一个问题看了讨论还是不明白~~急死啦！！！XY-1-9

eva808

 XY-1Q10
 

Over the 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,

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.

Q10:

It can be inferred from the passage that the occasional failure of some salmon to return to their natal streams in order to spawn provides a mechanism by which

A. pristine streams that are near polluted streams become polluted themselves

B. the particular adaptations of a polluted stream’s salmon population can be preserved without dilution

C. the number of salmon in pristine habitats decreases relative to the number in polluted streams

D. an environmentally degraded stream could be recolonized by new salmon populations should the stream recover

E. the extinction of the salmon populations that spawn in polluted streams is accelerated

答案是D~~~

请问后面should the stream recover是什么意思~~~是不是说这个stream不再environmentally degraded?

定位原文L36-L37~~~并没有说recolonize一定要发生在the stream recover的情况下阿~~

 

已经快晕死了~~求教各位！

[此贴子已经被作者于2007-8-27 15:25:24编辑过]

lavendery

我也刚好做这题。按照自己的理解试着翻一下：

a mechanism by which a location can be repopulated should the fish there disappear

少量的鲑鱼迷路，会使那些没有鲑鱼的溪流再次出现鲑鱼。也就是说，鲑鱼找不到既定产卵的溪流后，就不得不在陌生的地方产卵了，等到小鲑鱼孵化出来，这地方也就成了小鲑鱼的新家，又出现了越来越多的鲑鱼。

D an environmentally degraded stream could be recolonized by new salmon populations should the stream recover
        

       对应没有鲑鱼的溪流                                对应 repopulated       populations should recover 这样看结构比较清楚

lukeleng

should the stream recover=If the stream should recover

虚拟条件句的倒装,在虚拟条件状语中如果有were, should, had这三个词，通常将if省略，主语提前, 变成 were, should, had +主语的形式