How interpendent is the ecosystem?

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Last night I was watching a documentary about the Amazon Rain forest and i was amazed. A huge tree would secrete a sweet substance that would attract some tiny insects that swarm the forliage of the trees-little did I know that these insect were invited as security agents to protect the plant from other pests, because when another group of insects appraoched the tree, they were brutally attacked and chased away. How interdependent is the ecosystem?