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Biodiversity (a contraction of "biological diversity") is the variety of life at all levels of biological organization, from genes to species to ecosystems. The term was popularized in the 1980s, and it is defined in the Convention on Biological Diversity (1992) as the variability among living organisms from all sources, including terrestrial, marine, and other aquatic ecosystems and the ecological complexes of which they are part. Biodiversity is studied in ecology, evolutionary biology, conservation biology, and Earth system science. It is more than a count of species. It includes the differences among genes within species, the differences in form and function among species, and the variety of habitats and ecological processes.

Biodiversity contributes to the functioning of the biosphere. Organisms take part in the carbon cycle, the nitrogen cycle, and the water cycle (for example through transpiration by plants), and they affect soil formation, the composition of the atmosphere, and the properties of the hydrosphere. These processes are not controlled by biodiversity alone, because they also depend on climate, geology, and the physical environment, but biological communities can change them strongly. The loss of biodiversity is now recognized as a global environmental problem, and it is one of the topics of the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES).

Biodiversity is usually described at three connected levels:

• Genetic diversity is the variation in genes within and among populations of a species. It is the raw material for adaptation by natural selection. It can be measured by heterozygosity and by nucleotide diversity, and it is reduced by genetic drift in small populations.
• Species diversity is the number and relative abundance of species in a community or region.
• Ecosystem diversity is the variety of ecosystems and habitats, such as tropical forests, coral reefs, grasslands, and tundra, and of the biomes that they form. Some authors also distinguish functional diversity, the variety of traits and ecological roles of species, and phylogenetic diversity, the evolutionary history that a set of species represents.

Measuring Diversity

Species diversity has several components. Species richness is the number of species. Species evenness describes how equal their abundances are. Indices combine both. If p_i is the proportion of individuals that belong to species i among S species, the Shannon index is:

and the Simpson index of dominance is D = Σ p_i², with the Gini-Simpson index being 1 − D. The maximum of _H_′ is ln S, reached when all species are equally abundant. For example, a community of 4 species with equal abundance has _H_′ = ln 4 ≈ 1.39. The effective number of species (the Hill number of order 1) is exp(_H_′), which is 4 in this case. Diversity can be partitioned into alpha diversity (within a site), beta diversity (the turnover among sites), and gamma diversity (across a region), and in the multiplicative form of Whittaker, γ = α × β.

Patterns of Diversity

Biodiversity is unevenly distributed. The number of species generally increases from the poles to the tropics, a pattern called the latitudinal diversity gradient, with several proposed explanations that include greater energy, more stable climates, higher rates of diversification, and longer times for diversification in the tropics. Diversity also increases with area. The species-area relationship is often described by a power law:

where S is the number of species, A is the area, and c and z are constants. The exponent z is typically about 0.15 to 0.35, so that, for z = 0.25, a reduction of habitat area by 90 percent is expected to eventually reduce the number of species by about 1 − 0.1^0.25 ≈ 44 percent. The theory of island biogeography of Robert MacArthur and E. O. Wilson explains the number of species on an island as a balance between immigration and extinction. Biodiversity hotspots are regions with high numbers of endemic species and a high degree of habitat loss. The 36 hotspots that were identified cover about 2 to 3 percent of the land surface and contain a large fraction of the plant and vertebrate species that are found only there.

About 2 million species have been formally described, and the total number of eukaryotic species has been estimated at roughly 8 million, with a large uncertainty. The number of prokaryotic species is much less certain.

Biodiversity and Ecosystem Function

Experiments in grasslands and other ecosystems have found that more diverse communities, on average, have higher and more stable biomass production and use resources more completely. Several mechanisms are proposed. Species differ in their use of resources (niche complementarity), so that a mixture can use more of the available resources. A diverse community is also more likely to contain species with a large effect on function (the selection effect). The "insurance hypothesis" proposes that species differ in their responses to environmental change, so that a diverse community has a more stable function because the decline of one species can be compensated by others, which is a form of functional redundancy. The relationship between diversity and function is not the same in all systems. It usually saturates, so that the loss of the first species has a smaller effect than losses at low richness, and the loss of a species with unique traits can have a large effect. Keystone species, such as sea otters and some predators, have effects on their communities that are much larger than their abundance would suggest, and their removal can cause trophic cascades.

Ecosystem Services

Benefits that people get from ecosystems are called ecosystem services. They include provisioning services (food, fiber, fuel, and genetic resources), regulating services (climate regulation, flood control, water purification, pollination, and pest control), supporting services (nutrient cycling and soil formation), and cultural services. About three quarters of the leading food crops depend to some extent on animal pollination, and assessments by IPBES estimate that a substantial share of crop production is at risk if pollinators decline. Many medicines are derived from natural products, and biodiversity is also valued for its own sake and for cultural reasons.

Current Rates of Extinction

The background rate of extinction, estimated from the fossil record, is of the order of 0.1 to 1 extinction per million species-years. Studies of recent extinctions estimate rates that are tens to hundreds of times higher, and it is estimated that about 1 million species of animals and plants are threatened with extinction, many within decades, in the 2019 global assessment of IPBES. The IUCN Red List assesses the conservation status of species, and it has recorded more than 40,000 species as threatened, with about 41 percent of amphibians, a third of reef-building corals, and a quarter or more of mammals assessed as threatened. Many biologists describe the present loss as the beginning of a sixth mass extinction, although there is debate about the rates, because most species are undescribed and the data are incomplete, and the extinction has not yet reached the magnitude of the five mass extinctions in the geological record, in which about 75 percent or more of species were lost. The term Anthropocene is informal and has not been adopted as a formal geological epoch.

Main Drivers

IPBES (2019) ranked the direct drivers of biodiversity loss, in order of the size of their global impact on land and sea, as: changes in land and sea use, direct exploitation of organisms, climate change, pollution, and invasive alien species. These drivers interact. They are in turn driven by population, consumption, trade, and technology.

Habitat Loss, Degradation, and Fragmentation

Land-use change, mainly the conversion of forests, wetlands, and grasslands to agriculture, is the largest cause of loss on land. Agriculture covers about a third of the ice-free land surface, and about three quarters of the land has been significantly altered by human activity. Habitat fragmentation divides continuous habitat into small patches. Fragments are smaller, have a larger proportion of edge, and are isolated, so that populations in them are smaller and more subject to genetic drift, inbreeding depression, and local extinction, and dispersal between them is reduced. The effect of reduced area on the number of species follows the species-area relationship, and it can occur with a delay, called the extinction debt, so that species that persist in a fragment may be committed to eventual extinction. Fragmentation can also impede the movement of species that need to track changes in climate, but the extent depends on the species and the landscape.

Overexploitation

Hunting, fishing, logging, and harvesting of wild species have driven declines. The overfishing of marine stocks and the hunting of terrestrial vertebrates, including in tropical forests, have caused the loss of many large species, in some cases with effects on whole food webs, and the trade in wildlife products is a threat to some species. The extinction of species by hunting began in the late Pleistocene, in the megafaunal extinctions that followed the spread of modern humans.

Climate Change

The accumulation of greenhouse gases is changing temperature and precipitation, and the velocity of climate change is fast compared with the dispersal and adaptation rates of many species. Responses that are documented include shifts of ranges toward the poles and to higher elevations, and changes in phenology, the timing of seasonal events. If the timing of two interacting species changes at different rates, a phenological mismatch can result. An example is the mismatch between the date of hatching of caterpillars and the arrival of insectivorous birds that feed their young on them, which has been associated with declines of some bird populations in Europe, although the effect on populations is variable and not found in all studies. All organisms have limits of temperature tolerance. Heat waves have caused mass mortality events, as in marine organisms, in trees during drought, and in bats and birds in extreme heat, and species in the tropics and on mountain tops may have little room to shift. The extinction risk from climate change rises with the level of warming, and several syntheses estimate that a larger fraction of species is at risk at 3 °C of warming than at 1.5 °C, with estimates that range widely among studies. Climate change interacts with other drivers, such as habitat loss, which makes adaptation and movement harder.

Ocean Acidification and Marine Heatwaves

The ocean has taken up roughly a quarter to 30 percent of anthropogenic CO₂ emissions. The dissolved CO₂ forms carbonic acid, which lowers the pH of surface seawater (by about 0.1 pH units since the pre-industrial period, a roughly 30 percent increase in hydrogen ion concentration) and decreases the concentration of carbonate ions. This reduces the saturation state of aragonite and calcite, the minerals that are used by corals, mollusks, pteropods, and some plankton, and it makes the formation of shells and skeletons harder. The effects differ among organisms, and some are resilient. Marine heatwaves and warming have caused repeated mass coral bleaching, in which corals lose the symbiotic dinoflagellate algae (Symbiodiniaceae) that supply most of their energy. Severe bleaching events on the Great Barrier Reef occurred in 2016, 2017, 2020, 2022, and 2024, and a global bleaching event that began in 2023 has been the most extensive on record. Reef-building corals support a large number of species, so that their decline affects many other organisms.

Pollution and Invasive Species

Pollution includes nutrient enrichment from fertilizers and sewage, which causes eutrophication and oxygen-depleted "dead zones", plastics, pesticides, and other chemicals. Pesticides that affect insects, notably neonicotinoids, have been linked to harm to pollinators, and many studies have documented declines in insect abundance and diversity in some regions, although the global trend is uncertain and varies among groups. Invasive species introduced by trade and travel are a major cause of extinction on islands. Examples include the brown tree snake on Guam, rats and cats on many islands, and the chytrid fungus Batrachochytrium dendrobatidis, which has contributed to amphibian declines and the extinction of many species of frogs.

Genetic Drift and Effective Population Size

When populations are reduced, they lose genetic diversity through genetic drift and bottlenecks. Expected heterozygosity declines at a rate of 1/(2_N__e) per generation, where N_e is the effective population size:

For example, a population with N_e = 50 loses about 1 percent of its heterozygosity each generation, and over 20 generations it retains (0.99)²⁰ ≈ 82 percent. Small populations also suffer from inbreeding depression, the lower fitness of inbred offspring that results from the expression of deleterious recessive alleles, and from a reduced ability to adapt, because there is less genetic variation on which selection can act.

The Extinction Vortex and Genetic Rescue

The "50/500" rule of thumb proposed that an effective size of 50 is needed to avoid inbreeding depression in the short term and 500 to retain evolutionary potential, and later authors suggested larger values, such as 100 and 1,000, so these values are guides and not thresholds. Small populations may also enter an "extinction vortex", in which small size, inbreeding, and demographic fluctuations reinforce each other. Genetic rescue, by moving individuals from other populations, has restored fitness in some cases, for example in the Florida panther.

Terrestrial and Marine Biogeochemical Coupling

Changes in biodiversity can affect climate and biogeochemical cycles, and the effects differ among ecosystems. The magnitude of these feedbacks is under study, and some are not well constrained.

• Tropical forests. The Amazon rainforest stores a large amount of carbon and recycles water by evapotranspiration, so that a large fraction of the rain in the basin is derived from the forest. Deforestation, fire, and drought reduce evapotranspiration and carbon storage. Parts of the forest, especially in the southeast, have recently emitted more carbon than they absorbed. Some studies suggest that the forest could be pushed past a tipping point to a drier, savanna-like state if deforestation and warming increase enough. The threshold is uncertain, and estimates range from about 20 to 25 percent deforestation in some studies, with other factors such as fire and drought that contribute. Diversity is not the only factor. Forest structure, the length of the dry season, and land management are important.
• Marine biological pump. Phytoplankton fix CO₂ by photosynthesis, and part of the organic matter sinks to the deep ocean, where its carbon is stored for centuries or longer, a process called the biological pump. The efficiency depends on the composition of the plankton community, because organisms with mineral ballast such as diatoms and coccolithophores produce faster-sinking particles, and on temperature and the structure of the food web. Changes in the community with warming and stratification could change the rate of export of carbon, but the net effect on atmospheric CO₂ is uncertain, and it is not established that the loss of particular species reduces sinking.
• Other effects. Loss of large animals, such as whales, sharks, and large herbivores, and of predators, can change carbon storage, nutrient cycling, and fire regimes. Albedo and evapotranspiration change when the vegetation changes, as when shrubs spread into Arctic tundra.

Global Targets and Management Interventions

Conservation aims to prevent extinctions and to maintain ecosystems. Main approaches include:

• Protected areas. The Kunming-Montreal Global Biodiversity Framework (2022) set a goal of protecting 30 percent of land and sea by 2030 (the "30 by 30" target). The effectiveness of protected areas depends on management, funding, and connectivity.
• Restoration and corridors. Ecological restoration returns degraded ecosystems toward a more natural state, and corridors connect habitat patches.
• Species-specific action. Captive breeding, reintroduction, control of invasive species, and the regulation of trade, for example under CITES, have helped to recover some species.
• Sustainable use and policy. Sustainable agriculture, forestry, and fisheries, community-based management, and economic incentives. Climate mitigation is also important for biodiversity.
• Genetic management. Maintaining genetic diversity in small populations, in gene banks, and by genetic rescue.

Why does biodiversity increase toward the equator?

The latitudinal diversity gradient arises from the convergence of high incident solar radiation, long-term climatic stability, and larger contiguous biome areas in the tropics. Elevated temperatures and abundant precipitation support higher net primary productivity, allowing finer resource partitioning and narrower ecological niches. Furthermore, tropical regions escaped repeated Pleistocene glaciation, permitting lower background extinction rates and extended evolutionary time for speciation to accumulate lineages.

How does the species-area relationship predict extinction debt?

The species-area relationship (S = cA^z) demonstrates that species richness scales as a power function of contiguous habitat area, with z typically near 0.25. When habitat destruction removes 90% of an ecosystem's area, roughly 44% of resident species lose the minimum viable territory required for long-term persistence. Because long-lived species decline gradually over multiple generations, this delayed demographic collapse creates an "extinction debt" that unfolds over decades.

How does functional redundancy protect ecosystems during environmental shocks?

Functional redundancy occurs when multiple species within an ecosystem perform overlapping biogeochemical roles, such as nitrogen fixation, pollination, or organic decomposition. Under the ecological insurance hypothesis, coexisting species possess distinct physiological tolerances to temperature, drought, or pathogens. When an environmental perturbation suppresses one sensitive species, functionally redundant taxa with higher tolerance expand to compensate, stabilizing community biomass production and nutrient retention.

• Ecology
• Conservation biology
• Extinction
• Holocene extinction
• Ecosystem service
• Species richness
• Genetic diversity