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The scholarly article, “Predator-induced stress makes the pesticide carbaryl more deadly to gray treefrog tadpoles,” summarizes Relyea’s and Mills’ research on widespread pesticide use and its impact on the global decline in amphibians. Previously, there was not enough persuasive evidence to support the hypothesis that pesticide use was a major contributor to global amphibian decline, and any existing research was conducted under highly artificial conditions. However, Relyea’s and Mills’ research has shown that low concentrations of just one pesticide—in this case carbaryl—can cause high mortality rates in gray treefrogs.
The researchers stress the importance of follow-up studies that incorporate more natural experimental conditions to further explore the impact of environmental contaminants on amphibian populations.
The researchers conducted a pilot study in 1999 to “determine the chronic (longer-term) effect of carbaryl and predator stress on larval treefrog survival.” The researchers collected eggs from a pond, hatched the eggs in filtered tap water, and then randomly assigned groups of 10 tadpoles to tubs with filtered tap water.
Each tub was randomly assigned one of four chemical treatments (two control treatments and one high and one low-concentration carbaryl treatment) and one of two predator treatments (either a caged larval salamander or an empty cage). The experiment lasted ten days, and the tadpoles were fed ground fish food throughout the experiment, whereas short-term tests typically would not have provided food. The water was changed every three days and the chemicals were reapplied after each water change.
The researchers conducted two, more-thorough follow-up studies in 2000 to “determine the effects of carbaryl and predator stress on larval treefrog behavior, growth and survival.
” The researchers collected eggs from a different pond, and once again hatched them in filtered tap water. The researchers randomly assigned groups of 10 tadpoles to tubs filled with filtered tap water. The tubs were randomly assigned a factorial combination of two predator treatments and six chemical treatments. While the predator treatments remained the same as in experiment 1, the chemical treatments now had four concentrations of carbaryl and two controls (compared to two concentrations in the first experiment.) The experiment lasted 16 days, and the tadpoles were once again fed food throughout the test. The water was changed every four days, and the oxygen, temperature, pH and total ammonia was measured for each tub. In both experiments, the researchers applied both a chemical and biological approach by including both chemical treatments and predator treatments.
While skimming the discussion section, I learned that very low use of carbaryl killed up to 97% of tadpoles when given a few extra days to observe the effects. This is important to note, since previous studies determined that low-grade pesticide use had no effect on amphibian survival rates—only on tidepole activity and growth. However, it is now clear that these results were determined only because the effects were not studied for an adequate amount of time. This emphasizes the importance of well thought-out research methods, since details such as study length can greatly impact final results.
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