Mosquitofish
In the early 20th century, human beings caught western mosquitofish and took them to other places. They did this because they thought the western mosquitofish would eat mosquitoes and their young. This did not work. Sometimes there were more mosquitoes than before: The western mosquitofish killed other fish that were eating mosquitoes. It did this by either eating the other fish, killing them without eating them, or eating so much of the food that the other fish died. The western mosquitofish sometimes killed fish that were better at killing mosquitoes than it was. It also ate some of the insects that had been eating the mosquitoes and mosquito young. It also ate zooplankton, which left more food for the mosquito young to eat themselves.