What is the Evil Quartet in biology?

The Evil Quartet is NCERT's nickname for the four major causes of biodiversity loss. They are habitat loss and fragmentation, over-exploitation, alien species invasions and co-extinctions. All four are linked to human activity. A handy way to remember them is the letters H-O-A-C, in the same order as the textbook.

What is co-extinction? Give an example.

Co-extinction is the loss of a species because another species it depends on has died out. NCERT gives two examples. When a host fish species becomes extinct, the parasites found only on it disappear too. In a plant and its partner pollinator that evolved together, the extinction of one leads to the extinction of the other.

How many species are threatened with extinction?

The IPBES Global Assessment, released in May 2019, found that around 1 million animal and plant species are now threatened with extinction, many within decades. NCERT's older textbook figure is more than 15,500 threatened species worldwide, with over 650 of them from India. Quote the source and year with either number.

Loss of Biodiversity: Causes, Effects and Solutions

Q: What is the most important cause of biodiversity loss today?

NCERT's Class 12 Biology book names habitat loss and fragmentation as the most important cause. In practice, forests, wetlands and grasslands are cleared for farms, towns, roads and mines, and what remains is often cut into small patches. The IPBES global assessment of 2019 agrees, ranking changes in land and sea use as the top direct driver.

Q: What are the effects of loss of biodiversity?

NCERT lists three effects in a region: a fall in plant production, lower resistance to disturbances such as drought, and more variation in processes like water use and pest cycles. Losing species also weakens services people rely on, such as crop pollination, clean water and fertile soil, and it can trigger further extinctions.

Loss of biodiversity means the decline or disappearance of living variety on Earth: fewer genes, fewer species and damaged ecosystems. Today it is driven mainly by people. NCERT groups its causes as the “Evil Quartet”, led by habitat loss and fragmentation. The effects reach ecosystems and the people who depend on them.

This article explains the causes, effects and concerns in an exam-ready order. For the bigger picture, start with our main guide to biodiversity, its levels and threats.

What is loss of biodiversity?

Biodiversity is the variety of life at three levels: genes, species and ecosystems. Loss can happen at every level. A crop may lose its old varieties, a frog may vanish from a stream, or a forest may turn into farmland. Our guide to the three types of biodiversity: genetic, species and ecosystem explains each level with examples.

Extinction is the final stage of loss: the last member of a species dies. Extinction is natural. Fossils record five mass extinctions, episodes when species were lost on a very large scale. What worries scientists now is the speed. NCERT’s Class 12 Biology book (2026-27 reprint; its figures date from earlier editions) estimates current extinction rates at 100 to 1,000 times faster than in pre-human times. It places the blame on human activities. This present wave is often called the Sixth Extinction.

What does the NCERT textbook record?

NCERT’s chapter “Biodiversity and Conservation” gives a set of older figures that exams still use. Learn them as NCERT’s numbers:

The IUCN Red List (2004) documents 784 extinctions in the last 500 years: 338 vertebrates, 359 invertebrates and 87 plants.
Human settlement of tropical Pacific Islands is said to have caused the extinction of more than 2,000 native bird species.
Recent losses include the dodo, quagga, thylacine, Steller’s sea cow and three subspecies of tiger (Bali, Javan, Caspian).
In “the last twenty years” alone, 27 species disappeared.
More than 15,500 species worldwide faced the threat of extinction.

The same page notes that extinctions are not random across groups. Some groups, such as amphibians, appear more vulnerable. NCERT also reports ecologists’ warning that nearly half of all species might be wiped out within 100 years if present trends continue.

How fast is biodiversity being lost today?

The most widely quoted modern figures come from the IPBES Global Assessment, released in May 2019. IPBES is the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services. Its media release reports these findings:

Around 1 million animal and plant species are now threatened with extinction, many within decades.
The average abundance of native species in most major land habitats has fallen by at least 20%, mostly since 1900.
More than 40% of amphibian species, almost 33% of reef-forming corals and over a third of marine mammals are threatened.
Human actions have significantly altered three-quarters of the land-based environment and about 66% of the marine environment.
Global extinction rates are tens to hundreds of times higher than the average over the last 10 million years, and still rising.

Notice how the amphibian figure differs between sources. NCERT’s book puts threatened amphibians at 32 per cent. IPBES (2019) says more than 40%. Always name the source and year with a number.

What are the causes of loss of biodiversity?

NCERT states that today’s faster extinctions are largely due to human activities. It groups the causes under one memorable label, the Evil Quartet. Each cause is explained below with the textbook’s own examples.

Cause	What happens	NCERT example
Habitat loss and fragmentation	Natural homes are destroyed, degraded or broken into small pieces	Amazon rainforest cleared for soya beans and cattle pasture
Over-exploitation	People harvest a species faster than it can recover	Steller’s sea cow and passenger pigeon
Alien species invasions	Introduced species spread and harm native ones	Nile perch in Lake Victoria; Parthenium, Lantana, water hyacinth
Co-extinctions	A species dies out with the partner it depends on	Parasites of an extinct host fish; plant and pollinator pairs

1. Habitat loss and fragmentation

A habitat is the natural home where a species finds food, shelter and mates. When the habitat goes, the species goes with it. Tropical rainforests give NCERT its most dramatic example. They once covered more than 14 per cent of Earth’s land surface, but now cover no more than 6 per cent. The Amazon, called the “lungs of the planet”, is being cleared to grow soya beans or to make grassland for beef cattle.

Habitats can also be lost without being cleared. NCERT notes that pollution degrades many habitats and threatens the species living there.

Fragmentation is the breaking of one large habitat into small, separate patches. Roads, canals, farms and towns are typical causes. NCERT explains that mammals and birds needing large territories suffer most, along with migratory animals. Their populations decline. Forest clearing is a major source of this damage; see our article on the effects of deforestation on the environment.

2. Over-exploitation

Humans have always taken food and shelter from nature. NCERT puts the problem neatly: trouble begins when “need” turns into “greed”. Over-exploitation means hunting, fishing or collecting a species faster than it can breed.

NCERT blames over-exploitation for many extinctions of the last 500 years, including Steller’s sea cow and the passenger pigeon. It also warns that many marine fish populations are over-harvested today. Some commercially important fish species are now at risk.

3. Alien species invasions

An alien species is one brought into a region where it did not live naturally. Some introductions are deliberate, others accidental. Not every alien species causes harm. Those that spread fast and push out native species are called invasive.

The textbook’s most striking case is the Nile perch. After it was introduced into Lake Victoria in East Africa, more than 200 species of cichlid fish in the lake eventually became extinct. In India, NCERT points to invasive weeds: carrot grass (Parthenium), Lantana and water hyacinth (Eichhornia). It also warns about the illegal introduction of the African catfish (Clarias gariepinus) for aquaculture. This fish threatens the native catfishes of Indian rivers.

4. Co-extinctions

Some species are bound to one partner so tightly that they cannot live without it. A co-extinction occurs when such a dependent species dies out because its partner has gone. NCERT gives two examples:

When a host fish species becomes extinct, its unique set of parasites disappears too.
In a plant and a pollinator that evolved together, the extinction of one always leads to the extinction of the other.

Co-extinctions show that one loss can trigger others. The damage spreads quietly through a food web.

What is the most important cause of biodiversity loss today?

The most important cause of biodiversity loss today is habitat loss and fragmentation. NCERT’s Class 12 Biology book calls it the most important cause driving animals and plants to extinction.

A modern global review points the same way. The IPBES report of 2019 ranked five direct drivers of change in nature by their global impact so far, in descending order:

Changes in land and sea use
Direct exploitation of organisms
Climate change
Pollution
Invasive alien species

The first two closely match the first two members of the Evil Quartet. IPBES also lists climate change and pollution as separate drivers. In NCERT’s account, pollution appears under habitat degradation.

What are the effects of loss of biodiversity?

Losing species is not only sad. It changes how ecosystems work. NCERT lists three general effects of biodiversity loss in a region:

Decline in plant production. The plants of the area produce less.
Lower resistance to disturbances. The ecosystem copes less well with stresses such as drought.
More variability in ecosystem processes. Plant productivity, water use, and pest and disease cycles become less predictable.

Effects on people

People depend on many free services from nature. Pollination is a clear example. According to IPBES (2019), more than 75% of global food crop types rely on animal pollination. The same release puts the yearly value of crop output at risk from pollinator loss at US$235 to US$577 billion.

Biodiversity also supplies medicines, fibre, timber and fuel. When species disappear, these options disappear with them. Our article on why biodiversity matters for people and nature sets out the full list of benefits.

Chain reactions in food webs

One loss can lead to another, as co-extinctions show. Predators may run short of prey. Plants may lose the animals that spread their seeds. Pests may multiply when the species that ate them vanish. Each loss makes the ecosystem a little weaker.

Why is loss of biodiversity a concern for India?

India is rich in species, so it also has much to lose. NCERT’s summary notes that more than 650 of the world’s threatened species are from India. Its examples of invasive weeds and the African catfish describe threats to India’s own native species.

India’s vultures show how fast a group can crash. NCERT’s earlier Class 8 book, Resources and Development, records that vultures were dying of kidney failure. They had fed on dead livestock treated with diclofenac, a painkiller. According to a December 2024 Rajya Sabha reply (PIB), the use, sale and manufacture of veterinary diclofenac were banned by India’s Drugs Controller General in August 2006. Vultures are scavengers, and the same Class 8 book describes them as a vital cleanser of the environment.

NCERT notes that hotspots are also regions of accelerated habitat loss. Our guide to biodiversity hotspots in India describes the pressures on each one.

How can loss of biodiversity be prevented?

Stopping the loss means tackling each cause directly. The main approaches are:

Protect habitats through national parks, wildlife sanctuaries and biosphere reserves, and keep forest patches connected.
Harvest sustainably, by limiting fishing, hunting and collection to levels a species can recover from.
Control invasive species, and check plants and animals before bringing them into a new region.
Cut pollution and greenhouse gas emissions, which add stress to already weakened ecosystems.
Back up threatened species in zoos, botanical gardens and seed banks.

There are two broad strategies: protecting species where they live, and protecting them away from home. Compare them in our guide to in situ and ex situ conservation of biodiversity. For laws, community action and other methods, read biodiversity conservation: methods and examples.

Key points

Loss of biodiversity is the decline in the variety of genes, species and ecosystems.
NCERT says current extinction rates are 100 to 1,000 times faster than in pre-human times.
The Evil Quartet: habitat loss and fragmentation, over-exploitation, alien species invasions, co-extinctions.
Habitat loss and fragmentation is the most important cause; IPBES (2019) also ranks land and sea use change first.
NCERT examples: Amazon clearing, Steller’s sea cow, passenger pigeon, Nile perch, Parthenium, Lantana, water hyacinth, African catfish.
IPBES (2019): around 1 million species are threatened with extinction.
Effects include lower plant production, weaker resistance to drought and unstable ecosystem processes.
Solutions: protect habitats, harvest sustainably, control invasives and use ex situ back-ups.