Daily life in most environments, including the watery environments of salty oceans and freshwater streams, rivers, lakes, ponds, and wetlands is a balance between all the living things in the particular environment (ecosystem). Changing the mix of these living things can upset the ecosystem and have undesirable consequences.

Sometimes the change is accidental. A new species (a classification of related organisms that can reproduce) can happen to find its way into a new environment where the conditions include plenty of food, few enemies, and an ideal temperature. The change can also be deliberate, due to humans' attempts to control one undesirable species by adding another. Sometimes, the introduced species thrives in the environment and becomes the dominant species. This can reduce the biodiversity (the vast range and number of different species) of the environment, as the introduced species outcompetes other species. But, this is not always...