Loss of Biodiversity & the Evil Quartet

NCERT grounding

This subtopic is built directly on NCERT Class 12 Biology, Chapter 13, Section 13.1.4 — Loss of Biodiversity. The text opens with a stark observation: while it is doubtful whether any new species are being added to Earth's treasury through speciation, there is no doubt about their continuing losses. The biological wealth of the planet has been declining rapidly, and the accusing finger is clearly pointing to human activities. NCERT then quantifies this decline with the IUCN Red List figures, frames the present crisis as a "Sixth Extinction", and names the four major drivers collectively as the Evil Quartet.

"The accelerated rates of species extinctions that the world is facing now are largely due to human activities. There are four major causes — 'The Evil Quartet' is the sobriquet used to describe them."

Every fact on this page — the IUCN tally of 784 species, the named recent extinctions, the 100-to-1,000-times extinction rate, and the four-part Evil Quartet — is lifted from this NCERT section. NEET draws this content verbatim, so the priority is exact memory rather than interpretation.

The extinction crisis & the Sixth Extinction

It has taken millions of years of evolution to accumulate the rich diversity nature holds today, yet that wealth could be lost in less than two centuries if present rates of species loss continue. The evidence of accelerating loss is documented in hard numbers.

The colonisation of tropical Pacific Islands by humans is said to have led to the extinction of more than 2,000 species of native birds. On a global scale, the IUCN Red List (2004) documents the extinction of 784 species in the last 500 years. That total breaks down into 338 vertebrates, 359 invertebrates and 87 plants. The last twenty years alone have witnessed the disappearance of 27 species. Extinctions across taxa are not random — careful analysis of records shows that some groups, such as amphibians, appear to be more vulnerable than others.

Named recent extinctions

NCERT lists a small set of recent extinctions by name, and these are the most frequently quoted examples in NEET match-the-column questions. Each pairs a vanished organism with a place, so the geography is part of the fact.

Beyond the species already gone, the grim scenario is deepened by the fact that more than 15,500 species worldwide currently face the threat of extinction. Presently, 12 per cent of all bird species, 23 per cent of all mammal species, 32 per cent of all amphibian species and 31 per cent of all gymnosperm species in the world face that threat. The high amphibian figure echoes the earlier point about that group's particular vulnerability.

Why it is called the Sixth Extinction

Earth's fossil record shows that large-scale loss of species is not new. During the long period of more than 3 billion years since the origin and diversification of life, there were five episodes of mass extinction. The crisis now in progress is termed the "Sixth Extinction" — and NEET repeatedly tests how it differs from the earlier five.

The key distinction NCERT draws is one of rate and cause. The difference from the earlier five episodes lies in the rates — current species extinction rates are estimated to be 100 to 1,000 times faster than in pre-human times — and human activities are responsible for those faster rates. Ecologists warn that if present trends continue, nearly half of all the species on Earth might be wiped out within the next 100 years.

The Evil Quartet — four causes of loss

The accelerated extinction rates the world now faces are largely due to human activities. NCERT groups the major causes into four, and "The Evil Quartet" is the sobriquet used to describe them. The four are habitat loss and fragmentation, over-exploitation, alien species invasions, and co-extinctions. A NEET pitfall here is to slip a non-member — mutation or migration, for example — into this set, so the membership must be memorised exactly as a closed list of four.

Cause 1 — Habitat loss and fragmentation

This is the most important cause driving animals and plants to extinction — a single fact NEET has tested directly in 2016, 2019 and 2023. The most dramatic examples of habitat loss come from tropical rain forests. The Amazon rain forest, so vast that it is called the "lungs of the planet" and harbouring probably millions of species, is being cut and cleared for cultivating soya beans or for conversion to grasslands for raising beef cattle.

Habitat damage has two faces. Besides total loss, the degradation of many habitats by pollution also threatens the survival of many species. Then there is fragmentation: when large habitats are broken up into small fragments due to various human activities, mammals and birds that require large territories — and certain animals with migratory habits — are badly affected, leading to population declines. Fragmentation is therefore not merely "less habitat"; it is habitat broken into pieces too small to support wide-ranging species.

Cause 2 — Over-exploitation

Humans have always depended on nature for food and shelter, but when "need" turns to "greed", it leads to over-exploitation of natural resources. NCERT phrases this driver precisely as that shift from need to greed, and the wording is worth remembering.

Many species extinctions in the last 500 years were due to over-exploitation by humans — NCERT cites Steller's sea cow and the passenger pigeon as examples. The threat is not historical only: presently, many marine fish populations around the world are over-harvested, endangering the continued existence of some commercially important species. Steller's sea cow appears in this section as well as in the extinction list, which is consistent — it was lost specifically because of over-exploitation.

Cause 3 — Alien species invasions

When alien species are introduced — unintentionally or deliberately, for whatever purpose — some of them turn invasive and cause the decline or extinction of indigenous species. NCERT supplies a cluster of named examples that NEET draws on heavily.

The classic case is the Nile perch introduced into Lake Victoria in East Africa, which led eventually to the extinction of an ecologically unique assemblage of more than 200 species of cichlid fish in the lake. On land, the invasive weed species — carrot grass (Parthenium), Lantana and water hyacinth (Eichhornia) — cause environmental damage and pose a threat to native species. More recently, the illegal introduction of the African catfish Clarias gariepinus for aquaculture purposes is posing a threat to the indigenous catfishes in our rivers.

Cause 4 — Co-extinctions

When a species becomes extinct, the plant and animal species associated with it in an obligatory way also become extinct. The word "obligatory" is load-bearing — the linked species cannot survive without the lost one, so its extinction is forced rather than incidental.

NCERT gives two examples. When a host fish species becomes extinct, its unique assemblage of obligate parasites meets the same fate, because those parasites depend entirely on that host. The second example is a coevolved plant–pollinator mutualism: where two species have evolved together in a tight mutual dependence, the extinction of one invariably leads to the extinction of the other. Co-extinction is therefore a chain effect — one loss pulling its obligate partners down with it.

What loss of biodiversity does to a region

The Evil Quartet explains why species are lost; NCERT also states what that loss does to a region. In general, loss of biodiversity in a region may lead to three measurable consequences. First, a decline in plant production. Second, a lowered resistance to environmental perturbations such as drought. Third, an increased variability in certain ecosystem processes — for example, plant productivity, water use, and pest and disease cycles.

These outcomes connect loss of biodiversity to the wider question of why species diversity matters to ecosystem stability. They are a high-yield list because NEET can test them as a three-part recall just as it tests the four-part Evil Quartet.

Worked examples

Common confusion & NEET traps

Most errors on this subtopic come from mixing up the four members of the Evil Quartet, or from misreading the headline figures. The traps below isolate the recurring confusions.