Biodiversity is the variability of life on Earth. It can be measured on
various levels, for example, genetic variability, species diversity, ecosystem
diversity and phylogenetic diversity. Diversity is not distributed evenly on
Earth—it is greater in the tropics as a result of the warm climate and high
primary productivity in the region near the equator. Tropical forest
ecosystems cover less than one-fifth of Earth's terrestrial area and contain
about 50% of the world's species. There are latitudinal gradients in species
diversity for both marine and terrestrial taxa.
Since life began on Earth, six major mass extinctions and several minor events
have led to large and sudden drops in biodiversity.