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Narrator: Imagine if all the cats in the world suddenly disappeared. That would make cats extinct.

Extinction is when there are no more individuals of a certain species left alive anywhere in the world! But extinction is part of life on Earth.

In fact 99 percent of all living things that have ever existed are extinct. Species come and go all the time, but there are moments in Earths history when living things have died out at a much faster rate.

These are called mass extinctions and they've been caused by things like asteroids, giant volcanic eruptions and ice ages.

Some scientists think human activity might be causing a mass extinction now. Humans are destroying natural habitats and global warming is causing environments to change rapidly! And this is causing more and more species to become endangered.

A species is endangered when the number of individuals is going down. So they are at risk of becoming extinct. If a species goes extinct its role in the eco-system is often filled by another species. This unbalancing of an eco-system can lead to problems for humans too.

Healthy eco-systems clean our water and air; as well as provide food and protection from disease. So, although cats are not likely to disappear any time soon, there are lots of plants and animals that need us to change, so they can keep living happily and maintain a healthy, balanced eco-system.