A theory is a well-established, data-backed idea that has been shown true many times but cannot be 100% proven

Well if you were asking it was a true or false questions the answer is: True

the population of unionid mussels in the hudson river decreased after zebra mussels were introduced. zebra mussels, a nonnative species, competed with the unionid mussels for food. the zebra mussels were able to outcompete the unionid mussels, allowing the zebra mussel population to increase dramatically over six years. the unionid mussel population decreased due to a lack of food.

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