Tropical forests cover under a fifth of land but hold half of all species
Life is spread very unevenly. Warm, productive tropical forests occupy less than one-fifth of Earth's land yet shelter roughly half its species. Scientists estimate around 8.7 million species live on land and 2.2 million in the sea, and most have never been named, including perhaps tens of millions of insects.
Biologists usually define biodiversity as the sum of genes, species and ecosystems in a region, a framing that folds together taxonomic, ecological, morphological and functional variety. The term often stands in for older phrases like species richness, but it has no settled definition; in 2019 the UN's Food and Agriculture Organization described it as variation within and between species and across the ecosystems they form. Diversity generally rises towards the equator, in the oceans as well as on land.
The unknowns are enormous. A 2011 study by Mora and colleagues produced the land and sea estimates above, noting they are strongest for complex cells and probably understate bacterial diversity. Insects may number 10 to 30 million species, of which about 900,000 are known. Fungi may run from 1.5 to 3 million, yet only around 75,000 had been documented by 2001. Plankton sampling at sea between 2004 and 2006 uncovered so many new genes that it may force a rethink of how species are defined. Birds and mammals are well studied; fish and arthropods much less so.
Deep time shows how fragile variety can be. The Cambrian explosion, early in the last 540 million years, produced most animal body plans, but repeated mass extinctions followed. The worst, at the end of the Permian 251 million years ago, left vertebrates needing 30 million years to recover.
Many scientists now describe a sixth mass extinction driven by people, mainly through clearing habitat for farming, with climate change adding pressure; it may have begun with overhunting of ice age megafauna. A 2007 estimate suggested up to 30 per cent of species could vanish by 2050, and extinctions now run hundreds of times above background rates. The Living Planet Report 2014 claimed that monitored vertebrate populations had roughly halved in 40 years, with freshwater species and Latin America hit hardest.
Source: Biodiversity
