Why is there animal extinction?

Animal extinction is a bad thing because each animal has a function in an ecosystem. Ecosystems are delicate and removing a piece of them, no matter how small, can have dramatic consequences for the entire ecosystem. For example, if all the fish died a river, the animals that eat them would starve and eventually leave or die off. In addition, everything that the fish ate would be left without a predator and overrun the ecosystem, possibly forcing out other species and depleting a resource.