2. 外语
  3. Exotic insect pests can produce both short-and long-term effects on forest ecosystems. Short-term effects include the disturbanc

Exotic insect pests can produce both short-and long-term effects on forest ecosystems. Short-term effects include the disturbanc

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问题     Exotic insect pests can produce both short-and long-term effects on forest ecosystems. Short-term effects include the disturbances directly associated with the action of the pest, which may cause the defoliation, loss of vigor, or death of trees. Long-term effects involve changes in tree species composition and the consequent alterations of forest structure, productivity, and nutrient uptake. Exotic pests are more efficient than most abiotic disturbances (e.g., fire or wind) at producing long-term changes in species composition. Pests often target specific tree species and, if they become established, they usually remain as permanent components of the ecosystem. Shifts in forest species composition ramify through the ecosystem in many ways because tree species have different, often unique properties.
The author of the passage mentions the “unique properties” of tree species primarily in order to help explain
选项 A、why pests tend to target specific tree species
B、why pests can have long-term effects on entire ecosystems
C、how pests contribute to shifts in forest species composition
D、how pests are able to become established in an ecosystem
E、how some tree species are able to withstand the effects of pests
答案C
解析 根据题干中的unique properties初步定位到第六句。本题问unique properties用来解释什么，当A可以解释B的时候，意味着A可以导致B，所以这道题相当于问unique properties是谁的原因。所以本题的选择方向就是第六句because之前部分的同义改写，即解释in many ways。因为每个树木物种都有独特的性质，因此丛林生态系统的改变才会以不同的方式蔓延于生态系统中。C项how对应in many ways。A项文章没提害虫为什么以特定树木物种为目标。B项独特性质解释的应该是为什么会以不同方式蔓延于生态系统，而非解释为什么产生长期影响。D项文章没提害虫如何定居。E项文中未提及withstand。
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