Successin colonization depends to a great extent on there being a site available for colonization ---- a safe site where disturbance by fire or by cutting down of trees has either removed competing species or reduced levels of competition and other negative interactions to a level at which the invading species can become established. (TPO32, 54)

　　大家自己先读，不回读，看一遍是否能理解

　　Success in colonization depends to a great extent on there being a site available for colonization ---- (a safe site) (where disturbance by fire or by cutting down of trees has either removed competing species or reduced levels of competition and other negative interactions to a level) (at which the invading species can become established.)(TPO32, 54)

　　托福阅读长难句分析：

　　这个句子的主干就是：

　　Success in colonization depends (to a great extent) on there being a site (available for colonization)

　　注意depend on被短语to a great extent(很大程度)隔开了，断句不要出现问题，后面还有一个放在后面的形容词短语(available for colonization)修饰a site

　　修饰一：(a safe site) ，同位语，解说说明前面的a site

　　中文：一个安全的地点

　　修饰二：(where disturbance by fire or bycutting down of trees has either removed competing species or reduced levels of competition and other negative interactions to a level) ，从句

　　这个从句有点复杂：

　　where disturbance (by fire or by cutting down of trees) has either removed competing species or reduced levels of competition and other negative interactions to a level

　　(by fire or by cutting down oftrees)，介词修饰放在后面disturbance

　　注意这里有一个并列结构，either or

　　either removed competing species

　　or reduced levels of competition and other negative interactions to a level

　　中文：在那里由于焚烧或伐木要么移除了竞争物种，要么减低了竞争水平和其它负面的物种间相互影响的水平

　　修饰三：(at which the invading species can become established.) ，从句

　　中文：入侵物种能够定居下来。

　　托福阅读长难句参考翻译：

　　成功的移居很大程度上依赖于有一个可用的移居的地点，即一个安全的地点，在那里由于焚烧或伐木要么移除了竞争物种，要么把物种间的竞争和其他负面的物种间相互影响减少到让入侵物种能够定居下来。

　　这个句子的主要修饰成分就是从句、同位语，大家务必想清楚，每一个修饰成分修饰的是什么，才能看清楚这个句子的结构。