There are many answers, but the general idea is that humans are both positive and negative for the world. We are negative in that we cause damage to the ecosystems of the world. We cause eutrophication of lakes, acid rain, deforestation, loss of species, increased global temperature (greenhouse effect), and more. We are positive because we save dying animals, preserve endangered species, and more. We are more negative than positive and our efforts of positive are usually just to fix our mistakes.
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Lots of carbon dioxide in the atmosphere, pollution, extinct species and environmental damages.
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We participate in a complicated cycle of life.
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Body odor.