托福 托福 46 - Ecosystem Diversity and Stability
题目
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6.According to paragraph 2, which of the following increases the stability of an ecosystem?
  • A.Species in which producers outnumber predators
  • B.New or exotic species that increase ecosystem diversity
  • C. Heavily populated species that are free of disease
  • D. Species that are diverse but have similar nutritional roles
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    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.

    一直以来,生物保护学家一直关注的一个问题是,物种灭绝可能会对整个生态系统的稳定性产生严重的影响,影响到与它们有交集的生物以及它们所生活的物理环境。一个生态系统可以承受某些物种的损失,但如果损失的物种足够多,生态系统就会严重退化。事实上,一个重要的物种损失就可能引发一系列的物种灭绝,可能极大地改变整个生态系统。海獭被从太平洋海藻(海草)床生态系统中移除就是一个很好的例子:没有了海獭捕食,海胆数量暴增,吃掉了大量海带和其他大型海藻,海草床也因此不复存在。 一般来说,物种丰富的生态系统往往比物种贫乏的生态系统更稳定。有人提出,物种的多样性透过三种机制增加了生态系统的稳定性。首先,一个生态系统的物种越多,它的食物网就越复杂,物种间的营养作用方面就会有更多的重合。换句话说,在一个物种丰富的系统里,如果失去了一个物种,就给了其它物种一个很好的机会,代替它扮演猎物、捕食者、生产者、分解者、或任何它所扮演的角色。第二,多样的生态系统不太可能遭到新物种的入侵,特别是会破坏生态系统结构和功能的外来物种(生活在本土范围以外的物种)。第三,在一个物种丰富的生态系统中,疾病可能会传播得比较慢,因为每个物种的数量相对较少,从而增加了同一物种内个体间的平均距离,从而阻碍了个体间的疾病传播。 很多现象我们仍无法解释,但是已经慢慢有一些科学证据来证明这些观点。最早提供数据来支撑多样性和稳定性之间的关系的是一项关于草原植物如何对抗干旱的研究。D. 蒂尔曼和J A.唐宁两位研究员在美国明尼苏达州的雪松溪自然历史保护区,用207个地块在1988(经过两年的干旱)和1986(干旱前)的地表生物量的比值来反映生态系统受干旱破坏的程度。在一个1982年开始的实验中,他们比较了这些比值与每个取样点的植物物种数,发现取样点的植物物种越多,其生物数量在干旱中急剧下降得就越少。那些物种数量超过十种的取样点在1988年的生物量大概是1986年的一半,而那些物种数量少于五种的取样点在经过两年干旱之后,大概只剩下八分之一的生物数量。显然,物种丰富的地块很可能包含一些抗旱植物品种,这些品种在干旱年份生长得更好,弥补了那些不那么抗旱的品种的数量。 从更一般的意义上来说,一个物种丰富的生态系统可能更稳定,因为它更可能含有能对某种气候条件(如干旱)的做出各种反应的物种。此外,物种丰富的生态系统更可能包含有着类似生态系统的功能,因此,如果生态系统中的一个物种损失,另一个物种,可能是竞争对手,就有希望蓬勃发展,并取代它的作用。对恶劣条件的不同的反应,以及系统内的功能重合性,这两个特征都是对抗恶劣气候的保障。 明尼苏达草原的研究作为多样性-稳定性理论的有力证据已被广泛接受,但是,它的研究结果已经被质疑。另外,其他生态系统的类似的研究并没有找到多样性和稳定性之间积极的相关性。显然,这是一个复杂的问题,需要进一步的关于更多各种生态系统和物种的实地研究,对草地的研究和建立计算机模型只能证明部分问题。最后,尽管这些富有洞察力的尝试得出了一些概括性结论,我们可能会发现很难将这个话题缩小成一个简单的、普遍认可的事实。

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    最新提问
    • wx_6697
      觉得B C 意思一样,不知道选哪个
    • wx_5576
      这道题C为什么对,E为什么不对?
    • wx_5576
      B为什么不能选啊?
    • wx_6697
      TPO30 passage 2 Q5我选的 D,不明白为啥不对?
    • wx_6697
      鑫哥,TPO6passage3Q5 答案是给错了吗?好多人都选A
    • wx_6697
      这题也很容易选错选成了D
    • wx_6697
      这道题A为什么错了
    • 芊儿
      为什么这道题不选c??a中的variety不是应该对应文中的differentiating 吗??求解!
    • wx_1000
      这道题不选E是因为太细节了吗
    • 王金阁
      这个题为什么不选C啊。。。
    • 芊儿
      这道题的D选项不是和文中的better able to reproduce in open settings相对应么??
    • 风荨火
      有大佬解释一下这个为啥选D嘛?
    • 以沫
      请问这个D 在哪里提现?为什么D错?
    • 芊儿
      第六题 的C选择为什么不对,感觉A是明显驳斥啊...
    • wx_6697
      鑫哥,这道题D是从哪里看出来的
    • wx_6697
      这题选的A,根据是Joly’s calculations clearly supported those geologists who insisted on an age for Earth far in excess of a few million years.想问鑫哥为啥不选A
    • wx_6697
      这题我选的是C依据是into a new habitat outside of its natural range, it may adapt to the new environment and leave its enemies behind.C为啥错了呢?鑫哥
    • wx_8861
      F选项的weather-related destruction在哪里体现了呢?原文最后一段的开头Among the costs里的costs是不是打错了?应该是coast?
    • wx_6697
      求问这道题B为啥不选,原文依据:viable seeds of pioneer species can be found in large numbers on some forest floors.
    • 与托福的斗争史
      与托福的斗争史 去解答 去解答
      这题为什么选C?
    • 小雨淅沥哗啦的下
      小雨淅沥哗啦的下 去解答 去解答
      B哪里错了
    • 小雨淅沥哗啦的下
      小雨淅沥哗啦的下 去解答 去解答
      B为啥不对
    • 李浩然
      B选项错误,是因为残缺么?
    • wx_100
      请问在做题的时候如何排除c呢。看了答案,感觉是该选a的,但是当时做题脑子一热,就特别钟爱c,也没看其他选项。。求敲醒。。
    • wx xxxxx
      请问鑫哥,这段开头有写As one pesticide replaces another为什么不是对应a new pesticide is developed?
    • wx_7695
      鑫哥,从哪里看出来这个masks 不是use呀,原文说了wear呀
    • haiyuqiao
      @鑫哥,这题the damage will continue 不应该对应前面的 the target species evolves resistance to it,然后As one pesticide replaces another,不应该是结束了time cycle 吗
    • wx_2065
      鑫哥,想知道E错在哪里?
    • wx_7695
      鑫哥,B选项 cannot extended to earlier geological periods. 原文说的意思是后来的进化无法估计吧
    • wx_2163
      B为什么不选
    • wx_7780
      鑫哥,这个哪里看不use了。BD是修饰错,C是无中生有,怎么能直接选出A?
    • 100
      看到第一句话,以为是中心句就选了A... 为什么不能选A呢
    • 100
      为什么选b?
    • gu33
      请问下 这里选D的原因是 因为 evolutionary approach 对应着 原文的 Rates of evolution 嘛? 这里我选了C。。不是很懂 插入句和 D的关系 求解答
    • 我是啦啦啦
      我是啦啦啦 去解答 去解答
      这个题A哪里错了?是因为主语不对吗?这个C比A多一步推理啊
    • haiyuqiao
      鑫哥,D选项里的19世纪出现了很多假设,原文中并没有提到啊
    • wx_7060
      为什么选a 呢。我觉得a是细节。F哪错了?
    • wx_1105
      我想问一下,这道题为什么不能选A呢?
    • wx_8122
      D为什么不选
    • wx_1655
      f选项哪里说了
    • chaulaw
      鑫哥,原文是below经济损害水平,D是一触发经济损害就用,这也对吗?
    • chaulaw
      interclan婚姻是对的吗?不是只在自己的family结婚扩大家族吗?
    • wx_6697
      鑫哥,这道题答案是不是错了,好多人选D 我也选的D求解答
    • wx_6697
      这道题应该是一道易错题,每个选项的都有,然而我选的A错了,求解
    • wx_6697
      鑫哥,这题的C是怎么得到的?B也没有找到啊?难道不是应该选B
    • wx_6697
      鑫哥,还有这个,好多人选A答案是不是错了
    • wx_6697
      求问D是从哪里得出来的,我选的B呀哎呀
    • wx_4185
      it is difficult to say how far they were intended to be portraits rather than generalized images 这句话怎么理解呢
    • 此楠楠
      请问下这个插入题怎么选的呢?
    • 此楠楠
      求鑫哥讲解下A选项。。。 Even though in error, Joly’s calculations clearly supported those geologists who insisted on an age for Earth far in excess of a few million years.