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TPO托福阅读真题答案及解析：托福阅读文段的结构和出题点都有一定的规律，比如说托福阅读文段一般是总分总的格式，考生重点关注首段，尾段，转折句，开头句，基本上就能找到主题句，从这个方面来说，练习托福阅读真题，从中总结托福阅读考试经验是十分必要的，下面新东方网为大家整理一份“TPO托福阅读真题答案及解析”，希望对大家有所帮助。
托福阅读真题：
The Long-Term Stability of Ecosystems
Plant communities assemble themselves flexibly, and their particular
structure depends on the specific history of the area. Ecologists use the term
“succession” to refer to the changes that happen in plant communities and
ecosystems over time. The first community in a succession is called a pioneer
community, while the long-lived community at the end of succession is called a
climax community. Pioneer and successional plant communities are said to change
over periods from 1 to 500 years. These changes—in plant numbers and the mix of
species—are cumulative. Climax communities themselves change but over periods of
time greater than about 500 years.
An ecologist who studies a pond today may well find it relatively unchanged
in a year’s time. Individual fish may be replaced, but the number of fish will
tend to be the same from one year to the next. We can say that the properties of
an ecosystem are more stable than the individual organisms that compose the
ecosystem.
At one time, ecologists believed that species diversity made ecosystems
stable. They believed that the greater the diversity the more stable the
ecosystem. Support for this idea came from the observation that long-lasting
climax communities usually have more complex food webs and more species
diversity than pioneer communities. Ecologists concluded that the apparent
stability of climax ecosystems depended on their complexity. To take an extreme
example, farmlands dominated by a single crop are so unstable that one year of
bad weather or the invasion of a single pest can destroy the entire crop. In
contrast, a complex climax community, such as a temperate forest, will tolerate
considerable damage from weather to pests.
The question of ecosystem stability is complicated, however. The first
problem is that ecologists do not all agree what “stability” means. Stability
can be defined as simply lack of change. In that case, the climax community
would be considered the most stable, since, by definition, it changes the least
over time. Alternatively, stability can be defined as the speed with which an
ecosystem returns to a particular form following a major disturbance, such as a
fire. This kind of stability is also called resilience. In that case, climax
communities would be the most fragile and the least stable, since they can
require hundreds of years to return to the climax state.
Even the kind of stability defined as simple lack of change is not always
associated with maximum diversity. At least in temperate zones, maximum
diversity is often found in mid-successional stages, not in the climax
community. Once a redwood forest matures, for example, the kinds of species and
the number of individuals growing on the forest floor are reduced. In general,
diversity, by itself, does not ensure stability. Mathematical models of
ecosystems likewise suggest that diversity does not guarantee ecosystem
stability—just the opposite, in fact. A more complicated system is, in general,
more likely than a simple system to break down. A fifteen-speed racing bicycle
is more likely to break down than a child’s tricycle.
Ecologists are especially interested to know what factors contribute to the
resilience of communities because climax communities all over the world are
being severely damaged or destroyed by human activities. The destruction caused
by the volcanic explosion of Mount St. Helens, in the northwestern United
States, for example, pales in comparison to the destruction caused by humans. We
need to know what aspects of a community are most important to the community’s
resistance to destruction, as well as its recovery.
Many ecologists now think that the relative long-term stability of climax
communities comes not from diversity but from the “patchiness” of the
environment, an environment that varies from place to place supports more kinds
of organisms than an environment that is uniform. A local population that goes
extinct is quickly replaced by immigrants from an adjacent community. Even if
the new population is of a different species, it can approximately fill the
niche vacated by the extinct population and keep the food web intact.
Paragraph 1: Plant communities assemble themselves flexibly, and their
particular structure depends on the specific history of the area. Ecologists use
the term “succession” to refer to the changes that happen in plant communities
and ecosystems over time. The first community in a succession is called a
pioneer community, while the long-lived community at the end of succession is
called a climax community. Pioneer and successional plant communities are said
to change over periods from 1 to 500 years. These changes—in plant numbers and
the mix of species—are cumulative. Climax communities themselves change but over
periods of time greater than about 500 years.
1. The word “particular” in the passage is closest in meaning to
~Natural
~Final
~Specific
~Complex
2. According to paragraph 1, which of the following is NOT true of climax
communities?
~They occur at the end of a succession.
~They last longer than any other type of community.
~The numbers of plants in them and the mix of species do not change.
~They remain stable for at least 500 years at a time.
Paragraph 2: An ecologist who studies a pond today may well find it
relatively unchanged in a year’s time. Individual fish may be replaced, but the
number of fish will tend to be the same from one year to the next. We can say
that the properties of an ecosystem are more stable than the individual
organisms that compose the ecosystem.
3. According to paragraph 2, which of the following principles of
ecosystems can be learned by studying a pond?
~Ecosystem properties change more slowly than individuals in the
system.
~The stability of an ecosystem tends to change as individuals are
replaced.
~Individual organisms are stable from one year to the next.
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发表于 2016-7-10 15:22:17