Earth has witnessed 5 major mass extinctions in its 4.5-billion-year history — events where a large proportion of species were lost in a geologically short time. The Big Five: (1) Ordovician-Silurian extinction (443 mya): ~86% of species lost. Ice age + sea level fall. (2) Late Devonian extinction (375 mya): ~75% of species. Gradual, multiple pulses, possibly asteroid + volcanism. (3) Permian-Triassic extinction (252 mya): 'The Great Dying' — ~96% of marine species and ~70% of terrestrial lost. Siberian volcanism. Most severe. (4) Triassic-Jurassic extinction (201 mya): ~80% of species. Central Atlantic Magmatic Province volcanism. (5) Cretaceous-Paleogene extinction (66 mya): ~76% of species including non-avian dinosaurs. Chicxulub asteroid impact + Deccan Traps volcanism.

Currently underway: Holocene extinction (last ~10,000 years) or Anthropocene extinction (last ~200 years). Key difference from previous 5: entirely caused by human activities. Rate: background extinction rate = 1-5 species per million species per year. Current rate: estimated 100-1000× higher. Some scientists estimate current rates equivalent to 100-10,000 species lost per million per year. Timeline: accelerating since Industrial Revolution (~1800). Exponential increase with human population growth, urbanisation, globalisation. Scale: affecting all taxonomic groups — amphibians most threatened (1/3 of species), birds, mammals, reptiles, insects, plants. Note: most extinctions are poorly documented because many species are unknown to science when they go extinct (especially invertebrates, microbes, plants).

Multiple interacting human-caused drivers: Habitat loss and fragmentation: deforestation (especially tropics), wetland drainage, coral reef destruction, grassland conversion to agriculture. Most important single cause — affects most species. Over-exploitation: hunting, poaching, overfishing beyond sustainable yields. Steller's sea cow, passenger pigeon, dodo — all extinct from hunting. Marine fisheries collapse. Invasive alien species: intentional and accidental introductions — compete with, predate on, or transmit disease to native species. Lake Victoria cichlid extinction from Nile perch. Pollution: pesticides (DDT causing eggshell thinning in raptors), plastic pollution (marine debris), heavy metals, light and noise pollution affecting wildlife. Climate change: habitat shifts, phenological mismatches (species moving poleward or to higher altitude), coral bleaching, sea level rise, ocean acidification. Disease: Chytrid fungus (Batrachochytrium dendrobatidis) causing amphibian mass die-offs globally.

Biodiversity has value at three levels: Genetic diversity: variety of alleles within species → allows adaptation to environmental change, supports crop improvement (wild relatives of crops as gene banks). Species diversity: number and variety of species → provides ecosystem stability (species richness correlates with ecosystem productivity, resilience). Ecosystem diversity: variety of habitats → provides ecosystem services. Values of biodiversity: Utilitarian values: food, medicine (25% of drugs from plants — aspirin, quinine, taxol, penicillin), fibres, timber, industrial products, ecosystem services (pollination, water purification, climate regulation, carbon storage, soil formation, nutrient cycling). Ethical value (intrinsic): every species has right to exist independent of human use. Aesthetic/spiritual value: recreation, ecotourism, cultural value. 'Rivet-popper hypothesis' (Paul Ehrlich): each species lost is like a rivet popped from an airplane — too many lost = catastrophic failure.

Keystone species: species with disproportionately large effect on ecosystem relative to their abundance. When removed, entire ecosystem changes dramatically. Classic examples: Sea otters (Pacific coast): eat sea urchins → without otters → urchin population explodes → destroys kelp forests → loses habitat for many species. Wolves in Yellowstone: reintroduced 1995 → prey on elk → elk avoid rivers → riverside vegetation recovered → beavers returned → more ponds → fish returned → entire ecosystem transformed (trophic cascade). Bees (pollinators): pollinate 75% of crop species → if lost → food production collapses. African elephants: open forests through feeding and movement → create habitat for smaller species. Loss of keystone species → trophic cascade (top-down effects through food web) → ecosystem collapse. Identifying and protecting keystone species is critical conservation priority.

In-situ conservation (protecting in natural habitat): National parks, Wildlife sanctuaries, Biosphere reserves, Sacred groves, Marine protected areas. Most cost-effective — protects entire ecosystem + all species within. India: 104 national parks, 565 wildlife sanctuaries, 18 biosphere reserves, 26 Ramsar wetland sites. Ex-situ conservation (protecting outside natural habitat): Zoos, Botanical gardens, Aquaria, Seed banks (Svalbard Global Seed Vault), Cryopreservation (sperm, eggs, embryos), Tissue culture banks, DNA banks. Necessary when in-situ is insufficient. Integrated approach: IUCN Red List categorisation. CITES (Convention on International Trade in Endangered Species). CBD (Convention on Biological Diversity) — 1992. Nagoya Protocol on Access and Benefit Sharing. Kunming-Montreal framework (2022): 30×30 target (protect 30% of land and sea by 2030).

Alexander von Humboldt observed that species richness increases with area — more area = more species. Mathematical relationship: S = CA^Z or log S = log C + Z log A. S = species richness, A = area, Z = slope (regression coefficient), C = constant. Z values: For smaller areas (within a continent/region): Z = 0.1-0.2. For larger areas (between continents): Z = 0.6-1.2. Implication: if forest area reduced to 10% of original: species remaining = 10^0.3 = 50% (for Z=0.3). Half of all species lost when 90% of habitat destroyed! This explains why deforestation causes such rapid biodiversity loss. The species-area relationship is the scientific basis for predicting extinction rates from habitat loss. Also explains why islands have fewer species than mainlands (island biogeography — MacArthur and Wilson model).

Amphibians (frogs, toads, salamanders, caecilians) are the most threatened vertebrate class: ~32% of species threatened with extinction. ~120-200 amphibian species believed extinct since 1980. Why so vulnerable: Permeable moist skin → absorbs toxins, pesticides, UV directly. Biphasic life cycle → need both aquatic and terrestrial habitat → more exposure to disturbance. Temperature sensitive ectotherms → climate change impacts directly. Chytrid fungus (Batrachochytrium dendrobatidis, Bd): emerging infectious disease, first described 1998. Causes chytridiomycosis — disrupts skin function → electrolyte imbalance → cardiac arrest. Spread globally through amphibian trade. Has caused or contributed to extinction of ~90 frog species. Also B. salamandrivorans (Bsal) affecting European salamanders. Amphibian conservation: captive breeding programmes (Amphibian Ark), habitat protection, disease screening and treatment, anti-fungal treatment.