有关托福阅读真题练习
来源:才华咖 本文已影响1.89W人
来源:才华咖 本文已影响1.89W人
托福阅读真题练习2017
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
托福考试阅读历年真题
托福英语阅读真题
2017年新托福阅读理解考试练习题
2017年托福阅读模拟试题练习
托福考试机经阅读部分
商业托福英语考试中级阅读练习题
托福备考阅读精选试题练习2017
备考托福阅读理解题练习2017
新托福阅读真题解题策略
历年托福阅读真题精选
2017年托福备考阅读精选练习题
2017托福阅读考前练习题及答案
托福阅读真题步骤简介
2017商业托福试题练习阅读材料
托福考试阅读模拟题强化练习2017
历年托福考试阅读真题
2017年托福考试阅读题精选练习
托福阅读真题分析
初中英语外刊阅读练习题
托福阅读题专项练习及答案解析
女兵墓阅读真题练习
托福阅读真题
2017年托福考试阅读理解练习题
托福考试阅读真题
2017托福阅读理解预测练习题