托福閱讀中的總結(jié)題是導(dǎo)致很多學(xué)生不能拿滿分的原因，總結(jié)題考察考生抓取每個(gè)段落中心的能力。下面老師給各位考生寫出了TPO3 The Long-Term Stability of Ecosystems每個(gè)段落的中心，如果你們?cè)谡n下能夠每個(gè)段落抓成這樣，那就意味著你抓住了這個(gè)段落的中心，進(jìn)而說明你能夠做對(duì)托福閱讀中讓很多考生云山霧罩的托福后一題總結(jié)題也就是6選3題。
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： 介紹了兩個(gè)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.
第二段段落中心：生態(tài)系統(tǒng)比組成它的單個(gè)生物體更穩(wěn)定。
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.
第三段段落中心：生態(tài)學(xué)家曾經(jīng)認(rèn)為物種多樣性使生態(tài)系統(tǒng)更穩(wěn)定。
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.
第四段段落中心：根據(jù)穩(wěn)定性的定義，climax community可以是穩(wěn)定也可以是不穩(wěn)定的。
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.
第五段段落中心：當(dāng)穩(wěn)定性被定義為缺乏變化的時(shí)候，也不總是與大的多樣性有關(guān)。
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.
第六段段落中心：生態(tài)學(xué)家想知道什么對(duì)群落的恢復(fù)力有幫助。
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.
第七段段落中心：生態(tài)學(xué)家現(xiàn)在認(rèn)為群落的長期穩(wěn)定不是來自于多樣性而是因?yàn)榄h(huán)境的斑塊分布。