Biodiversity is the variety of living things that exist in the world. You can measure biodiversity by counting how many creatures or plants there are of a certain type in an area. For example, if you measure the biodiversity of a forest, you could count the number of different types of trees, or the different types of animals that live in the forest. You could also measure the number of different species in an area. That way, you can tell how much diversity exists in that area.