The three levels you must be able to separate

Biodiversity is the variety of living organisms, biological systems and heritable variation found in an area or on Earth as a whole. The key point is that it goes beyond a simple “number of species” count. Biodiversity exists at several levels of biological organization.

Ecosystem diversity is the variety of ecosystems in a region, differing in their physical conditions, communities and ecological processes. A coral reef, a peat swamp forest, a temperate grassland and a mangrove forest are not just different scenery; each one has its own abiotic conditions, food webs and cycling of matter.

Species diversity is the variety of species in a community or region, usually considering both how many species are present and how evenly individuals are distributed among them. So, a forest with 100 tree species has more species diversity than a plantation of one tree species, even if both contain the same number of individual trees.

Genetic diversity is the variety of alleles and genotypes within a species or population. Students often forget this level. A species reduced to a few surviving individuals may still exist, but it may have lost much of the variation needed for resistance to disease, adaptation to environmental change and avoidance of inbreeding.

A global map of land vertebrate diversity shows a clear pattern: diversity is not spread evenly across Earth. It is generally highest in tropical regions near the equator and lower toward the poles. That matters for conservation, because the decisions are spatial. Losing a hectare of habitat in one place may remove far more species than losing a hectare somewhere else.

Unity and diversity together

The linking idea here is that life is unified by shared features — cells, DNA, genetic code, metabolism and evolution by natural selection — but diversified by variation at every scale. DNA sequence variation can produce protein variation. Variation among individuals can produce population-level adaptation; variation among species produces different niches in communities; and variation among ecosystems produces the biosphere’s enormous range of habitats.

Variation also helps stabilize communities. If several species perform similar ecological roles, others may partly buffer the loss or decline of one species. If genetic variation is high within a population, some individuals are more likely to survive a new disease, pollutant or climate stress. Diversity, then, is not just a catalogue of “interesting organisms”; it is part of how living systems persist through change.

Present-day species numbers are estimates, not a final total

Species discovery means finding, naming and describing species so other scientists can recognize them. Around two million species have been formally described, but that does not mean only two million species exist. Many insects, fungi, protists, deep-sea organisms and microorganisms are still poorly sampled.

For eukaryotes, estimates of the total number of species often reach into the millions, and the figures vary because researchers use different methods. For prokaryotes, there is even more uncertainty. Many cannot be cultured easily, and defining a bacterial “species” is less straightforward than defining many animal or plant species.

Fossils suggest biodiversity is probably higher now than in the past

Fossil evidence means preserved material or traces of past organisms that scientists can use to infer the history of life. Fossils do not provide a complete list of every species that ever lived. Soft-bodied organisms, small organisms and organisms from habitats with poor preservation are under-represented. Even so, fossils do show broad trends.

The fossil record shows five previous mass extinctions, each followed by long periods during which biodiversity increased again as new groups evolved and diversified. Since the last major mass extinction, at the end of the Cretaceous period, biodiversity has had tens of millions of years to build up. The best fossil evidence suggests that more species are alive today than at any previous time in Earth’s history.

Classification is pattern recognition, not a perfect mirror of nature

Classification is the arrangement of organisms into groups based on shared characteristics or evolutionary relationships. It works as a kind of pattern recognition: scientists compare similarities, differences and ancestry patterns in the evidence.

The same observations can sometimes be classified in more than one defensible way. Splitters are taxonomists who recognize more separate species within a group by emphasizing differences between populations. Lumpers are taxonomists who recognize fewer species by emphasizing similarities and combining populations into broader species. Neither label automatically means “right” or “wrong”; what matters is which evidence is being given more weight.

DNA sequencing has often encouraged splitting, since populations that look similar may still be genetically distinct and reproductively isolated. Older classifications based mainly on appearance sometimes encouraged lumping instead. Strong evidence in biology is therefore not just a single observation. It is a pattern supported by multiple methods, open to checking and revision.

Extinction is natural; the present rate is not

Extinction means the permanent loss of every living individual of a species. Extinctions have happened throughout Earth’s history. They don’t always lower biodiversity, as long as speciation balances them over long periods. The problem today is speed: many present-day extinctions are anthropogenic, meaning caused directly or indirectly by human activity.

In this topic, keep your attention on the causes of the current sixth mass extinction, rather than the asteroid impacts, volcanism or long-term climate shifts linked with earlier mass extinctions.

Main anthropogenic causes

Over-exploitation happens when organisms are removed from a population faster than reproduction can replace them. This includes hunting, fishing, logging and harvesting plants for food, medicine or trade. Large animals that reproduce slowly are especially vulnerable.

Habitat destruction means removing or severely altering the place where a species normally lives and obtains resources. Agriculture, settlement, roads, mining and logging can push a population below the size needed for survival.

Invasive alien species are non-native organisms introduced by humans that spread and harm native species or ecosystems. They may cause extinction through predation, competition, disease transmission or hybridization with native relatives.

Pollution is the release of substances or energy into the environment at levels that harm organisms or ecological processes. Pesticides, heavy metals, oil, plastics, excess nutrients and pharmaceutical residues can all reduce survival or reproduction.

Rapid climate change is human-driven change in long-term temperature, rainfall, seasonality or extreme events, happening faster than some species can adapt or migrate. Species restricted to islands, mountains, polar regions or isolated reefs have few escape routes.

Case studies of anthropogenic extinction

The North Island giant moa, Dinornis novaezealandiae, shows how terrestrial megafauna can be lost. It was a huge flightless bird from New Zealand. After human settlement, hunting pressure and associated habitat change led to the extinction of moa species within a short historical period. Being large did not protect it; slow-breeding large animals are often easy to overexploit.

The Caribbean monk seal, Neomonachus tropicalis, gives a marine extinction example. It lived in the Caribbean Sea and Gulf of Mexico. Humans killed it for oil and meat, disturbed its breeding sites and competed with it for fish. The species was last reliably seen in the twentieth century and is now extinct. This case helps because students sometimes assume extinction is mostly a land problem. Marine mammals can disappear too.

For a familiar regional case, choose a species that has gone extinct from your own country or local area if your class has one. A widely taught example is the thylacine, Thylacinus cynocephalus, which disappeared from Tasmania after hunting, government bounties, habitat change and conflict with livestock farming. The key skill is linking the extinction to named human pressures, rather than just saying the species “died out”.

Comparison of three anthropogenic extinction case studies and their main human causes.

When writing about organisms, either the common name or the scientific name is acceptable, as long as the organism is clear. Scientific names help when common names vary between countries.

Ecosystem loss is more than species loss

An ecosystem is a biological system made of a community of organisms interacting with each other and with the physical environment. Ecosystem loss happens when an ecosystem disappears or collapses in function, so its characteristic community, abiotic conditions and ecological processes no longer persist.

Loss can be direct, as when forest is cleared for agriculture. It can also be indirect: water flow, salinity, nutrient level or climate may shift beyond the tolerance of key organisms. A keystone species is a species with a disproportionately large effect on community structure compared with its abundance. Remove one, and wider ecosystem change can follow.

Anthropogenic causes of ecosystem loss

The causes studied here should be directly or indirectly anthropogenic. The main ones are:

• agricultural expansion, including clearance of forest, grassland and wetland;
• urbanization, including buildings, roads, railways and other infrastructure;
• over-exploitation of natural resources, such as overfishing, bushmeat hunting or fuelwood collection;
• mining and smelting, which remove habitat and can pollute air, soil and water;
• dams and water extraction, which alter river flow, flooding and lake levels;
• drainage or diversion of water, especially damaging to wetlands;
• fertilizer leaching, causing eutrophication and algal blooms in freshwater and coastal ecosystems;
• climate change, which shifts temperature, rainfall, fire regimes and sea level.

Case study: mixed dipterocarp forest in Southeast Asia

Mixed dipterocarp forest is a tropical rainforest ecosystem dominated by many tree species from the family Dipterocarpaceae. In Southeast Asia, these forests can contain very high tree diversity and large amounts of valuable timber.

Logging, conversion to oil palm plantations and wider land-use change are the main causes of loss. Lowland forest on deep peat is especially vulnerable: drainage lets peat decompose, releasing carbon dioxide and making the land more fire-prone. Once forest structure, soil water level and species composition have changed, the original ecosystem is not simply “waiting to grow back”.

Case study: the Aral Sea ecosystem

The Aral Sea was once one of the world’s largest lakes. During the twentieth century, rivers feeding it were diverted for irrigation. The lake shrank, salinity rose dramatically, and its fish and invertebrate communities collapsed. Here, ecosystem loss was caused by water diversion, not direct hunting of the species involved.

A local ecosystem-loss case should be added where possible — for example, a drained wetland, cleared woodland, converted grassland, polluted river or reclaimed coastal habitat. The strongest case studies name the ecosystem, identify the human driver, and explain the mechanism of collapse.

What counts as evidence of a crisis?

A biodiversity crisis is a period when species, genetic diversity and ecosystems are being lost unusually quickly across many regions. That claim needs evidence behind it, not just a few dramatic examples.

Useful evidence can come from IPBES reports and from reliable biodiversity surveys in many habitats around the world. IPBES is an intergovernmental science-policy body that assesses evidence on biodiversity and ecosystem services for governments and the public.

Surveys can record population size, species range, area of habitat, ecosystem degradation, number of threatened species, genetic diversity, species richness and species evenness. Species richness is the number of species present in a sample or area. Species evenness is the similarity in the abundances of the species present. A site with ten species in similar numbers is more even than a site where one species dominates and the other nine are rare.

Repeated surveys are essential

A single survey shows what was present at one time. On its own, it cannot prove a decline. To show change, surveys must be repeated using comparable methods: the same site boundaries, similar search effort, similar season, similar time of day where relevant, and consistent identification methods.

For example, a fixed transect can be walked each year to count butterflies, or quadrats can be used repeatedly to record plant species in a grassland. Remote sensing can also monitor ecosystem area and degradation, such as forest loss, burning or conversion to agriculture.

A commonly used diversity measure is Simpson’s reciprocal index: $D = \frac{N(N - 1)}{\sum n(n - 1)}$, where $D$ is the reciprocal diversity index (dimensionless), $N$ is the total number of organisms of all species in the sample (organisms, a dimensionless count) and $n$ is the number of organisms of one species in the sample (organisms, a dimensionless count). Higher values occur when richness is high and individuals are spread more evenly among species.

Expert scientists and citizen scientists

Citizen science is scientific data collection or analysis carried out by members of the public, usually following a shared protocol. It can be extremely valuable because many observers can collect data across huge areas and over long time periods. Bird counts, butterfly transects and species-recording apps can reveal changes that a small research team couldn't monitor alone.

There are methodological concerns too. Citizen observers may vary in expertise, sampling effort may be uneven, popular sites may be over-recorded, and absence of a record does not always mean absence of a species. Good citizen-science projects reduce these problems with training, validation, photographs, expert checking, standardized protocols and transparent databases.

To be verifiable, evidence usually needs to come from a published source, ideally peer-reviewed. Peer review is evaluation of scientific work by other specialists before publication, so that methods, analysis and conclusions can be checked. Strong evidence in biology is repeatable, methodologically clear, open to scrutiny and consistent across independent sources.

Human population growth is the overarching pressure

The current biodiversity crisis has worsened as human activity has grown in scale. Human population growth is the increase in the number of humans alive over time; it raises total demand for food, water, energy, land, transport and materials.

Population growth doesn’t act by itself. Its impact depends on consumption, technology, governance and inequality. Even so, a larger population usually puts more pressure on land and ecosystems unless resource use changes sharply.

Specific causes required for this topic

Several interacting causes drive the current crisis:

• Hunting and other forms of over-exploitation: populations fall when removal exceeds replacement by reproduction. This includes commercial fishing, wildlife trade, logging and harvesting.
• Urbanization: expanding towns, cities and infrastructure replace natural habitats and break up what remains.
• Deforestation and agricultural land clearance: forests, grasslands and wetlands are converted to cropland, pasture and plantations, causing direct habitat loss.
• Pollution: pollutants can reduce survival, fertility and development, and nutrient pollution can transform aquatic ecosystems through eutrophication.
• Spread of pests, diseases and invasive alien species through global transport: ships, aircraft, roads and trade carry organisms beyond natural barriers. Introduced predators, competitors, pathogens and parasites can then spread rapidly.

These causes often strengthen one another. A forest cut up by roads becomes easier for hunters to reach, more vulnerable to invasive species, and more exposed to drying and fire. Climate change can add extra stress by shifting temperature and rainfall beyond the range to which species are adapted.

The present situation is often called the sixth mass extinction because extinction rates are far above background rates and many groups are declining at the same time. Earlier mass extinctions were caused by non-human geological or astronomical events; this one is largely caused by human choices, so prevention and reduction are possible.

No single conservation method is enough

Conservation means protecting and managing biodiversity so extinction risk falls and ecosystems keep functioning. Species do not all face the same threats, so one method will not fix every case. A plant with viable seeds, a migratory bird, a coral reef and a large carnivore all need different measures.

In situ conservation and reserve management

In situ conservation conserves species within their natural habitats. Organisms stay where they interact with their normal abiotic environment, food sources, competitors, predators, mutualists and pathogens. This is usually the preferred approach because it protects whole ecological relationships, rather than only isolated individuals.

Protected areas include national parks, marine reserves, wildlife refuges and community-managed forests. But a boundary on a map rarely does much by itself. Nature reserve management means actively intervening in a protected area to maintain biodiversity and ecological function. It may involve anti-poaching patrols, control of invasive species, habitat burning or grazing regimes, reintroduction of locally extinct species, predator management, visitor control and monitoring.

Rewilding and reclamation

Rewilding restores a degraded ecosystem toward a more self-sustaining state by re-establishing natural processes and, where suitable, missing species. It is not just planting a few trees. The aim is for ecological interactions to do more of the work over time.

Reclamation is the recovery of degraded land or water so that it can support a functioning ecosystem again. Examples include restoring wetlands by raising water levels, reconnecting rivers to floodplains, removing pollutants, replanting native vegetation and reducing erosion.

These approaches link to the question of stability. Ecological communities with more variation in species, genes and niches often have more ways to absorb disturbance. Restoring habitat structure and species interactions can therefore increase resilience, although recovery may take decades and may not recreate the original ecosystem exactly.

Ex situ conservation and germ plasm storage

Ex situ conservation conserves species outside their natural habitats. Zoos can breed endangered animals, botanic gardens can propagate endangered plants, and carefully managed programmes may later return individuals to suitable habitat.

There are limits. Captive populations can lose genetic diversity, adapt to captivity, or fail to learn natural behaviours. Ex situ conservation also does not protect the original ecosystem. Use it when wild survival is currently unsafe, when numbers are extremely low, or when a backup population is needed.

Germ plasm is living reproductive or genetic material that can be used to produce new individuals. Seed banks store seeds at low temperature and low moisture to maintain viability. Tissue banks, sperm banks and egg storage can preserve genetic material from animals and plants. This is valuable, but it is a backup, not a substitute for conserving living ecosystems.