托福阅读真题及答案解析.pdf

智 课 网 托 福 备 考 资 料 托福阅读真题及答案解析 托福阅读真题及答案解析：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，下面智 课小编为大家整理一份“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 ”，希望对同学们有所帮助。下面和智课小编一起来看看吧： 托福阅读真题答案及解析： 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 inv