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21. In 1960, an invasive species of fish was introduced into the stable ecosystem of a river.
Since then, the population of a native fish species has declined. This situation is an example of
an
1) Ecosystem that has recovered
2) Ecosystem altered through the activities of humans.
3) Environmental impact caused by physical factors
4) Ecological niche without competition
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21. In 1960, an invasive species of fish was introduced into the stable ecosystem of a river.
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