Over the last 150 years, large stretches of salmon

[JbhB682]

Source : GMAT prep

Over the last 150 years, large stretches of salmon habitat have been eliminated by human activity: mining, livestock grazing, timber harvesting, and agriculture as well as recreational and urban development. The numerical effect is obvious: there are fewer salmon in degraded regions than in pristine ones; however, habitat loss also has the potential to reduce genetic diversity. This is most evident in cases where it results in the extinction of entire salmon populations. Indeed, most analysts believe that some kind of environmental degradation underlies the demise of many extinct salmon populations. Although some rivers have been recolonized, the unique genes of the original populations have been lost.

Large-scale disturbances in one locale also have the potential to alter the genetic structure of populations in neighboring areas, even if those areas have pristine habitats. Why? Although the homing instinct of salmon to their natal stream is strong, a fraction of the fish returning from the sea (rarely more than 15 percent) stray and spawn in nearby streams. Low levels of straying are crucial, since the process provides a source of novel genes and a mechanism by which a location can be repopulated should the fish there disappear. Yet high rates of straying can be problematic because misdirected fish may interbreed with the existing stock to such a degree that any local adaptations that are present become diluted. Straying rates remain relatively low when environmental conditions are stable, but can increase dramatically when streams suffer severe disturbance. The 1980 volcanic eruption of Mount Saint Helens, for example, sent mud and debris into several tributaries of the Columbia River. For the next couple of years, steelhead trout (a species included among the salmonids) returning from the sea to spawn were forced to find alternative streams. As a consequence, their rates of straying, initially 16 percent, rose to more than 40 percent overall.

Although no one has quantified changes in the rate of straying as a result of the disturbances caused by humans, there is no reason to suspect that the effect would be qualitatively different than what was seen in the aftermath of the Mount Saint Helens eruption. Such a dramatic increase in straying from damaged areas to more pristine streams results in substantial gene flow, which can in turn lower the overall fitness of subsequent generations.


1. The primary purpose of the passage is to

(A) argue against a conventional explanation for the extinction of certain salmon populations and suggest an alternative
(B) correct a common misunderstanding about the behavior of salmon in response to environmental degradation caused by human activity
(C) compare the effects of human activity on salmon populations with the effects of natural disturbances on salmon populations
(D) differentiate the particular effects of various human activities on salmon habitats
(E) describe how environmental degradation can cause changes in salmon populations that extend beyond a numerical reduction


Question

What is the reason why C is incorrect .. i chose C because
a) the blue clearly shows the author comparing the effects of human activity vs. natural disturbances -- is it not ? the author clearly brought up the blue because he wanted to compare human activity to natural environmental degradation
b) the passage started per the red -- saying humans have caused salmon populations to decline ..

E (which is the OA) seems to very focused specifically on environmental degradation ..no mention on human degradation which i thought for the purpose of the whole passage (reviewing the first line in red -- clearly the author is talking about human degradation and comparing it to naturally causing environmental degradation)

[Sage Pearce-Higgins]

Thanks for your clear, precise questions here. Let me address them in turn:

Why C is not correct:
You're right that the blue part does make a comparison between human activity and natural disturbances. However, the author doesn't investigate this comparison or say anything more about it. In fact, the eruption of Mount Saint Helens is simply used as an example of a 'severe disturbance', whether human or natural. The comparison between human activity and natural disturbances is part of the argument that 'habitat loss also has the potential to reduce genetic diversity'. If we map the passage, then we can see the following themes:
Paragraph 1: human activity has reduced salmon habitat, and that's reduced salmon numbers
Paragraph 2: disturbances also affect the genes of populations through 'straying', e.g. Mt. St. Helens
Paragraph 3: presumably human activity has a similar effect to natural disturbances

Why E is correct:
They're not going to make your life easy by mentioning humans. It's a typical trap of RC general questions that the correct answer is vague-sounding. But, if we think about it, environmental degradation includes both human activity and the eruption of Mount Saint Helens. For me, the phrase 'extend beyond numerical reduction' was a big clue: see how the second paragraph builds on the argument started in the first.