The Earth’s biodiversity includes the entire range of living species (species diversity), the genetic variation that occurs among individuals within a species (genetic diversity), and, at a higher level, the biological communities in which species live and their associations with the physical and chemical environment (ecosystem diversity). A disproportionately large amount of the world’s biodiversity is hosted by tropical forests, coral reefs, and Mediterranean-type ecosystems. For practical purposes, most ecologists and conservationists identify species in the field according to their morphology, although improvements in genetic techniques are allowing more species to be identified according to their evolutionary past, revealing many cryptic species that people did not realise were there. There are several ways to measure and compare this biodiversity. The most popular of which is species richness in a particular community, such as a forest or grassland (alpha diversity), species richness across a larger landscape, such as a mountain range (gamma diversity), and the rate of change of species composition as one crosses a large region (beta diversity). Patterns of species richness are affected by variation in climate, topography, and geological age. Geological age and complexity provide environmental variation, which in turn allows opportunities for genetic isolation, local adaptation, and speciation, given enough time. It is estimated that there may be as many as 2 billion species on Earth, most of which already described are insects, while the best-known species include birds and mammals. The majority, however, still need to be discovered.