Answers to Hone Your Problem-Solving Skills Questions

1. The five tanks with no atrazine serve as the control. By comparing results from control tanks to results from tanks with atrazine, an investigator could test whether the presence of atrazine affected one or more of the six variables measured in the experiment (phytoplankton abundance, attached algae abundance, water clarity, eosinophil number, tadpole survival, and number of Ribeiroia cysts).

2. Compared to the controls, when atrazine is added phytoplankton abundance decreases more than three-fold, the abundance of attached algae increases, and water clarity increases. To interpret these results, note that atrazine may have caused phytoplankton abundance to drop, which would cause water clarity to increase (because fewer phytoplankton were suspended in water), and that, in turn, would cause more sunlight to reach the algae attached to rocks, causing their abundance to increase.

3. Compared to the controls, when atrazine is added the number of eosinophils decreases more than two-fold, tadpole survival drops from 72% to 45%, and the number of Ribeiroia cysts increases more than four-fold. Atrazine may have impaired the tadpole's immune response, thereby causing the number of Ribeiroia cysts to increase, which would harm the tadpoles and cause their survival to drop.

4. The addition of atrazine to a pond could cause phytoplankton abundance to drop, thereby increasing the sunlight available to attached algae, hence increasing the growth of attached algae. Snails eat attached algae, so an increase in the abundance of those algae could cause snail abundance to increase, and that, in turn, could cause Ribeiroia abundance to increase (because Ribeiroia depends on snails to complete its life cycle). Atrazine also impairs the tadpole's immune response. Overall, since atrazine increases Ribeiroia abundance and impairs the tadpole's immune response, that could cause the number of Ribeiroia cysts to increase and tadpole survival to decrease.