The variety of life, measured at the level of genes, species and ecosystems. It is unevenly distributed, incompletely catalogued, and declining at a rate that has prompted comparison with past mass extinctions.
Genetic diversity is variation within a species. It determines the capacity to adapt, and its loss is why small isolated populations remain vulnerable even after their numbers recover.
Species diversity is the number and relative abundance of species. It is the most commonly reported measure, and abundance matters as well as count, since a community with one dominant species and many rare ones differs from an even one.
Ecosystem diversity is the variety of habitats and of the processes within them.
The levels are connected: losing genetic diversity raises extinction risk, losing species alters ecosystem function, and losing habitat drives both.
Around two million species have been formally described. Estimates of the total range widely, with figures around eight to ten million commonly cited for eukaryotes, and far higher numbers when bacteria and archaea are included.
The uncertainty is not carelessness. Most undescribed species are small, live in inaccessible places such as soil, deep sea and forest canopy, or belong to groups with few specialists. Insects, fungi and nematodes are heavily under-described.
Known diversity is also skewed by attention. Birds and mammals are nearly completely catalogued, while new species of insect and fungus are described continually.
Diversity is very unevenly distributed. The latitudinal gradient, in which species richness increases toward the tropics, holds across most groups and has been recognised since Humboldt and Darwin. Explanations include greater energy input, longer evolutionary time without glaciation, and higher speciation rates, and no single account is agreed.
Biodiversity hotspots, regions with exceptional endemism under severe threat, cover a small percentage of land and hold a large share of endemic plant and vertebrate species, which is what makes them a conservation priority.
Provisioning. Food, timber, fibre and medicine come from living things. A substantial share of pharmaceuticals derives from natural products or was inspired by them, and crop breeding depends on wild relatives as a source of genes for disease resistance and stress tolerance.
Regulation. Pollination, decomposition, water purification, flood control and carbon storage are performed by organisms, and pollination alone is required by a large fraction of global food crops.
Stability. Field experiments have found that more diverse communities are generally more productive and more resilient to disturbance, because different species respond differently to a given stress and can compensate for one another.
The precautionary consideration is that ecosystems are not well enough understood to predict which losses matter. Some species can be removed with little effect and others are keystone species whose removal restructures the whole community, and identifying which is which reliably in advance is not currently possible.
Extinction rates are estimated to be far above the background rate inferred from the fossil record, with figures of tens to hundreds of times commonly reported, and the current episode is frequently described as a sixth mass extinction.
The drivers are well characterised and are ranked broadly consistently. Habitat loss and conversion, principally for agriculture, is the largest. Direct exploitation, including fishing and hunting, follows. Climate change is rising in importance and is expected to become dominant. Pollution and invasive species complete the list.
Vertebrate population declines have been documented across monitoring programmes, and insect declines have been reported in several regions, though the insect data are patchier and their extrapolation has been contested.
Formal assessment of extinction risk is maintained through the IUCN Red List, which categorises species by defined criteria and provides the standard reference for status.
Protected areas are the primary tool, and international targets have progressively raised the share of land and sea to be protected. Their effectiveness depends heavily on whether protection is enforced.
Species-level intervention, including captive breeding and reintroduction, has recovered species that were functionally extinct in the wild, and it is expensive and applicable to few.
Addressing drivers matters more than protecting sites in isolation, since a reserve surrounded by converted land loses species over time as populations become isolated.
Biodiversity is the accumulated result of about four billion years of evolution, and the current rate of loss is compressing a substantial part of that into a period of decades.
The practical case rests on dependence: agriculture, medicine and the physical habitability of much of the planet rely on processes performed by organisms, and those processes were never designed and cannot be straightforwardly replaced.
