How long does it take for changed in an ecosystem to be reversed

That certain change from that ecosystem will require 50 years or longer because big ecosystems need a long time to restablize the living ecosystem.

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The answer is water molecules are split.cellular respiration and photosynthesis are complement  reactions to each other. these are reverse reactions. in cellular respiration glucose and oxygen yield water and carbon dioxide. so, when respiration begins,  glucose molecules are split. in the opposite reaction - in photosynthesis, carbon dioxide and water yield glucose and oxygen. thus,  when respiration begins, water molecules are split.