Article: AR Blaustein and PTJ Johnson. 2003. The complexity of deformed amphibians. Frontiers in Ecology and the Environment 2(1): 87-94


The emphasis in TIEE Issues is use of figures and tables for discussion and other types of student-active teaching and learning. These notes will give you ideas about using the figures in this paper in your ecology class. The Student-Active Teaching table will introduce you to a variety of approaches you can use in your class to actively engage your students. To see an essay on leading good discussions, go to Guided Class Discussions.

Blaustein and Johnson’s paper

This is an interesting paper that is quite accessible to undergraduates. You can use it to teach several ecological topics including the life cycle of a parasite involving several animal hosts, causes for biodiversity loss, and the concept of interaction between environmental and biological variables in an ecological system.

The amphibian deformity question will intrigue students for several reasons. First, seeing pictures (see resources) of the deformities is just interesting (and for some, gross at the same time). Stories about school children finding deformed frogs and bringing the issue to national attention please students as well.

Second, this is a puzzle about which scientists disagree and puzzles pique our curiosity. Many students may well be surprised that good scientists working on the same phenomenon can disagree. Most students have an immature understanding of the nature of scientific study; they are inclined to think that there are right and wrong answers to questions and that in a scientific disagreement one scientist is right and the other wrong. This paper allows these students to begin to appreciate the grey, i.e., differences of opinion may exist because of the different types of evidence examined.

(To get at the “why scientists might disagree” issue, you can use some of the questions below to stimulate discussion about the types of data ecologists use as supporting evidence and the observations and data that contradict each argument.)

Finally, students are drawn to questions about loss of diversity and most relate positively to frogs and other amphibians. This paper can help students better understand the complexity of species loss, which in many cases is not due to a single cause that can be fixed. The concept of interactions is important in ecology but difficult for some students to grasp. The examples of interactions here are concrete and readily understandable. They can help students appreciate why loss of diversity can accelerate with time.

Tips on how to use this Issue

Focus on some aspect of this Issue (e.g., lab versus field evidence) and select a figure or discussion question dealing with this point. Then select a teaching approach. The Table of Student-Active Approaches includes suggestions for all class sizes.

Comments about the figures

Blaustein and Johnson’s paper does not contain numerically-based figures. I find that discussion of figures is an effective way to engage students more deeply in an ecological question. In addition, describing and interpreting figures helps students better understand the process of science and gives them more confidence about their skill with data. (See Figure Sets Overview for more about this). I have therefore included three figures in the “Figure” section.

The first, Johnson Figure 1, from Johnson et al. 1999, has two panels. Panel A shows the inverse relationship between trematode (Ribeiroia sp.) density in tadpoles of the frog Hyla regilla and tadpole survivorship. Panel B is data from an experiment comparing effects of two different trematodes on Hyla tadpole survivorship and abnormality frequency. These data are strong evidence that Ribeiroia is an agent of the deformities. (The other trematode, Alaria, penetrated tadpoles but did not cause deformities). This is straightforward figure for students to describe and interpret.

The second set of figures are from a study by Kiesecker (2002) that links increased trematode infection, limb deformities of woods frogs, (Rana sylvatica), and pesticide exposure (Atrazine, Malathion, and Esfenvalerate).

Exposure to low concentrations of contaminants had dramatic effects on the immune response and rates of cercarial encystment. For each of the three contaminants, exposure increased the proportion of both Ribeiroia sp. and Telorchis sp. that successfully encysted. Likewise, exposure also altered woods frogs’ immune response, as measured by circulating leucocytes. Here I will focus on the response of eosinophils for simplicity and because of the role they play in macroparasite infections. As expected, exposure to pesticides resulted in a decrease in the number of eosinophils circulating in the blood.

In the discussion, the author says:

My findings support the hypothesis that parasite infection explains the development of limb deformities observed in frog populations in nature. By preventing access of cercariae to the developing amphibians, I was able to prevent developmental abnormalities. These results provide definitive evidence that links deformities under natural conditions to trematode infection. The occurrence of trematode-mediated limb deformities, however, depended on the context of the interaction. Stress in the form of pesticide exposure decreased the host tadpoles’ ability to resist infection, resulting in higher parasite loads and higher risk of limb deformities. Laboratory experiments revealed association between pesticide exposure and increased infection. The decreased immunocompetency associated with pesticide exposure could explain the observed differences in limb deformities at field sites given the link between increased cyst formation and increased limb deformities observed in other studies. However, although chemical contaminants can contribute to the occurrence of deformities, I found that it was necessary for developing amphibians to be exposed to trematode infection for limb deformities to occur. Parasite removal experiments conducted at natural breeding sites, like those conducted in this study, should be used as a first step to assess the etiology of limb deformities. Development of limb deformities in treatments exposed only to trematodes would guide researchers to focus on the role of trematode infection and factors that modify this host–pathogen interaction. However, occurrences of limb deformities in all treatments would then warrant more sophisticated chemical analyses and studies designed to determine which contaminants are responsible for deformities.

Note: Kiesecker Figure 2 is more complicated than Kiesecker Figure 1 and will take students more time to figure out. It is a nice example of ecological interactions and how controlled experiments develop from field observations, i.e., deformities were more frequent near agricultural fields.

A few definitions:

Discussion Questions

  1. Amphibians have skin without scales or feathers and eggs without shells. They live in a wide range of habitats. How might these characteristics result in their being “bioindicators” of environmental problems?
  2. Amphibian limb deformities may result from interactions between several factors, for example between pesticides and parasite infection. What is meant here by an interaction and how is this important in environmental research on the problem of limb deformation?
  3. Deformed limbs may be a secondary cause of amphibian mortality. Give an example of a primary cause.
  4. Three likely causes of amphibian deformities are UV radiation, chemical contamination, and trematode infection. What kinds of field evidence would you look for to support each of these as the causal agent(s)?
  5. Scientists disagree about the causes of amphibian limb deformities. For instance, some point to pesticide contamination and others to parasitic trematode infection. Why might scientists disagree about this question? Is one group wrong and the other right? How might you assess this?
  6. To assess whether the trematode Ribeiroia sp. caused frog limb deformities, Johnson et al. (1999) exposed tadpoles to Ribeiroia and another trematode Alaria sp. Why did they expose tadpoles to two species of trematode worms? What question were they asking?
  7. Most biologists accept the idea that host/parasite interactions often lead to coevolution between the two partners in the relationship. To what degree would parasitic trematodes act as a selective agent to frogs, and would you expect frogs to evolve in response to parasite-induced deformities? What evolutionary significance would you attribute to the fact that some frogs subjected to parasitic infestation do not suffer from deformities? How would you test whether the latter frogs are somehow more evolutionarily “fit” than the ones showing deformities?
  8. After reading the Kiesecker paper, examine the manner in which Blaustein and Johnson reported their key findings. Do you believe that Blaustein and Johnson accurately represented the information given by Kiesecker? Did you see any critical points that Kiesecker made in his paper that Blaustein and Johnson overlooked in their review?
  9. Some people may perceive that biologists are wasting their time studying limb deformities in frogs. They would argue that intelligent people should focus their energies on other problems like global terrorism, skyrocketing budget deficits, and rampant drug abuse among teenagers. Do you agree with that line of reasoning? Explain your answer.