Ecology 7th Edition by Manuel Molles

Exercise 7

One of the most powerful ways to test a hypothesis is through an experiment. Experiments used by ecologists generally fall into one of two categories-field experiments and laboratory experiments. Field and laboratory experiments generally provide complementary information or evidence, and differ somewhat in their design. Here we discuss the design of laboratory experiments.
In a laboratory experiment, the researcher attempts to keep all factors relatively constant except one. The one factor that is not kept constant is the one of interest to the experimenter and it is the one that the experimenter varies across experimental conditions. Let's draw an example of a laboratory experiment discussed in this chapter. Based upon published studies, Michael Angilletta (2001) concluded that geographically separated populations of the eastern fence lizard, Sceloporus undulatus, may differ physiologically or behaviorally.
Angilletta designed a laboratory experiment to test the hypothesis that populations of S. undulatus from regions with significantly different climates differ in how temperature affects their rates of metabolizable energy intake. The results of that experiment are summarized by figure 5.10. What we want to consider here is the design of the experiment that produced those results. What factors do you think Angilletta may have attempted to control in this experiment First, he used similar numbers of lizards from the two populations. He tested 20 lizards from both populations at 33°C, 13 from New Jersey at 30 8 and 36° C, and 14 from South Carolina at 30° and 36° C. A second factor that Angilletta controlled was lizard size. Lizards from both populations used in the experiments had an average body mass of approximately 5.4 g. Since males and females may differ physiologically, Angilletta included approximately equal numbers of males and females in his experiments. He also was careful to expose all the lizards to the same quality of light and to the same numbers of hours of light and darkness and he maintained them in the same kinds of experimental enclosures. Angilletta also fed all the lizards in his experiment the same type of food: live crickets. The list could go on but these are the major factors controlled in this experiment.

Now, what factors did Angilletta vary in that experiment For each study population, New Jersey or South Carolina, he varied a single factor: temperature. In the experiment, Angilletta maintained lizards from New Jersey and South Carolina at three temperatures: 30° , 33° , and 36° C and estimated their rates of metabolizable energy intake at these three temperatures. Angilletta's experiment revealed that lizards from both populations have a maximum metabolizable energy intake at 33° C. This result suggests, contrary to the study's hypothesis, that the optimum temperature for feeding does not differ for the two populations. However, the experiment also showed that at 33° C S. undulatus from South Carolina have a higher metabolizable energy intake compared to lizards from New Jersey. This result provides evidence of the geographic differences that Angilletta thought might exist across the range of S. undulatus. The power of this experiment to reveal the influence of temperature on lizard performance resulted from the ability of the researcher to control all significant factors but the one of interest. In this case the main factor of interest was temperature.
What is the greatest strength of laboratory experiments in ecological research