It is significant that the earliest living things that built communities on these islands are examples of symbiosis, a phenomenon that depends up on the close cooperation of two or more forms of life and a principle that is very importantin island communities. (TPO9, 43)

community n. 社区，团体，社会，(动植物的)群落

symbiosis /sɪmbaɪ'osɪs/ n. 共生。

It is significant that the earliest living things (that built communities on these islands) are examples of symbiosis, a phenomenon (that depends upon the close cooperation of two or more forms of life) and a principle (that is very important in island communities). (TPO9, 43)

分析：

修饰一：(that built communities on theseislands)，从句，修饰living things

中文：在这些岛上建立生物群落

修饰二：(a phenomenon and a principle)，同位语，修饰symbiosis，注意这里的a phenomenon和a principle被从句隔开了，注意这里的断句

修饰三：(that depends upon the close cooperation of two or more forms of life) ，从句，修饰a phenomenon

中文：依赖两种或两种以上的生物紧密合作

修饰四：(that is very important in island communities)从句，修饰aprinciple

中文：在岛上生物群落非常重要

参考翻译：

在这些岛上建立生物群落的最早的生物以共生方式存在是非常重要的，共生是一种依赖两种或两种以上的生物紧密合作而生存的现象，也是在岛上生物群落非常重要的原则。

这个句子的主要修饰成分就是从句和同位语，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。