Biodiversity refers to the variety and number of living organisms within an ecosystem or on Earth. It exists at three levels - genetic, species, and ecosystem biodiversity. Genetic biodiversity refers to the diversity within species and between populations of a single species. Species biodiversity is measured as the variety of species within a region. Ecosystem biodiversity refers to the variety of ecosystems like forests and wetlands within a region. Threats to biodiversity include habitat destruction, overexploitation, pollution, invasive species, climate change, overhunting, and natural catastrophes.