A4.2 Conservation of biodiversity

Biodiversity (biological diversity) means the full range of living things and the ways they differ, from whole communities down to individual genes. Biologists usually break it into three levels, from the largest scale to the smallest.

Why each level matters

• Ecosystem diversity exists because Earth has so many different combinations of climate, soil, water and geography, and because each species has its own geographical range.
• Species diversity reflects the huge range of body plans, life cycles and modes of nutrition that evolution has produced (see A4.1 Evolution and speciation).
• Genetic diversity is the raw material for evolution. A population with many alleles is more likely to contain individuals that survive a new disease or a change in climate. Small, isolated populations lose alleles by chance and through inbreeding, so they become less able to adapt.

The three levels are linked: losing an ecosystem removes the species that depend on it, and losing populations of a species removes genetic diversity even before the species itself disappears.

Species alive today

About 2 million species have been discovered, named and described, and roughly 15,000–20,000 new ones are added each year. Most estimates of the true total are much higher. One widely cited estimate (2011) suggested about 8.7 million species of eukaryotes, implying that most species have not yet been described. Undiscovered species are mostly insects, other invertebrates, fungi and species in poorly explored habitats such as the deep sea and rainforest canopy; bacteria and archaea would add many more.

Comparing with the past

It is harder still to count species in the past. Fossils are incomplete: soft-bodied organisms rarely fossilize, and older rocks have often been destroyed. But counts of fossil families and genera, especially of marine animals, show that diversity has risen in the long run, with five sharp falls (the mass extinctions) each followed by recovery. The evidence suggests that more species are alive on Earth today than at any time in the past.

Lumpers and splitters

Deciding where one species ends and the next begins is a matter of pattern recognition, and two careful taxonomists can look at identical specimens and still group them differently. Taxonomists who are “splitters” recognize many species, treating small consistent differences as enough to separate them; “lumpers” recognize fewer, broader species. For example:

• In 2017 orangutans in one small area of Sumatra were described as a separate species, the Tapanuli orangutan, giving three orangutan species instead of two.
• The Chinese giant salamander has been proposed to be several species rather than one.

So species totals depend partly on decisions about where to draw boundaries, not only on what exists.

Five mass extinctions have happened in the past, each removing roughly three-quarters or more of all species, with natural causes such as volcanism and an asteroid impact. Many biologists think that a sixth mass extinction is now under way, and this one is anthropogenic (caused by humans). Three case studies show the range of causes.

North Island giant moa: loss of terrestrial megafauna

The North Island giant moa (Dinornis novaezealandiae) was one of nine species of moa, large flightless birds found only in New Zealand. Females were much larger than males, reaching about 2 m at the back (more with the neck raised) and well over 100 kg. Moa evolved with no land mammals to prey on them. On the South Island their only major predator was Haast’s eagle, a huge eagle that did not live on the North Island, so the North Island giant moa had no significant predator at all.

• Polynesian settlers, the ancestors of the Māori, reached New Zealand around 1250–1300 CE.
• Moa were hunted for meat, and their eggs were collected. Forest was also burned, and the dogs and rats the settlers brought may have taken eggs and chicks.
• Moa were easy to catch, had no fear of humans and reproduced slowly: they took years to mature and laid few eggs.
• All moa species were extinct within about 150–200 years, by about 1400–1450. On the South Island, Haast’s eagle, which depended on moa as prey, died out at about the same time.

Caribbean monk seal: loss of a marine species

The Caribbean monk seal (Neomonachus tropicalis) lived in the Caribbean Sea, the Gulf of Mexico and nearby Atlantic coasts.

• From the 1490s, and on a large scale in the 18th and 19th centuries, seals were killed for the oil in their blubber, and for meat and skins.
• They hauled out and bred on beaches and islets, moved slowly on land and were not afraid of people, so whole colonies could be killed.
• Over-fishing of the reef fish they fed on reduced their food supply.
• The last confirmed sighting was in 1952, and the species was officially declared extinct in 2008 after searches found none.

A local example

Your teacher may choose a species lost from your own region. One illustrative example is the passenger pigeon (Ectopistes migratorius) of eastern North America, probably once the most numerous bird on the continent, with flocks of hundreds of millions. It was netted and shot in huge numbers for cheap meat, and its forest habitat was cleared. Because it bred in large colonies, it could not breed successfully once numbers fell. The last individual, “Martha”, died in Cincinnati Zoo in 1914. Other examples include the great auk, the thylacine and the dodo.

Ecosystems have always changed naturally, for example as glaciers advanced and retreated. What is new is the speed and scale of ecosystem loss over the past few centuries, and its anthropogenic causes. Causes can be direct (the activity destroys the ecosystem on the spot) or indirect (it creates the pressure or conditions for destruction).

Mixed dipterocarp forest, Southeast Asia

Dipterocarps are a family of tropical hardwood trees, named for their winged fruits (“two-winged”). Depending on the classification used there are roughly 500–700 species. In mixed dipterocarp forest many dipterocarp species grow together with other trees, forming tall rainforest with emergent trees often over 50 m high; it is the main lowland rainforest of Borneo, Sumatra and the Malay Peninsula. These forests are among the most species-rich on Earth and are home to orangutans, gibbons, hornbills and clouded leopards. Many dipterocarps fruit together only every few years (“mast fruiting”).

Direct causes of loss:

• Logging: dipterocarp timber is valuable, and forests were heavily logged (legally and illegally) from the 1970s. Logging roads open the forest to further clearance and hunting.
• Conversion to plantations: logged land is cleared for oil palm and fast-growing pulpwood trees.
• Fire: forests damaged by logging dry out and burn, especially in El Niño droughts; fire is also used to clear land.
• Drainage: where lowland forest grows on peat, the peat is drained for plantations, so it decomposes and releases large amounts of CO₂.

Indirect causes: global demand for timber and palm oil; growing human populations; government policies favouring plantations; and climate change, which makes droughts and fires more likely.

Lowland forest has been lost fastest because it is easiest to reach and to plant. On Borneo, satellite images show that about 30% of the forest present in 1973 had gone by 2010, and much of the rest had been logged. Most undisturbed dipterocarp forest now survives on steep upland sites.

A lost ecosystem near you

If possible, study a lost ecosystem from your own area. As an illustration, the tallgrass prairie of central North America once covered a vast area; ploughing for crops, the end of natural fires and the near-extermination of bison mean that only a few per cent of it remains. Other examples include drained wetlands and the Aral Sea, which shrank after its rivers were diverted for irrigation.

A claim that biodiversity is in crisis needs evidence. Scientists look for reliable data, collected with clear methods, published and peer-reviewed so that others can check the methodology and the conclusions.

IPBES reports

The Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) brings together experts from many countries to assess the published evidence for governments. Its 2019 Global Assessment, based on thousands of studies, concluded that:

• about 1 million animal and plant species are threatened with extinction;
• about 75% of the land surface has been significantly altered by human actions;
• more than 85% of wetlands present in 1700 had been lost by 2000;
• the extinction rate is already at least tens to hundreds of times higher than the average over the past 10 million years, and rising.

Repeated surveys: richness and evenness

A single survey shows what is present now. Evidence of change needs repeated surveys of the same places using the same methods (same sampling area, effort, season and identification standards). Comparing them shows changes in species richness and evenness, in population sizes and in the range of species. Surveys are needed in a wide range of habitats around the world, because one site may not be typical.

A worked example (hypothetical data from a meadow, surveyed in the same way ten years apart):

Richness fell from 5 to 4, and evenness fell too: in survey 1 no species made up more than 25% of individuals, but in survey 2 species A made up 82%. The total number of individuals is the same (100), which shows why counting individuals alone can hide a loss of diversity.

Citizen science

Much long-term data come from citizen scientists: volunteers who count birds, record butterflies or upload photos to recording apps. In Germany, standardized insect traps run mainly by members of an amateur entomological society showed that the biomass of flying insects in protected areas fell by over 75% between 1989 and 2016.

• Benefits: many observers can cover huge areas over many years, cheaply, and people become engaged with conservation.
• Methodological concerns: identification skill varies, effort is uneven (more records from accessible places and popular species), and methods may change over time. These are reduced by standard protocols, training, expert checking of records and statistical analysis in peer-reviewed papers.

Humans have caused extinctions for thousands of years, but the intensity of the causes has risen sharply in the past century. The overarching cause is human population growth: the world population roughly quadrupled between 1920 and 2020 and passed 8 billion in 2022. Combined with rising consumption per person, this increases demand for land, food, water, timber and energy.

The specific causes:

• Hunting and other over-exploitation: killing or harvesting faster than populations can reproduce, as with the moa and the Caribbean monk seal, and today with over-fishing, the bushmeat trade and poaching (pangolins, rhinos).
• Urbanization: towns, cities and transport networks replace natural habitats and split the rest into fragments.
• Deforestation and clearance of land for agriculture: the largest single cause of habitat loss on land, as in the mixed dipterocarp forests.
• Pollution: fertilizer run-off causing eutrophication, pesticides, industrial chemicals and plastics, on land and in the sea.
• Pests, diseases and invasive alien species spread by global transport: introduced predators (rats, cats, stoats) and competitors, and diseases such as the chytrid fungus that has caused declines and extinctions of amphibians worldwide.

Climate change acts on top of these, shifting the conditions species are adapted to (see D4.3 Climate change). IPBES ranks the direct drivers, from largest global impact: changes in land and sea use, direct exploitation, climate change, pollution and invasive alien species.

Conservation needs a mix of methods. What works depends on the species’ biology and on the threats it faces, so relying on one method alone will fail many species.

In situ conservation

In situ conservation protects species in their natural habitat. They stay adapted to their environment and keep interacting with other species, so whole ecosystems, food webs and natural selection are conserved.

• Nature reserves and national parks protect habitat, but usually need active management: removing invasive species (for example, clearing rats and stoats from New Zealand islands to protect birds such as the kākāpō), controlling hunting and poaching, managing human access, and controlling the populations of some species.
• Rewilding restores degraded ecosystems to as natural a state as possible, often by reintroducing species that had been lost, such as grey wolves to Yellowstone National Park in 1995. Afterwards the area is managed as little as possible, so that natural processes keep the ecosystem going.
• Reclamation of degraded land includes replanting forest, re-flooding drained wetlands and restoring mined sites.

Ex situ conservation

Ex situ conservation keeps species outside their natural habitat, usually when wild populations are too small or threats cannot quickly be removed.

• Zoos run captive breeding programmes, ideally with releases back to the wild. In 1987 the last 27 California condors were taken into captivity; captive-bred birds have been released since 1992. The Arabian oryx, extinct in the wild by 1972, was reintroduced from captive herds.
• Botanic gardens grow and propagate rare plants and keep collections of many species.
• Seed banks store seeds that have been dried and frozen (at about −20 °C) so they stay viable for decades; tissue banks store cells, eggs and sperm in liquid nitrogen (about −196 °C). This stored living material is called germ plasm.

Why several approaches are needed

There is not enough money or time to save every threatened species, so conservationists must decide priorities. The EDGE of Existence programme, launched by the Zoological Society of London in 2007, focuses on species that are both:

• Evolutionarily Distinct (ED): they have few or no close living relatives, so they sit at the end of a long, separate branch of the evolutionary tree. ED is measured in millions of years: the length of the species’ own branch, plus a share of each deeper branch that it shares with its relatives (shared more ways when more species descend from that branch).
• Globally Endangered (GE): they are in a threatened IUCN Red List category (Vulnerable, Endangered or Critically Endangered).

The rationale

Losing an EDGE species loses a whole branch of the tree of life: unique genes, anatomy, physiology and behaviour that evolved over tens or hundreds of millions of years and have no near equivalent. A species with many close relatives also matters, but much of its evolutionary history is shared with surviving species. The programme publishes ranked lists for groups such as mammals, birds, amphibians, reptiles, sharks and rays, and corals, and supports local conservationists working on EDGE species.

Who decides?

Picking which species get help is not a purely scientific choice: it raises ethical, environmental, political, social, cultural and economic questions, and people with different values will disagree, so the choice needs open discussion. Other ways to set priorities include:

• keystone species, whose loss would change the whole ecosystem;
• flagship species, charismatic animals that attract funding and protect habitat for many other species;
• species valued by local and Indigenous communities, or of economic importance;
• cost-effectiveness: spending where success is most likely, rather than on species that may already be too rare to save.

EDGE lists are one evidence-based contribution to this debate, not the only answer.