All living things require energy to carry out life functions such as growth, movement, and reproduction. For nearly all ecosystems—diverse collections of species that interact with each other and their physical environment—the major source of energy is the sun. The flow of energy tends to follow the same basic pattern whether the ecosystem is a tropical rainforest or a coral reef.
To portray the transfer of energy through an ecosystem, ecologists use simple models called food chains. Organisms can be organized into different trophic levels, or positions in a food chain; organisms at higher trophic levels feed on those at lower levels.
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