1The idea in one paragraph

Biodiversity is the variety of life at every level: the variety of ecosystems, of species, and of alleles within species. The fossil record suggests that more species are alive now than at any time in Earth's history, yet they are disappearing far faster than new ones form. This is the sixth mass extinction, and unlike the first five it is caused by one species: us. Hunting, the clearing of forests for farms and cities, pollution and the species we carry around the globe are the direct causes, and a growing human population drives all of them. We know the loss is real from surveys repeated over years, collected by professional and citizen scientists and published so that others can check them. No single method can stop it, so conservation combines protecting and restoring habitats with keeping species in zoos, gardens and seed banks, and the EDGE programme argues that the species carrying the most unique evolutionary history should be saved first.

2What biodiversity is

Biodiversity is the variety of life in all its forms, levels and combinations: ecosystem diversity, species diversity and genetic diversity.

Figure 1 sets out the three levels.

• Ecosystem diversity is the variety of habitats and communities in an area: a landscape with forest, wetland, grassland and river has more than one covered in a single crop.
• Species diversity is the variety of species. It has two parts: species richness, the number of different species, and species evenness, how equally the individuals are shared between them. Section 6 shows why both matter.
• Genetic diversity is the variety of alleles within a species. A population with many alleles is more likely to contain individuals that can survive a new disease or a changed climate; a population with few can be wiped out by one.

The three levels can be lost separately. A species can survive while losing most of its genetic diversity, if it is reduced to a few individuals. An area can keep the same species list while its ecosystems are simplified. An answer about biodiversity loss should say which level is being lost.

3More species now than ever before

About two million species have been discovered, named and described. That is certainly not all of them. Every year thousands of new species are described, mostly insects, fungi and small marine animals, and estimates of the total number of species on Earth run into many millions: one widely quoted estimate is close to nine million for eukaryotes alone, and the true figure for all life, including bacteria, is unknown.

How does that compare with the past? The fossil record, read carefully, shows diversity rising over hundreds of millions of years, interrupted by five mass extinctions in which a large fraction of species vanished in a geologically short time. Figure 2 shows the pattern schematically.

After each crash, diversity recovered and then went higher. The evidence from fossils suggests that there are more species alive today than at any time in the past. That is what makes the present so serious: the richest biosphere there has ever been is losing species at a rate that fossil evidence suggests is many times the normal background rate, and this time the cause is human activity.

Nature of science: lumpers and splitters. Classification is pattern recognition, and the same observations can be sorted in different ways. Splitters recognise many species, separating populations that differ even slightly; lumpers recognise fewer, treating such populations as varieties of one species. The same specimens can yield quite different species counts, which is one reason estimates of the number of species vary so much. Linking this back to A3.1: where the line falls between populations and species is partly a judgement.

4Extinctions caused by people: three case studies

The guide asks for the causes of the current extinction, not the natural causes of the earlier five, through at least three brief case studies. Figure 3 puts them on timelines.

North Island giant moa (Dinornis novaezealandiae): loss of terrestrial megafauna. Moa were large flightless birds found only in New Zealand, and the giant moa females were among the tallest birds that have ever lived. New Zealand had no land mammals except bats, so moa had no predators on the ground and no reason to fear a hunter. Polynesian settlers arrived around 1300 CE, and within about 150 years every species of moa was extinct. The causes worked together: hunting of adults and collection of eggs; slow reproduction, since moa laid few eggs and took years to mature, so losses could not be replaced; and burning and clearing of forest, which removed habitat. A large, slow-breeding, fearless animal meeting human hunters for the first time is the typical profile of a megafauna extinction.

Caribbean monk seal (Neomonachus tropicalis): loss of a marine species. This seal lived on beaches and reefs across the Caribbean and the Gulf of Mexico. The first recorded killing was by the crew of Christopher Columbus in 1494. Over the following centuries seals were hunted on a large scale, above all for the oil from their blubber, and also for meat and skins; they hauled out on beaches, which made them easy to kill. Overfishing of the reef fish they ate, and disturbance of the beaches they bred on, added to the pressure. The last confirmed sighting was in 1952, at Serranilla Bank between Jamaica and Nicaragua, and the species was declared extinct in 2008.

A third species, from an area you know. The guide asks for one extinction from an area familiar to you, so your teacher may choose a different one. Our model is the great auk (Pinguinus impennis) of the North Atlantic, a large flightless seabird that bred on a few rocky islands. From the sixteenth century onwards it was killed in huge numbers for meat, feathers, oil and fishing bait, and its eggs were taken. Because it bred in a few crowded colonies and could not fly, whole colonies could be emptied. As it grew rare, collectors paid high prices for skins and eggs, which sped the end: the last known pair was killed in Iceland in 1844. Replace it with a species from your own region and ask the same three questions: what made it vulnerable, what did people do, and why could it not recover?

The pattern across all three is worth stating in an answer: over-exploitation of a species that reproduces slowly and has no defence against a new predator, often combined with habitat loss.

5Ecosystem loss: two case studies

Whole ecosystems can be lost as well as species, and the guide asks for causes that are directly or indirectly human.

Mixed dipterocarp forest in Southeast Asia. The lowland rainforests of Borneo, Sumatra and the Malay Peninsula are dominated by trees of the family Dipterocarpaceae: very tall trees, many of great timber value, that fruit in irregular mass years. These forests are among the most species-rich on Earth, and home to orangutans, hornbills and thousands of other species. Over the last fifty years large areas have been lost, for several linked reasons:

• logging for valuable timber, which opens the forest and builds roads into it;
• conversion to plantations, above all oil palm, and also fast-growing trees for pulp and paper;
• clearance for farming as roads bring settlers;
• fire, much of it started to clear land, which spreads far in drought years, especially where peat swamps under the forest have been drained and dry out.

Once cleared, a dipterocarp forest does not simply regrow: plantations are replanted in rows of one species, and the mass fruiting cycle and the animals that depend on it are gone. That is loss of ecosystem diversity, not just of trees.

A local ecosystem. Again your teacher may choose one near you. Our model is the Aral Sea in Central Asia, once one of the largest lakes in the world. From the 1960s the two rivers that fed it were diverted to irrigate cotton and other crops. The lake shrank to a small fraction of its former area, its remaining water became far saltier, its native fish disappeared and the fishing industry that depended on them collapsed. The river deltas and their riverside forests dried out, and dust from the exposed lakebed, carrying salt and farm chemicals, blew over the surrounding land. The cause was indirect but entirely human: a decision about water use made far from the lake.

6Evidence for a biodiversity crisis

Claims of a crisis need evidence that others can check. The guide names the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES), which pulls together thousands of studies. Its 2019 global assessment concluded that around one million animal and plant species are threatened with extinction, that about three-quarters of the land surface has been significantly altered by human actions, and that most of the world's wetlands present in 1700 had been lost by 2000. Other sources point the same way: the Living Planet Index, for example, reports an average decline of about 70% in the sizes of monitored vertebrate populations since 1970.

Surveys must be repeated. One survey tells you what is there now. Only the same survey, repeated at the same sites by the same method years apart, shows change. And surveys must come from a wide range of habitats around the world, or a local decline could be mistaken for a global one.

Richness and evenness. A good survey records both. Figure 4 shows why: two invented woods with the same richness, five tree species each, but very different evenness.

Wood A is even: each species makes up 20% of the trees. Wood B is dominated by oak, 92% of the trees, and the other four species hang on with two trees each. Wood B is less diverse, because a small shock could remove four species at once, and a walker there meets almost only oak. A biodiversity crisis can show up as falling evenness long before species richness drops.

Some studies combine richness and evenness in one number. One common choice, which you will not be asked to recall but may meet in a data question, is Simpson's reciprocal index, D = N(N − 1) ÷ Σn(n − 1), where N is the total number of individuals and n the number of each species. The higher D, the more diverse.

Experts and citizen scientists. Much of the evidence comes from citizen scientists, members of the public who record species. In a famous case, members of an amateur entomological society in Krefeld, Germany, trapped flying insects by a standard method at protected sites for 27 years; when the data were analysed and published, they showed a decline of about three-quarters in the total mass of flying insects caught. Birdwatchers' records, garden bird counts and online recording apps add millions of observations every year.

Nature of science: what makes evidence verifiable. To count as verifiable, evidence usually has to come from a published, peer-reviewed source that sets out its methods so that others can check them and repeat them. Citizen science brings real benefits: huge numbers of observations, over wide areas and long periods, far more than professional scientists could collect. It also brings specific concerns: volunteers vary in skill at identifying species; effort varies (more records may mean more observers, not more animals); records cluster where people live; and methods may change over time. Good citizen-science projects deal with these by using standard methods, training and verifying records, and by professional scientists analysing the data and publishing them through peer review, as the Krefeld study did.

7Causes of the current biodiversity crisis

The guide names one overarching cause and several specific ones. Figure 5 shows how they connect.

Human population growth is the overarching cause. More people need more food, water, timber, housing, energy and transport, and every specific cause below scales with that demand. Consumption per person matters too, so the pressure grows even faster than the number of people.

• Hunting and other over-exploitation. Taking individuals faster than the population can replace them: the moa, the monk seal and the great auk, and today overfishing of the oceans and the trade in wild animals.
• Urbanisation. Cities, roads and infrastructure cover habitat and cut what remains into small, isolated fragments. Small fragments hold small populations, which lose genetic diversity and are easily wiped out.
• Deforestation and clearance of land for agriculture. The biggest single driver on land. Forests, grasslands and wetlands are replaced by farmland, and the natural habitat and the species that depend on it are lost, as in the dipterocarp forests.
• Pollution. Pesticides kill insects beyond the target pests; fertiliser running off farmland over-enriches rivers and lakes; plastics tangle and are swallowed by marine animals; carbon dioxide emissions warm the climate and acidify the oceans.
• Pests, diseases and invasive alien species, spread by global transport. Ships, planes and trade move organisms to places they never reached before. Brown tree snakes, arriving on Guam by accident, wiped out most of the island's native forest birds; a chytrid fungus spread around the world has caused the decline or extinction of many frog species. Island species, which evolved without these enemies, are the most at risk.

A good Section B answer names population growth as the driver, explains at least three specific causes with the mechanism by which each reduces biodiversity, and uses a named example for each.

8Conservation needs several approaches

No single approach is enough, and different species need different measures. Figure 6 lays the options out from the wild to the freezer.

In situ conservation keeps species in their natural habitat.

• Protected areas, such as national parks and marine reserves, where hunting, fishing, logging or building is restricted.
• Managing nature reserves. Protection alone is often not enough, because reserves are now islands in a human landscape. Managers remove invasive species, control grazing, manage water levels, prevent poaching, and link reserves with corridors so that populations are not isolated.
• Rewilding. Restoring natural processes by bringing back missing species, often large animals, and then stepping back. The return of grey wolves to Yellowstone National Park in the 1990s is the best-known example; the reintroduction of beavers, whose dams recreate wetlands, is another.
• Reclamation of degraded ecosystems. Repairing land that has been damaged: re-wetting drained peat bogs, replanting mangroves, restoring old mines and quarries.

In situ conservation is preferred because it keeps the species inside its ecosystem, with its food, predators, pollinators and partners, and allows it to go on evolving with them. Protecting a habitat also protects every other species that lives there.

Ex situ conservation keeps species away from their natural habitat.

• Zoos and botanic gardens hold living populations and run captive-breeding programmes. Some species survive only because of them: the Arabian oryx was extinct in the wild by the early 1970s and was returned to the wild from captive herds.
• Storage of germ plasm, the material that carries genes, in seed banks and tissue banks. Seeds of many plants survive for decades when dried and frozen; the Millennium Seed Bank in the United Kingdom and the Svalbard Global Seed Vault in Norway store seeds from all over the world. Frozen sperm, eggs, embryos and tissue samples do the same job for animals.

Ex situ methods are a safety net when a wild population is too small or its habitat is gone. Their limits are real: captive populations are small, so they lose genetic diversity; animals bred in captivity may lack the behaviour needed to survive in the wild; and a species kept in a zoo or a freezer plays no part in its ecosystem. That is why the approaches work best together: ex situ breeding to rebuild numbers, in situ protection and restoration so there is somewhere to return them to.

9Choosing what to save first: EDGE

Money and time for conservation are limited, so someone has to decide which species come first. The EDGE of Existence programme, run by the Zoological Society of London, ranks species by two measures, and Figure 7 shows both.

• Evolutionarily distinct (ED). How much unique evolutionary history a species carries. A species with no close living relatives sits at the end of a long, lonely branch of the tree of life; if it goes extinct, all of that branch goes with it. A species with many close relatives shares most of its history with them.
• Globally endangered (GE). How close the species is to extinction, taken from its category on the IUCN Red List.

Panel (a) of Figure 7 shows how distinctiveness is measured on an invented tree with branch lengths in millions of years. Each branch is shared equally among the species that descend from it.

Species Q carries more than twice the unique history of S or T. If Q is also endangered, it is an EDGE species, and it goes to the top of the list.

The rationale. Losing an evolutionarily distinct species loses unique genes, unique features and a whole lineage that took tens of millions of years to evolve and cannot be replaced. Many such species are small, strange or little known, such as the aye-aye of Madagascar, the Chinese giant salamander or the purple frog of India, and receive little attention compared with familiar large mammals. Focusing on EDGE species saves the most evolutionary history for each unit of effort.

Nature of science: a decision that has to be debated. Which species to save is not only a scientific question. It has ethical, environmental, political, social, cultural and economic sides. Should a species that is distinctive but plays a small role in its ecosystem come before a less distinctive keystone species? Should a species important to a local culture or economy be ranked by a formula made elsewhere? Is it right to let some species go? Science can supply the measures; the choice still needs open debate.

10Linking questions

In what ways is diversity a property of life at all levels of organisation? At the level of molecules, the variety of alleles and proteins; of cells, the many cell types and the three domains (A3.2); of organisms, variation between individuals and species (A3.1); and of ecosystems, the variety of communities and habitats in this subtopic. Biodiversity loss can happen at every one of these levels.

How does variation contribute to the stability of ecological communities? A community with many species, evenly represented, has several species doing similar jobs, so if one declines another can take its place; genetic diversity within each species lets populations adapt to change. Communities dominated by one species, or made of genetically uniform populations, are more easily disrupted by disease or climate. This is why falling evenness is a warning sign.