<P> Biodiversity, a portmanteau of "bio" (life) and "diversity", generally refers to the variety and variability of life on Earth . According to the United Nations Environment Programme (UNEP), biodiversity typically measures variation at the genetic, the species, and the ecosystem level . Terrestrial biodiversity tends to be greater near the equator, which seems to be the result of the warm climate and high primary productivity . Biodiversity is not distributed evenly on Earth, and is richest in the tropics . These tropical forest ecosystems cover less than 10 percent of earth's surface, and contain about 90 percent of the world's species . Marine biodiversity tends to be highest along coasts in the Western Pacific, where sea surface temperature is highest and in the mid-latitudinal band in all oceans . There are latitudinal gradients in species diversity . Biodiversity generally tends to cluster in hotspots, and has been increasing through time, but will be likely to slow in the future . </P> <P> Rapid environmental changes typically cause mass extinctions . More than 99.9 percent of all species that ever lived on Earth, amounting to over five billion species, are estimated to be extinct . Estimates on the number of Earth's current species range from 10 million to 14 million, of which about 1.2 million have been documented and over 86 percent have not yet been described . More recently, in May 2016, scientists reported that 1 trillion species are estimated to be on Earth currently with only one - thousandth of one percent described . The total amount of related DNA base pairs on Earth is estimated at 5.0 x 10 and weighs 50 billion tonnes . In comparison, the total mass of the biosphere has been estimated to be as much as 4 TtC (trillion tons of carbon). In July 2016, scientists reported identifying a set of 355 genes from the Last Universal Common Ancestor (LUCA) of all organisms living on Earth . </P>

What is the measure of the number of different species present in an area