这年头，连ETS都玩起了套娃，你有被套路过吗？

所谓的“套娃”指的是文章会陈述多个对象，彼此之间形成直接或间接的形成环环相扣的因果关系。由于牵涉的对象较多，读这类文章的关键就在于从文字当中提炼出对象之间的正负关联，从而更直观地反映出它们之间的联系。
比如下面这段文字：
Geese can often be seen grazing in coastal salt marshes. Unfortunately, their intense grazing removes the grassy covering, exposing marsh sediment; this increases evaporation, which in turn increases salt concentration in marsh sediments. Because of this increased concentration, regrowth of plants is minimal, leading to increased erosion, which leads to a decrease in the fertile topsoil, leading to even less regrowth. In time, the salt marsh becomes a mudflat.
鹅群在盐沼中高强度的觅食行为会剥离表层植被，暴露出沼泽沉积物，这会导致水分蒸发增多，提高沉积物盐浓度，继而引发植被重新生长变少，从而导致水土流失增加，表面肥沃的土壤减少，而这又进一步减少了植被重新生长的数量。
好家伙，鹅群的觅食就像多米诺骨牌引发了一连串的连锁反应，最后形成了一个self-perpetuating的恶性循环
为了更好地理解这段文字，
可以把其中的因果关系转换为直观的逻辑链：
geese grazing↑— grassy covering↓— evaporation↑— salt concentration↑— plant regrowth↓— erosion↑— fertile topsoil↓— plant regrowth↓↓
把文字转换为逻辑链之后，我们就能很容易得知因素之间的正负相关性，有利于理清做题思路。
比如下面这道机经真题：
In 1995, after an absence of nearly 70 years, wolves were reintroduced into Yellowstone National Park. During the wolf-free era, heavy browsing of aspen trees by elk populations spelled doom not only for trees themselves but for a host of other creatures dependent on them, such as beavers, whose population in Yellowstone crashed after wolves were removed. Without beavers to create ponds, wetland ecosystems—aquatic plants, amphibians, birds—were devastated. When wolves returned, grazers and browsers resumed normal patterns of behaviors, preferring safer, open areas over the dense cover and streamsides where carnivores can lurk. Keeping elk wary and on the move, wolves gave aspen and other young trees the opportunity to grow and become reestablished.
这篇文章探讨了一些生物，它们之间环环相扣：
没有狼的时期，麋鹿elk大量啃食山杨树aspen trees，导致树木大量减少，同时依赖山杨树生存的海狸beaver等生物也大量减少，一旦失去海狸建造池塘，那么包括水生植物、两栖动物和鸟类在内的湿地生态系统便遭到了破坏；重新引入狼可以让麋鹿四处迁徙，从而让树木有机会恢复。
不妨把其中的逻辑链整理出来，做题时便一目了然啦：
wolf↓— elk browsing↑— aspen trees↓— beavers↓— ponds↓— wetland↓(aquatic plants↓ amphibians↓ birds↓)
❓1. The passage asserts which of the following about beaver populations in Yellowstone?
A. They have rebounded since the reintroduction of wolves.
B. They were adversely affected by the feeding habits of elk population.
C. They increased during the period when wolves disappeared from the park.
D. They have historically had an adverse effect on the park’s wetland ecosystems.
E. They are essential to the health of the park’s aspen trees.
题干：关于beaver的数量，文章表达了以下哪个观点？
分析：
❌A. 重新引入狼之后，beaver数量已经回升。错误，“have rebounded”表示已经发生的状况，但原文并未明确指出beaver数量确实已经回升。
B. beaver数量过去遭到elk觅食行为的负面影响。正确，1995年之前没有狼的时期，elk啃食树叶导致beaver数量减少，对其造成负面影响。
❌C. 狼消失之后，beaver数量增加了。错误，beaver消失了（removed）而非增加了。
❌D. beaver数量过去（historically）对湿地生态系统有负面影响。错误，beaver对于湿地有利。
❌E. beaver对于山杨树aspen trees的健康不可或缺。错误，aspen不需要依赖beaver。
❓2. The author would most likely agree with which of the following claims about the reintroduction of wolves to Yellowstone?
A. It indirectly harmed some of the park’s amphibian habitats
B. It reduced the number of elk feeding along streamsides
C. It led to greater species diversity among the park’s grazers and browsers.
D. It significantly increased competition for food among the park’s carnivores.
E. It fostered the resurgence of tree species that once flourished in the park’s open areas.
题干：关于重新引入狼群，作者最有可能同意哪个说法？
分析：
❌A. 它间接损害了某些两栖类动物栖息地。错误，通过逻辑链可知，狼群数量和湿地生态系统呈正相关，所以并不会损害两栖类动物栖息地。
B. 它减少了在河边（streamsides）觅食的麋鹿数量。正确，植被茂密的地方（dense cover）以及河边都可能有肉食动物潜伏，狼回归之后麋鹿更喜欢在开阔的地区（open areas）活动。
❌C. 它导致草食动物物种多样性的提高。错误，原文并未提及狼和草食动物物种数量之间的联系。
❌D. 它极大地增加了肉食动物对食物的竞争。错误，原文未提到狼与其他肉食动物的竞争。
❌E. 它导致曾经在开阔地区（open areas）繁茂的树木复苏。错误，重新引入狼群后，麋鹿更喜欢在开阔地区活动，这里的树木很难复苏。
总 结
题目中出现多个对象之间因果关系时，我们需要梳理对象之间逻辑链及其正负相关性，当其中某个变量发生改变时，便可以很容易判断其他因素改变的方向。
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In explaining his opposition to efforts aimed at stabilizing moose populations and at suppressing fires in northern forests, Pastor points out that moose, by feeding on hardwood trees, contribute to a characteristic process whereby hardwood trees are eventually overtaken and shaded out, as conifers such as spruces block sunlight. A decline in hardwood trees leads to fewer nitrogen-rich, quick-rotting leaves on the forest floor, which is increasingly carpeted with slower-rotting conifer needles. Hardwoods require more nitrogen for growth than do conifers, so the decline in soil fertility in areas where moose feed reinforces the advance of conifers. But conifers cannot dominate the forest for long, because mature conifer forests are more combustible than are hardwood forests. Fire combined with high winds, attacks of spruce budworm, or even a greatly diminished nitrogen supply will destroy expanses of spruces, allowing hardwoods to recolonize in the nutrient-rich ash. Such oscillations in forest composition and consequently in moose population are, Pastor believes, essential to the integrity and functioning of northern forest ecosystems.
❓The passage suggests which of the following about conifers?
A. They enrich the soil with nitrogen more than do hardwood.
B. They are relatively long-lived compared with hardwoods.
C. They require nutrient-rich ash in order to recolonize.
D. They are adversely affected by increases in moose populations.
E. They are adversely affected by extremely low levels of nitrogen.
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