Which of the following, if true, most helps explain the difference in the rates of decline between 1980 and 1990 in population of puffins and arctic terns, two kinds of seabirds for which sand eels serve as a primary source of food?
Show Hint
When a question asks you to explain a difference between two things, look for an answer choice that presents a difference between them. The cause of a different outcome is almost always a difference in attributes or behavior.
Puffins switched in part from their preferred food of sand eels to rockfish and other fish, but arctic terns did not.
The marked decline in the populations of puffins and arctic terns that occurred on Alair Island did not occur on other similar islands nearby, where there are substantial populations of both species.
The decline in sand eels was due to changes in environmental conditions that affected the reproduction of eels rather than to overfishing by people.
The main diet of puffin and arctic tern chicks on Alair Island in 1980 consisted of young sand eels.
Unusual severe weather that disrupted the breeding cycle of the sand eels of Alair Island in 1989 also damaged the nests of puffins but not those of arctic terns.
Hide Solution
Verified By Collegedunia
The Correct Option isA
Solution and Explanation
Step 1: Understanding the Concept:
This is an "Explain the Discrepancy" question based on a graph. The graph shows three populations declining from 1980 to 1990.
\begin{itemize}
\item Sand Eels (food source) decline moderately.
\item Arctic Terns (predator) decline steeply, more than the sand eels.
\item Puffins (predator) decline, but much less steeply than the Arctic Terns.
\end{itemize}
The discrepancy is: why did the puffins fare so much better than the arctic terns when their common primary food source was declining? We need to find a difference between the two bird species that accounts for this difference in outcome. Step 2: Detailed Explanation:
\begin{itemize}
\item (A) This provides a direct explanation for the difference. If puffins were able to find and switch to an alternative food source (rockfish), their population would be less affected by the decline in sand eels. If arctic terns were unable to switch, their population would be almost entirely dependent on the declining sand eels, leading to a much steeper population crash. This perfectly explains the different slopes on the graph.
\item (B) This explains that the problem was local to the island but does not explain the different outcomes between the two species on that island.
\item (C) This explains the cause of the sand eel decline but does not explain why the puffins and terns reacted differently to it.
\item (D) This establishes the baseline condition in 1980 but does not explain the subsequent divergence in population trends.
\item (E) This would lead us to expect the opposite result. If puffin nests were damaged and tern nests were not, the puffin population should have declined more steeply, not less.
\end{itemize}
Step 3: Final Answer:
The ability of one species to adapt by finding alternative food, while the other could not, is the most logical explanation for their different rates of decline. Option (A) provides this crucial differentiating factor.
