The studies of amphibian deformities that we discussed earlier in this chapter illustrate several ways in which ecologists seek to answer questions about the natural world.
The study by Johnson and his colleagues (1999), for example, had two key components: observational studies in the field and a controlled experiment in the laboratory. In the observational part of their work, the researchers surveyed ponds, noted the species present, and observed that tree frog deformities were found only in ponds that contained both tree frogs and a snail that harbored the Ribeiroia parasite.
These observations suggested that Ribeiroia might cause deformities, so Johnson and his colleagues performed a laboratory experiment to test whether that was the case (it was).Kiesecker (2002) extended these results in two experiments, one performed in the field, the other in the laboratory. To examine the effects of pesticides on frog deformities, Kiesecker compared results from three ponds containing pesticides with results from three ponds without detectable levels of pesticides. While this approach had the advantage of allowing the effects of Ribeiroia to be examined under different field conditions (in ponds with and without pesticides), Kiesecker could not control the conditions as precisely as he did in his laboratory experiment. The constraints of working in the field meant, for example, that he could not start out with six identical ponds, then add pesticides to three of them but not to the other three—an experiment that would test more directly whether pesticides were responsible for the results he obtained. As this example suggests, no single approach works best in all situations, so ecologists use a variety of methods when seeking to answer ecological questions.