Ecologists hypothesized that a decline in sea otter populations in the North Pacific led to a collapse of kelp forests. Sea otters do not eat kelp; however, they prey heavily on sea urchins, which are the primary grazers of kelp.
Which finding, if true, would most directly substantiate the researchers' hypothesis?
Sea otters in the region have recently begun preying on smaller fish species due to a scarcity of sea urchins.
Kelp density is generally higher in areas where both sea otters and sea urchins are absent compared to areas where only otters are present.
The population of sea urchins in the region increased dramatically shortly after the decline in the sea otter population began.
Sea urchins have been found to grow larger in size in regions where kelp forests are most dense.
Correct Answer: C
Choice C is the best answer. The hypothesis implies a chain reaction: Otters eat urchins → Urchins eat kelp. Therefore, fewer Otters should mean more Urchins (and thus less kelp). Choice C confirms the link that a decline in otters led to an increase in urchins.
Choice A is irrelevant to the kelp forest hypothesis. Choice B discusses areas where both are absent, which does not explain the interaction between the species. Choice D discusses urchin size, not population density or the causal link to otters.