frogs, by mimicking sex hormones–which is problematic and may be involved in a decline of reproductive capabilities, but they seemed to have no effect in limb development. Mind you–at much higher doses, this environmental pollutant can have similar effects in humans, but let us not worry about that now–the EPA says that there isn’t enough of it in the environment to be causing the documented decline in fertility among human beings that has been going on for the past few decades.)
As the years went on, the most likely candidate to pin the blame for the frog problem became parasitic infection. When I first heard that scientists had found that trematodes, a flatworm or fluke, were the likely cause of the deformities, I was fascinated. These tiny parasites are carried by snails, who are a host species, and then are transferred to frog embryos, where they attach themselves and form cysts, near the posterior end of the body. Studies using tiny glass beads to act as artificial trematode cysts have shown the same hind limb malformations, showing that the parasites are the most likely widespread cause. Analysis of areas where frog malformations were high showed high populations of trematodes in the environment.
Coincidentally, these areas also tended to be areas where agricultural runoff was also high.
For some people, the trematode is the end of the story, taking agricultural chemicals off the hook.
However, some scientists theorized that the reason why instances of parasitic malformation was high in wetland areas where agricultural runoff was prevalent may have to do with pesticides’ probable link between trematode infestation of frog embryos–scientists theorized that pesticides lowered the frogs’ immune systems and cause them to become more susceptible to infestation. This promising lead also ended up being at least partially discredited; while it was found that frog embryos exposed to various pesticides and herbicides had lower white blood cell counts and thus compromised immune systems, scientists failed to study the effects of these chemicals on the hosts of the trematodes–snails–and the trematodes themselves.
Finally, this week, it seems that a direct link has been established between trematode infestation in snails and frogs and agricultural chemical contamination through runoff: it appears to be the use of high levels of chemical fertilizers, carried from fields and deposited in wetlands by rainwater, which is the root of the problem.
How can this be?
Well, when large amounts of pure nitrogen and phosphorus are washed into ponds and other wild waterways, this extra fertilizer causes a very fast growth of algae, called “bloom.” This attracts many snails to eat the algae, and the higher population of snails gives trematodes a larger pool of possible hosts, which then causes a population boom in the parasites, which leads to more frogs being affected. (It is still also possible that runoff of pesticides and herbicides is also affecting the situation by lowering the frog’s resistance to parasitic infestation.)
The reason I found this issue imperative to write about today is because I realized that years ago, my knee-jerk reaction to blame agricultural chemical pollutants was only partially correct. My assumption that there was a direct link between the mutations and the chemicals was false, although my belief that there was a causal link between the two was true.
The lesson of the malformed frogs is this: when humans introduce powerful chemicals into complex ecological systems, it is very nearly impossible to predict all of the consequences. Every action humans take upon an environment causes a ripple effect that changes things far beyond the factors which humans wanted to control in the first place, and these changes may not become apparent until far into the future.
This is part of why I support the use of organic agricultural methods which have smaller, more predictable effects on the environment at large. For example, using organic matter as fertilizer, which must be broken down by the bacteria present in the soil, releases nutrients slowly into the soil, without causing overloads of fertilizer runoff in the waterways. (Algae bloom also causes large fish die-offs in streams, rivers, lakes and ponds by deoxygenating the water and suffocating the fish.) It also helps build healthy soil, which is not just rock dust, but which is in itself a complex ecosystem consisting of organic matter, insects, minerals, bacteria and other microorganisms, worms, nematodes and burrowing mammals. Healthy living topsoil is something that is so complex that many environmental and agricultural scientists say it would take generations of study just to come close to understand it.
These malformed frogs are telling us something–they are telling us to slow down and think before we act. They are telling us to stop doing uncontrolled and uncontrollable experiments on our land, our water, our air, our bodies, and on the whole of the earth, by dumping chemicals hither and yon, because we simply don’t know all of the implications of what we are doing.
I hope we listen, because if we don’t–what happens to the frogs may eventually happen to us.
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