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托福阅读TPO3(试题+答案+译文)第3篇:The Long-Term Stability of Ecosystems
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Plant communities asmble themlvesflexibly, and their particular structure depends on the specific history of thearea. Ecologists u the term “succession” to refer to the changes that happenin plant communities and ecosystems over time. The first community in asuccession is called a pioneer commu
nity, while the long-lived community at theend of succession is called a climax community. Pioneer and successional plantcommunities are said to change over periods from 1 to 500 years. Thechanges—in plant numbers and the mix of species—are cumulative. Climaxcommunities themlves change but over periods of time greater than about 500years.
An ecologist who studies a pond today maywell find it relatively unchanged in a year’s time. Individual fish may bereplaced, but the number of fish will tend to be the same from one year to thenext. We can say that the properties of an ecosystem are more stable than theindividual organisms that compo the ecosystem.
At one time, ecologists believed that speciesdiversity made ecosystems stable. They believed that the greater the diversitythe more stable the ecosystem. Support for this idea came from the obrvationthat long-lasting climax communities usually have more complex food webs andmore species diversity than pioneer communities. Ecologists concluded that theapparent stability of climax ecosystems depended on their complexity. To takean extreme example, farmlands dominated by a single crop are so unstable thatone year of bad weather or the invasion of a single pest can destroy the entirecrop. In contrast, a complex climax community, such as a temperate forest, willtolerate considerable damage from weather to pests.
The question of ecosystem stability iscomplicated, however. The first problem is that ecologists do not all agreewhat “stability” means. Stability can be defined as simply lack of change. Inthat ca, the climax community would be considered the most stable, since, bydefinition, it changes the least over time. Alternatively, stability can bedefined as the speed with which an ecosystem returns to a particular formfollowing a major disturbance, such as a fire. This kind of stability is alsocalled resilience. In that ca, climax communities would be the most fragileand the least stable, since they can require hundreds of years to return to theclimax state.
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Even the kind of stability defined assimple lack of change is not always associated with maximum diversity. At leastin temperate zones, maximum diversity is often found in mid-successionalstages, not in the climax community. Once a redwood forest matures, forexample, the kinds of species and the number of individuals growing on theforest floor are reduced. In general, diversity, by itlf, does not ensurestability. Mathematical models of ecosystems likewi suggest that diversitydoes not guarantee ecosystem stability—just the opposite, in fact. A morecomplicated system is, in general, more likely than a simple system to breakdown. A fifteen-speed racing bicycle is more likely to break down than achild’s tricycle.
Ecologists are especially interested toknow what factors contribute to the resilience of communities b
ecau climaxcommunities all over the world are being verely damaged or destroyed by humanactivities. The destruction caud by the volcanic explosion of Mount St.Helens, in the northwestern United States, for example, pales in comparison tothe destruction caud by humans. We need to know what aspects of a communityare most important to the community’s resistance to destruction, as well as itsrecovery.
Many ecologists now think that the relativelong-term stability of climax communities comes not from diversity but from the“patchiness” of the environment, an environment that varies from place to placesupports more kinds of organisms than an environment that is uniform. A localpopulation that goes extinct is quickly replaced by immigrants from an adjacentcommunity. Even if the new population is of a different species, it canapproximately fill the niche vacated by the extinct population and keep thefood web intact.
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1. The word “particular” in the passage(paragraph 1) is clost in meaning to
A.natural孕妇可以吃橄榄菜吗
B.final
换奶粉C.specific
