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　　托福阅读素材：物种丰富度与群落稳定性

　　内容回忆：TPO46-3 Ecosystem Diversity and Stability 物种多样性与生态系统的稳定性之间的关系;

　　第一段：不同生态的稳定性不同。例如，河流因其快速流动的水，比起湖的稳定性要好，因为能够更快的冲走污染物质。其次，沙漠这种极端环境，影响因素只有水，因此稳定性也比较好;

　　第二段： R 首次提出了物种多样性与群体稳定性的关系，认为物种越是多样，群体就越不容易受到外界的影响。例如，在孤岛上，生物的多样性比较小，引入兔子等生物时，甚至会导致本地的某个物种灭绝;

　　第三段：其它一些专家不认为这样。他们指出可能是因为在洲际(continental)的环境中，因为害虫(pest)与本地物种(native species)存在共同进化的历史，才没有导致本地物种被威胁。也就是说海岛的生物减少并不是因为物种单一而是因为 pest 和本地物种并不存在一起进化的历史;

　　第四段：支持 Tilman 根据 11 年的提供动力或能量的生物量(biomass)与 species 关系的观察得出：生物多样性与群体稳定性呈正相关。因为当一个群体的占主导地位的物种(dominant species)受到伤害时，就会有其他的 disturbance-resistant 的物种弥补其影响。

　　参考阅读：

　　Title: Ecosystem Diversity and Stability

　　Conservation biologists have long been concerned that species extinction could have significant consequences for the stability of entire ecosystems—groups of interacting organisms and the physical environment that they inhabit. An ecosystem could survive the loss of some species, but if enough species were lost, the ecosystem would be severely degraded. In fact, it is possible that the loss of a single important species could start a cascade of extinctions that might dramatically change an entire ecosystem. A good illustration of this occurred after sea otters were eliminated from some Pacific kelp (seaweed) bed ecosystems: the kelp beds were practically obliterated too because in the absence of sea otter predation, sea urchin populations exploded and consumed most of the kelp and other macroalgae.

　　It is usually claimed that species-rich ecosystems tend to be more stable than species-poor ecosystems. Three mechanisms by which higher diversity increases ecosystem stability have been proposed. First, if there are more species in an ecosystem, then its food web will be more complex, with greater redundancy among species in terms of their nutritional roles. In other words, in a rich system if a species is lost, there is a good chance that other species will take over its function as prey, predator, producer, decomposer, or whatever role it played. Second, diverse ecosystems may be less likely to be invaded by new species, notably exotics (foreign species living outside their native range), that would disrupt the ecosystem’s structure and function. Third, in a species-rich ecosystem, diseases may spread more slowly because most species will be relatively less abundant, thus increasing the average distance between individuals of the same species and hampering disease transmission among individuals.

　　Scientific evidence to illuminate these ideas has been slow in coming, and many shadows remain. One of the first studies to provide data supporting a relationship between diversity and stability examined how grassland plants responded to a drought. Researchers D. Tilman and J A. Downing used the ratio of above-ground biomass in 1988 (after two years of drought) to that in 1986 (predrought) in 207 plots in a grassland field in the Cedar Creek Natural History Area in Minnesota as an index of ecosystem response to disruption by drought. In an experiment that began in 1982, they compared these values with the number of plant species in each plot and discovered that the plots with a greater number of plant species experienced a less dramatic reduction in biomass. Plots with more than ten species had about half as much biomass in 1988 as in 1986, whereas those with fewer than five species only produced roughly one-eighth as much biomass after the two-year drought. Apparently, species-rich plots were likely to contain some drought-resistant plant species that grew better in drought years, compensating for the poor growth of less-tolerant species.

　　To put this result in more general terms, a species-rich ecosystem may be more stable because it is more likely to have species with a wide array of responses to variable conditions such as droughts. Furthermore, a species-rich ecosystem is more likely to have species with similar ecological functions, so that if a species is lost from an ecosystem, another species, probably a competitor, is likely to flourish and occupy its functional role. Both of these, variability in responses and functional redundancy, could be thought of as insurance against disturbances.

　　The Minnesota grassland research has been widely accepted as strong evidence for the diversity stability theory; however, its findings have been questioned, and similar studies on other ecosystems have not always found a positive relationship between diversity and stability. Clearly, this is a complex issue that requires further field research with a broad spectrum of ecosystems and species: grassland plants and computer models will only take us so far. In the end, despite insightful attempts to detect some general patterns, we may find it very difficult to reduce this topic to a simple, universal truth.

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