Explanation
Correct answer: D.
Choice D is the best answer because it provides the finding that, if true, would most directly support Aburto-Oropeza and colleagues’ explanation that species composition differs between Alacranes and Bajos del Norte because of a difference in the regulation of fishing at the two reefs. The text explains that isolated oceanic reefs are often unprotected and are increasingly attracting commercial and sport fishing activity. According to the text, when Aburto-Oropeza and colleagues surveyed Alacranes, an isolated reef where fishing is prohibited, and Bajos del Norte, an isolated reef that is unprotected, they found that species at the highest trophic level (that is, top predators) make up a smaller proportion of the total mass of all living organisms at Bajos del Norte (10%) than they do at Alacranes (34%). If it were true that commercial and sport fishers tend to disproportionately remove top predators—that is, that they catch them at a higher rate than they catch other species—this would support the explanation that the allowance of fishing at Bajos del Norte and the prohibition of fishing at Alacranes has led to a lower proportion of top predator species remaining in the total biomass at Bajos del Norte.
Why the other choices are wrong
Choice A
Choice A is incorrect. To support the researchers’ claim that the role of fishing regulations accounts for Alacranes having a greater proportion of species at the highest trophic level (top predators) than Bajos del Norte does, a finding would need to help explain how fishing affects the presence of top predator species in particular; the finding that the two reefs have similar biomass densities (that is, about the same mass of living organisms per unit) and that Alacranes has a much greater total biomass (total mass of living organisms) wouldn’t reveal anything about the species composition at each reef or help explain why the proportion of top predators in the total biomass at each reef differs. This finding might simply suggest that Alacranes is a larger reef, with more units at that density.
Choice B
Choice B is incorrect because the finding that there are species at the highest trophic level (top predators) at Bajos del Norte that aren’t found at Alacranes would reveal only that there are some different species in the reefs’ total biomasses (total mass of living organisms) but wouldn’t explain why that’s the case; this finding wouldn’t be clearly connected to the role of fishing regulations, which is what, according to the researchers, accounts for the reefs having different proportions of top predators in their total masses.
Choice C
Choice C is incorrect. To support the researchers’ claim that the role of fishing regulations accounts for Alacranes having a greater proportion of species at the highest trophic level (top predators) than Bajos del Norte does, a finding would need to help explain how fishing affects the presence of top predator species in particular, but different costs of reaching the two reefs wouldn’t affect fishing behavior at all because fishing isn’t even allowed on one of the reefs.