gwd 1-9 看不懂的句構

Kath1985

GWD-1-Q9 to GWD-1-Q12:

Over the last 150 years, large
Stretches ofsalmon habitat have
been eliminated by human activity:
Line mining, livestock grazing, timber
(5) harvesting, and agriculture as well
as recreational and urban devel-
opment.(第一段第一句肯定最重要) 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
(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.
(很明顯第一段是說一種事實，至少是部分科學家的看法)

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 ofsalmonto 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. (這一句肯定是重點，而且要是讀懂，下一句一定是講例子，就簡單讀過即可)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 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.


擷取出這句
the process
provides a source of novel
genes and a mechanism
by which a location can be
(40) repopulated should the fish
there disappear.

請問
1.黃色部分的by which  的 which到底指誰
2.請求黃色部分整句的還原