托福阅读中的总结题是导致很多学生不能拿满分的原因，总结题考察考生抓取每个段落中心的能力。下面大象教育的汪娟老师给各位考生写出了TPO3 The Long-Term Stability of Ecosystems每个段落的中心，如果你们在课下能够每个段落抓成这样，那就意味着你抓住了这个段落的中心，进而说明你能够做对托福阅读中让很多考生云山雾罩的托福最后一题总结题也就是6选3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.
第一段段落中心： 介绍了两个概念Pioneer community和Climax community。
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 stabilityjust 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.