Question 18·Hard·Command of Evidence
Percentage of Available Eggs Eaten by Cane Toad Tadpoles
| Amphibian species (common name) | Percentage of eggs eaten | Native to Australia | Produces bufadienolide |
|---|---|---|---|
| Little red tree frog | 1% | yes | no |
| Cane toad | 90% | no | yes |
| Short-footed frog | 7% | yes | no |
| Striped burrowing frog | 10% | yes | no |
| Dainty green tree frog | 1% | yes | no |
Native to Latin America, the cane toad was introduced to Australia in the 1930s. In recent decades, tadpoles in the Australian population have been shown to consume eggs of their own species. A 2022 study showed that when presented with cane toad eggs as well as eggs of native Australian amphibians, cane toad tadpoles disproportionately consumed eggs of their own species. This behavior results from their attraction to bufadienolide, a chemical produced by cane toad eggs but not by the eggs of native amphibians.
However, using data from this study, a student wishes to argue that the presence of bufadienolide doesn’t entirely explain the cane toad tadpoles’ preference for certain eggs over others.
Which choice best describes data from the table that support the student’s argument?
For table-based evidence questions, first restate the claim you’re trying to support or challenge in simple terms (here: “bufadienolide doesn’t completely explain preference”). Then scan the table for patterns that directly speak to that idea—often, you should compare rows where one key variable is the same (like all species without bufadienolide) and see if the outcome still differs. Finally, test each answer choice against this pattern: the correct choice will clearly describe a comparison or trend in the data that matches the claim in the question, while wrong choices will be irrelevant, too general, or even support the opposite point.
Hints
Clarify the student’s claim
The student wants to argue that bufadienolide does not completely explain which eggs the tadpoles prefer. Think about what kind of pattern in the table would show that something besides this chemical must also be affecting their choices.
Focus on the bufadienolide column
Look carefully at which species produce bufadienolide and which do not. How many species have this chemical? What would we expect the percentages to look like if bufadienolide were the only factor that mattered?
Compare species with the same chemical status
Try comparing the percentages eaten for species that either both have bufadienolide or both don’t have it. If tadpoles treat such species differently, what does that suggest about bufadienolide being the sole explanation?
Test each answer against the student’s claim
For each choice, ask: does this statement show that tadpoles behave in a way that cannot be fully explained just by whether the eggs have bufadienolide? Eliminate choices that are neutral, irrelevant, or that actually support bufadienolide as the full explanation.
Step-by-step Explanation
Restate what the question is really asking
The passage says cane toad tadpoles are attracted to bufadienolide, which only cane toad eggs have, and that this explains why they disproportionately eat their own species’ eggs. The student wants to argue that bufadienolide doesn’t entirely explain which eggs they prefer. So we need data showing tadpoles’ preferences that cannot be fully explained just by the presence or absence of bufadienolide.
Identify which species produce bufadienolide
Look at the table’s last column:
- Cane toad: produces bufadienolide.
- All four native Australian frogs (little red tree frog, short-footed frog, striped burrowing frog, dainty green tree frog): do not produce bufadienolide.
So if bufadienolide were the only factor, we would expect:
- Cane toad eggs to be treated differently from all the others.
- Among the native species (none with bufadienolide), bufadienolide would not predict any differences, because they’re all the same in that respect.
Look for preference differences where bufadienolide is the same
Now focus on the native species, which all lack bufadienolide. If the percentages eaten vary across these species despite all having the same bufadienolide status, that would suggest something besides bufadienolide is influencing which eggs the tadpoles eat.
Match the reasoning to the answer choices
Evaluate which option describes a difference in consumption between species that both lack bufadienolide.
- Choice A describes a higher percentage eaten for one native species than for another native species. Since neither produces bufadienolide, that difference supports the argument that bufadienolide doesn’t fully explain preference.
- Choice B summarizes the overall range for native species but doesn’t highlight a preference difference that challenges bufadienolide as a complete explanation.
- Choice C compares a non-bufadienolide species to cane toads (which do have bufadienolide), which is consistent with bufadienolide driving preference.
- Choice D describes equal consumption for two non-bufadienolide species, which does not show an unexplained preference.
Therefore, the best choice is: The tadpoles consumed a higher percentage of the striped burrowing frog eggs than they did of the eggs of the dainty green tree frog.