The elimination of an organism in an ecosystem takes out all balance. There may be an overpopulation of some organisms, and an underpopulation of others. For an example, if rabbits were taken out of the food chain, there would be an overpopulation of grass, and underpopulation of what eats rabbits.
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A. Increase in soil fertility
If there was more than one planet on each orbit, eventually they would collide with each other.