Human beings rely on a host of invisible services that ecosystems provide. For example: they provide foods, medicines and energy; they regulate nutrient recycling and waste; they lock up carbon; they maintain clean air and water, they supply crop pollination, seed dispersal, pest and disease control; they preserve genetic diversity and provide recreation for us all. The concept of an ecosystem is important because it conveys one of the key insights that we have gained from the science of ecology, that everything is related to everything else. Everyday perception tells us that we live in a world composed of distinct units: trees, rocks, animals, buildings, and so on. Yet all of these seemingly unconnected fragments are in fact part of one system; they are interrelated, and this interrelation is essential for life. Since no piece exists independently of another, none can be modified without affecting the others. It is this idea that is behind the term "ecosystem."
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