Ecosystems disturbed by a nature will be eventually restored unless it's a really big cataclysm, but most of the ecosystems are now a human caused disturbance. For example there are plenty of species which are already extinct or nearly extinct because of hunting industry.
Answer:
A dam that could allow fish to still pass through is a fishway/fish pass its a "fish ladder" that is structured on or around artificial or natural barriers to allow for fishes natural migration and the movement for potamodromous species.
That the oxygen levels in the test tube increased and carbon dioxide levels decreased because plants produce oxygen using carbon dioxide
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