1. 讲生物的2个阶段，怎麽改善土地之流

第1段：任何ecosystem都有生物组成的轮换 (turn) 每种组成都有不同stage 然后都在它们最后一个stage达到climax (有题)。

第2段：第一个阶段从一片荒芜之中(severe=extreme condition有题) [高或低温，土地不存储水，大风?] 勇敢生长出来的是pioneer plants，此物可抵抗一切不良条件。之后他们用他们的根改善土壤条件，然后死了之后又成为有机养料(organic material)。有了这些植物之后会有昆虫、小动物来吃他们。鸟会来吃虫子。于是最后达到climax。

第3段：此时，捡便宜的subsequent植物出现鸟，他们往往是高大的植物(比如某berry)，通过来觅食的鸟的粪便中留下种子。他们生长起来之后，通过叶子产生大量shadow，然后让楼下的暗无天日，于是长江后浪推前浪，先锋植物死光光。之后楼下就会生成一堆无阳光也能活的小植物出来。到达stage 2的climax了。

第4段：以沙丘(sand dune)来举例，沙丘不仅是不毛之地（旱 温差大），还经常打滚，一打滚就把东西全埋进去了。(这里还提到它们离海往往还很近，忘了是要说啥，然后还有题，问为啥要提离水近) 突然有种无比牛x的草，被埋进去也死不掉，根还特别长，于是就把沙丘固定住了。后面就像上面所说各种后续植物生出来。

Ecological Succession 生态演替。

主要讲植物对环境的改善，将不毛之地变成动物天堂，有很多stages。首先讲了第一个阶段，在不毛之地，气候和环境都很恶劣，只有适应性很强的植物可以苟且生存。（考点，同义句）这些低等植物抵御乾旱和强烈光照等恶劣环境，慢慢改善着脚下的土地，死后就变成腐殖质，给土地营养。它们还吸引来了哺乳动物和鸟类。（考点，它们是怎麽吸引的）然后环境进一步改善。进入了第二阶段，大型的植物来了，它们靠自己的叶子给土地提供庇护，湿度和温度得到保证（考点，大型植物的作用），动物数量和种类也多了起来。最后达到了一个climax（考点，关于climax状态的阐释）。

关于生物分佈，pioneer植物的概念。通常都在严峻的环境下，比如相对比较极端的温度，乾旱缺水等等。然后讲了PIONEER植物来了之后如何（有辞彙题，*考的那个词*接decay什麽什麽。。。我貌似选了=later）。后来pioneer植物慢慢把这种极端环境改造成了较为适宜的环境，慢慢动物植物也都来了。举例一个沙漠里的pioneer植物的过程。总体来说我觉得就是一些天不怕地不怕比较小强型的植物先遣部队，在没有花香没有树高的荒芜地带认识世界改造世界造福后来动植物兄弟的血泪史。

托福阅读真题题源：Ecological succession 生态演替

Ecological succession

The structure of communities is constantly changing. All communities are subject to periodic disturbances, ranging from events that have only localized effects,such as the loss of a tree that creates a gap in the canopy of a forest, to those that have catastrophic consequences, which include wildfires that sweep across vast landscapes or storms that pound immense stretches of shoreline. Each new disturbance within a landscape creates an opportunity for a new species to colonize that region. New species also alter the character of the community, creating an environment that is suitable to even newer species. By this process, known as ecological succession, the structure of the community evolves over time.

Types of succession

Two different types of succession, primary and secondary, have been distinguished. Primary succession occurs in essentially lifeless areas—regions in which the soil is incapable of sustaining life as a result of such factors as lava flows, newly formed sand dunes, or rocks left from a retreating glacier. Secondary succession occurs in areas where a community that previously existed has been removed; it is typified by smaller-scale disturbances that do not eliminate all life and nutrients from the environment. Events such as a fire that sweeps across a grassland or a storm that uproots trees within a forest create patches of habitat that are colonized by early successional species. Depending on the extent of the disturbance, some species may survive, other species may be recolonized from nearby habitats, and others may actually be released from a dormant condition by the disturbance. For example, many plant species in fire-prone environments have seeds that remain dormant within the soil until the heat of a fire stimulates them to germinate.

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