[求助]我问，我再问GWD22---1---9

lianyi03

     Overthe last 150 years, large
                    

       stretches of salmon habitat have

       been eliminated by human activity:

Line      mining, livestock grazing, timber

  (5)     harvesting, and agriculture as well  
            

as recreational and urban devel-

opment.  The numerical effect is

obvious:  there are fewer salmon

in degraded regions than in pris-

 (10)     tine 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

 (15)     populations.  Indeed, most

analysts believe that some kind

of environmental degradation

underlies the demise of many

extinct salmon populations.

 (20)     Although some rivers have

       been recolonized, the unique

       genes of the original populations

have been lost.

      Large-scale disturbances in  
            

 (25)     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

 (30)     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

 (35)
      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

 (40)    repopulated should the fish

there disappear.  Yet high rates

of straying can be problematic

because   misdirected fish may

interbreed with the existing stock

 (45)     to such a degree that any local

adaptations that are present

become diluted.  Straying

rates remain relatively low when

environmental conditions are

 (50)     stable, but can increase dramati-

cally when streams suffer severe

disturbance.  The 1980 volcanic

eruption of Mount Saint Helens,

[S7] for example, sent mud and debris

 (55)     into several tributaries of the

Columbia River.  For the next

couple of years, steelhead trout

(a species included among the

salmonids) returning from the

 (60)     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

 (65)     overall.

      Although no one has quantified

changes in the rate of straying 

as a result of the disturbances

caused by humans, there is no

 (70)     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 dra-

 (75)     matic 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.Such a dra-，

 (75)     matic 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. 
                        

 

The primary purpose of the passage is to
                                        

             

1. argue against a conventional explanation for the extinction of certain salmon populations and suggest an alternative

2. correct a common misunderstanding about the behavior of salmon in response to environmental degradation caused by human activity

3. compare the effects of human activity on salmon populations with the effects of natural disturbances on salmon populations

4. differentiate the particular effects of various human activities on salmon habitats

describe how environmental degradation can cause changes in salmon populations that extend beyond a numerical reductio

The primary purpose of the passage is to
                                

             

1. argue against a conventional explanation for the extinction of certain salmon populations and suggest an alternative

2. correct a common misunderstanding about the behavior of salmon in response to environmental degradation caused by human activity

3. compare the effects of human activity on salmon populations with the effects of natural disturbances on salmon populations

4. differentiate the particular effects of various human activities on salmon habitats

describe how environmental degradation can cause changes in salmon populations that extend beyond a numerical reductio

 

The primary purpose of the passage is to
                                

             

1. argue against a conventional explanation for the extinction of certain salmon populations and suggest an alternative

2. correct a common misunderstanding about the behavior of salmon in response to environmental degradation caused by human activity

3. compare the effects of human activity on salmon populations with the effects of natural disturbances on salmon populations

4. differentiate the particular effects of various human activities on salmon habitats

describe how environmental degradation can cause changes in salmon populations that extend beyond a numerical reductio

The primary purpose of the passage is to
                        

             

1. argue against a conventional explanation for the extinction of certain salmon populations and suggest an alternative

2. correct a common misunderstanding about the behavior of salmon in response to environmental degradation caused by human activity

3. compare the effects of human activity on salmon populations with the effects of natural disturbances on salmon populations

4. differentiate the particular effects of various human activities on salmon habitats

describe how environmental degradation can cause changes in salmon populations that extend beyond a numerical reductio

 不明白不明白不明白，为什么第9题选E啊？？  environmental degradation 只是文章里的一个方面啊，还有human activity 和 disturbance 呢。og 中说主题题应该是看全篇啊？human activity ，environmental
          degradation,disturbance三者之间究竟是什么关系啊，看起来也不是并列。难道存在因果关系？我万般无奈下选了A，A的语气太强了，但是我是根据however那个转折选择的，这道题我没看出那个选项合适。请高人指点一下吧。

 不明白不明白不明白，为什么第9题选E啊？？  environmental degradation 只是文章里的一个方面啊，还有human activity 和 disturbance 呢。og 中说主题题应该是看全篇啊？human activity ，environmental
          degradation,disturbance三者之间究竟是什么关系啊，看起来也不是并列。难道存在因果关系？我万般无奈下选了A，A的语气太强了，但是我是根据however那个转折选择的，这道题我没看出那个选项合适。请高人指点一下吧。

[此贴子已经被作者于2006-12-31 3:23:54编辑过]