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Over the last 150 years, large stretches of salmon habitat

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Over the last 150 years, large stretches of salmon habitat [#permalink] New post 17 Feb 2013, 07:08
Complete discussion is provided at the below mentioned link
over-the-last-150-years-large-stretches-of-salmon-habitat-135658.html#p1103703


GMATPrep wrote:
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. 1ndeed, 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 ifthose 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.

According to the passage, human activity has had which of the following effects on salmon population?

1. An increase in salmon population numbers in some previously polluted rivers.

2. A decline in salmon numbers in some rivers.



I answered #1, which is apparently wrong:

GMATPrep wrote:
While human efforts may have been able to clean up some rivers and encourage some salmon to return to them, the passage does not discuss them.


My Argument: Clearly the correct answer has to be about rivers, which are mentioned only once in the passage (highlighted in bold) - and the one mention discusses recolonising or increasing population numbers.

The correct answer is apparently #2:

GMATPrep wrote:
According to the passage, there are fewer salmon in areas that have been degraded by human activity.


My Argument: The question asks about rivers, not areas. The passage mentions regularly the declining population in "areas", "locales" and "habitats" - which are not limited to rivers. The passage does not explicitly discuss reducing populations in rivers.



Please correct me.
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Re: Numbers of river salmon, GMAT Prep [#permalink] New post 17 Feb 2013, 07:16
Hi Stormbind,

My advice to you in general would be to look first at the general picture, and see wht that tells you before looking into the minutiae.

It is clear that the overall tone of the first paragraph is that the human influence has been negative. This should guide you in your answer choice.

Whilst you are correct in pointing out that the term 'rivers' is not used in the main passage when referring to human influence, it takes only the smallest bit of common sense to work out that 'habitats' and so forth must mean rivers when talking about salmon.

In answer 1, there is as the passage says no reference to HUMAN inflluence in re-building population. Your highlighted sentence could refer to any number of possible causes for this (global warming changing the temperature of rivers etc)

James
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Re: Numbers of river salmon, GMAT Prep [#permalink] New post 17 Feb 2013, 07:28
Hi James,

Thank you for discussing this with me. Sometimes the questions focus on small details, and other times not. I agree that in this example the answer does not consider the small details.

plumber250 wrote:
.. it takes only the smallest bit of common sense to work out that 'habitats' and so forth must mean rivers when talking about salmon.


To the best of my knowledge, habitat refers to an organisms' home or regular residents. A salmon's home is an ocean and not a river. This fact is difficult to ignore because the text discusses extinction, re-population, and straying (an impossible position if salmon did not have habitats beyond their river).

This was of course in the back of my mind while reading the question, and I focused on the details (i.e. river) because the question asks about rivers in a text that is concerned mostly with population change between salmon generations.

Glen
Re: Numbers of river salmon, GMAT Prep   [#permalink] 17 Feb 2013, 07:28
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Over the last 150 years, large stretches of salmon habitat

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