Biogeography

In short

Biogeography is the study of the distribution of plants, animals and ecosystems over space and time, and of the processes — dispersal, evolution, barriers, climate and tectonics — that produce that distribution. It joins physical geography to ecology, so it supplies both the global pattern (realms, biomes) and the local measurement (quadrats, diversity indices, succession). In Indian universities it carries full questions in Physical Geography and Environmental Geography papers, and it is heavily examined in UPSC and State PSC (Geography and Environment), UGC-NET Geography, IFS and Forest Ranger exams. On the job it is the knowledge base for EIA and biodiversity-impact studies, forest working plans, wildlife management plans, People's Biodiversity Registers and GIS-based habitat mapping.

Explained

What biogeography studies, and its branches

• Definition: the science of where organisms live, why they live there, and how those ranges change — descriptive, analytical and historical.
• Phytogeography (plant distribution) and zoogeography (animal distribution) are the two classical halves; ecological biogeography explains present ranges by current conditions, historical biogeography by past events (tectonics, glaciations, evolution).
• Island biogeography and conservation biogeography are the applied modern branches; palaeobiogeography uses fossils and pollen.
• Range: the total area a species occupies. Cosmopolitan = near-worldwide (house sparrow); endemic = confined to one area (Nilgiri tahr, lion-tailed macaque, Asiatic lion in Gir); relict/disjunct = isolated survivors of a formerly wider range.
• Centre of origin: the region where a taxon arose and from which it spread (Vavilov's centres for crop plants — India is one, the Hindustani centre).
• Niche: the role plus the range of conditions a species uses. Fundamental niche = where it could live; realised niche = where competition and predation actually allow it. Gause's competitive exclusion principle: two species with identical niches cannot coexist indefinitely.
• Tolerance limits: Shelford's law of tolerance (range of each factor) and Liebig's law of the minimum (the scarcest essential factor limits growth).

Dispersal, barriers and the historical pattern

• Dispersal types: active (flight, walking, swimming) and passive — anemochory (wind), hydrochory (water), zoochory (animals), anthropochory (humans).
• Dispersal routes (Simpson): corridor (easy passage, e.g. the Indo-Gangetic plain), filter bridge (lets some taxa through, e.g. the Isthmus of Panama, the arid Thar belt), sweepstake route (chance crossing of a wide barrier, as to oceanic islands).
• Barriers: physical (ocean, mountain, desert), climatic (temperature, rainfall), edaphic (soil, salinity) and biological (predator, competitor, absence of pollinator).
• Vicariance vs dispersal: vicariance explains a split range by a barrier arising across a continuous one (Gondwana breaking up); dispersal explains it by crossing an existing barrier. Both operate; modern cladistic biogeography tests which fits.
• Continental drift and plate tectonics underlie many patterns: the Gondwanan ancestry of India's flora (Dipterocarps, cycads), the Austro-Asiatic exchange after India docked with Asia, and the Himalaya's rise creating a new barrier and new habitats.
• Wallace's six zoogeographic realms (1876): Palaearctic, Nearctic, Neotropical, Ethiopian (Afrotropical), Oriental and Australian; Udvardy's later scheme adds Antarctic and Oceanian. India lies mainly in the Oriental realm, with the Trans-Himalaya and Ladakh in the Palaearctic — making India a transition zone.
• Wallace's Line between Bali and Lombok separates Oriental from Australian fauna; the transitional zone between Wallace's Line and Lydekker's Line (its eastern edge) is Wallacea, with Weber's line marking the point of faunal balance inside it.
• Latitudinal diversity gradient: species richness generally rises from poles to equator — explained by greater energy and productivity, climatic stability, longer evolutionary time and smaller niche width in the tropics.

Biomes — the major terrestrial units

• Aquatic/transitional systems examined with biomes: mangroves (halophytes with pneumatophores and vivipary — Sundarbans, Bhitarkanika), coral reefs, wetlands, estuaries.
• Altitudinal zonation compresses the latitudinal sequence; the treeline in the Himalaya lies roughly at 3,500-4,000 m, higher on drier inner ranges.

Ecosystem structure, succession and energy flow

• Structure: abiotic components (energy, climate, soil, nutrients) and biotic components — producers, consumers (herbivores, carnivores, omnivores), decomposers (saprotrophs).
• Trophic levels and food webs: grazing food chain starts with living plants; detritus food chain starts with dead organic matter and dominates forests and mangroves.
• Lindeman's 10% law: roughly 10% of energy transfers between successive trophic levels, limiting chains to 4-5 links. Ecological pyramids of number, biomass and energy; the energy pyramid is always upright, biomass pyramids can be inverted in the sea.
• Productivity: GPP = total energy fixed; NPP = GPP − respiration; measured in g/m²/yr or kJ/m²/yr. Highest in tropical rainforests, coral reefs and estuaries; lowest in deserts and open ocean.
• Biogeochemical cycles: gaseous (carbon, nitrogen, oxygen) and sedimentary (phosphorus, sulphur); nitrogen fixation, nitrification, denitrification; phosphorus has no gaseous phase, hence it limits many systems.
• Ecological succession: orderly change in community at a site. Primary (bare rock, new island, cooled lava) through xerarch (lithosere, psammosere) or hydrarch (hydrosere) series; secondary succession after fire, shifting cultivation or clear-felling — faster, as soil and seed bank survive.
• Stages: nudation → invasion (migration, ecesis, aggregation) → competition and coaggregation → reaction (pioneers modify the habitat) → stabilisation (climax). Clements' monoclimax, Tansley's polyclimax and Whittaker's climax-pattern are the three theories.
• Ecotone: transition belt between two communities; the edge effect gives it higher species richness and density, but edges also expose interiors to invasion — important in fragmented Indian forests.
• Keystone species (tiger, elephant, fig trees) have disproportionate influence; flagship species mobilise support; umbrella species protect wide habitat; indicator species signal condition (lichens for air quality).

Island biogeography and the measurement of diversity

• MacArthur and Wilson (1967) equilibrium theory: the number of species on an island settles where immigration rate = extinction rate.
  • Immigration falls as the island fills and with distance from the mainland (distance effect).
  • Extinction rises with species number and falls with area (area effect, since large islands hold larger, safer populations).
  • Therefore: large-near islands hold most species, small-far islands fewest; species composition keeps turning over even when the number is constant.
• Species-area relationship: S = cA^z, with z usually 0.15-0.35 for mainland samples and higher (0.25-0.45) for true islands — the basis for estimating extinctions from habitat loss.
• Relaxation / faunal collapse: a newly isolated fragment loses species over decades until it reaches its smaller equilibrium — the theoretical case for large, connected protected areas and corridors, and the origin of the SLOSS (single large or several small) debate.
• Three levels of biodiversity: genetic (within species), species (richness and evenness) and ecosystem/community diversity.
• Alpha, beta, gamma (Whittaker): alpha = within-habitat diversity, beta = change in species between habitats (turnover), gamma = total landscape diversity.
• Field measurement: quadrats (herbs 1×1 m, shrubs 5×5 m, trees 10×10 m), line and belt transects, point-centred quarter method; density, frequency, abundance, basal area and Importance Value Index for vegetation; mark-recapture, camera-trap capture-recapture, line transects with distance sampling, pugmark and dung counts for animals.
• Indices: Shannon-Wiener (richness + evenness), Simpson (dominance), Margalef (richness), Pielou's evenness, Jaccard and Sørensen (similarity).

India's biogeography — zones, forest types, richness

• Rodgers and Panwar (1988) ten biogeographic zones: 1 Trans-Himalaya, 2 Himalaya, 3 Desert, 4 Semi-arid, 5 Western Ghats, 6 Deccan Peninsula, 7 Gangetic Plain, 8 Coasts, 9 North-East, 10 Islands — the official framework for protected-area planning.
• Champion and Seth (1968) forest classification: 16 type groups in 5 major groups — tropical moist (wet evergreen, semi-evergreen, moist deciduous, littoral and swamp), tropical dry (dry deciduous, thorn, dry evergreen), montane subtropical, montane temperate (including sub-alpine), and alpine (moist and dry alpine scrub).
• Phytogeographic regions (Hooker/Chatterjee tradition — nine): Western Himalaya, Eastern Himalaya, Indus plain, Gangetic plain, Central India, Deccan, Malabar (Western Ghats), Assam, and the Andaman-Nicobar Islands.
• Richness: India is one of the 17 megadiverse countries, with about 2.4% of world land area but roughly 7-8% of recorded species — some 55,000+ plant and over 1,00,000 animal species described (figures rise with each ZSI/BSI annual update).
• Four global biodiversity hotspots touch India: the Himalaya, Indo-Burma, the Western Ghats-Sri Lanka and Sundaland (Nicobar Islands). A hotspot needs ≥1,500 endemic vascular plants and ≥70% loss of primary vegetation.
• ISFR 2023 (Forest Survey of India, released December 2024): forest cover 7,15,343 km² (21.76% of geographical area); tree cover 1,12,014 km² (3.41%); forest and tree cover together 8,27,357 km² = 25.17%. Mangrove cover 4,992 km². The National Forest Policy, 1988 target is 33%.
• Flagship population estimates: tigers 3,682 (All-India Tiger Estimation 2022), Asiatic lion 891 (Gujarat census 2025), greater one-horned rhino around 2,600 in Kaziranga, Asian elephant roughly 27,000-30,000 (a fresh national estimate has been under release, so quote the year with the figure).
• Invasive alien species are a major driver of change: *Lantana camara*, *Prosopis juliflora*, *Eichhornia crassipes* (water hyacinth), *Mikania micrantha*, *Parthenium hysterophorus*, African catfish; they suppress native regeneration in dry deciduous forests and wetlands.

Conservation — Indian law, institutions and global agreements

• In-situ: national parks, wildlife sanctuaries, conservation reserves, community reserves, tiger reserves, biosphere reserves, sacred groves, Ramsar wetlands. Ex-situ: zoos, botanical gardens, gene and seed banks (NBPGR, NBAGR), cryopreservation, tissue culture.
• Protected Area network: about 106 national parks, 570+ wildlife sanctuaries, and over 100 conservation and 200 community reserves, together roughly 5-6% of India's area; counts change with every new notification, so verify on the ENVIS/WII database.
• Wild Life (Protection) Act, 1972 — as amended in 2022, the schedules were reorganised: Schedules I and II for animals (I = highest protection), Schedule III for plants and Schedule IV for CITES-listed scheduled specimens. It creates NPs, sanctuaries, the National Board for Wildlife, the NTCA and the Wildlife Crime Control Bureau.
• Biological Diversity Act, 2002, amended by the Biological Diversity (Amendment) Act, 2023: three-tier structure — National Biodiversity Authority (Chennai), State Biodiversity Boards and Biodiversity Management Committees at local-body level, which maintain People's Biodiversity Registers; it governs access and benefit sharing and recognises Biodiversity Heritage Sites.
• Forest (Conservation) Act, 1980, renamed by the 2023 amendment as the Van (Sanrakshan Evam Samvardhan) Adhiniyam, requires central approval to divert forest land; compensatory afforestation funds are handled under the CAMPA Act, 2016.
• Forest Rights Act, 2006 recognises individual and community forest rights, including Community Forest Resource rights, and conditions relocation from critical wildlife habitats on consent.
• Environment (Protection) Act, 1986 supports Eco-Sensitive Zones and the EIA Notification, 2006; the Wetlands (Conservation and Management) Rules, 2017 and CRZ Notification, 2019 protect wetlands and coasts.
• Programmes: Project Tiger (1973), Project Elephant (1992), Project Snow Leopard (2009), Project Dolphin (2020), Project Lion, Integrated Development of Wildlife Habitats, National Mission for a Green India, Nagar Van Yojana.
• International: CBD (1992; India party 1994), Cartagena and Nagoya Protocol on ABS, the Kunming-Montreal Global Biodiversity Framework (2022) with the 30×30 target, CITES (India party since 1976), CMS, Ramsar Convention (India has about 90 Ramsar sites — the most in Asia, with the count still rising), World Heritage natural sites (Kaziranga, Manas, Keoladeo, Sundarbans, Nanda Devi-Valley of Flowers, Western Ghats, Great Himalayan NP, Khangchendzonga), UNESCO World Network of Biosphere Reserves (12 of India's 18 biosphere reserves are listed; Nilgiri was the first, 1986). India's updated NBSAP (2024) aligns national targets with the GBF.
• IUCN Red List categories: Extinct, Extinct in the Wild, Critically Endangered, Endangered, Vulnerable, Near Threatened, Least Concern, Data Deficient, Not Evaluated — the first three threatened grades are assigned on quantitative criteria of population size, decline rate and range.

Key points

• Biogeography - spatial and temporal distribution of organisms and the processes behind it.
• Endemism - species restricted to one area; highest on islands, mountains and old isolated blocks like the Western Ghats.
• Dispersal routes - corridor (free), filter bridge (selective), sweepstake (chance).
• Vicariance - a range split by a new barrier, as against dispersal across an old one.
• Wallace's realms - six (Oriental includes most of India; Trans-Himalaya is Palaearctic).
• Wallace's Line - Bali-Lombok boundary between Oriental and Australian faunas.
• Biome - largest ecological unit, defined by climate and dominant life form, repeated on every continent.
• Altitudinal zonation - vertical repetition of the latitudinal vegetation sequence on mountains.
• Succession - nudation → invasion → competition → reaction → climax; primary on bare substrate, secondary after disturbance.
• Ecotone and edge effect - transition zone with raised richness but higher exposure to invasion.
• 10% law - about one-tenth of energy passes to the next trophic level; limits food-chain length.
• NPP - GPP minus plant respiration; the energy actually available to consumers.
• Island biogeography - species number equilibrates where immigration equals extinction; large and near islands are richest.
• Species-area curve - S = cA^z; used to project extinctions from habitat loss and to size reserves.
• Alpha, beta, gamma diversity - within-habitat, between-habitat turnover, and landscape total.
• Rodgers and Panwar - ten biogeographic zones of India, the basis of protected-area planning.
• Champion and Seth - 16 forest type groups in 5 major groups; the standard Indian forest classification.
• Hotspots in India - Himalaya, Indo-Burma, Western Ghats-Sri Lanka, Sundaland (Nicobars).
• ISFR 2023 - forest cover 21.76%; forest and tree cover together 25.17%; policy target 33%.

Formulas and examples

Example 1 — diversity indices for one quadrat. Four tree species with 40, 30, 20 and 10 individuals, N = 100.

• pᵢ = 0.4, 0.3, 0.2, 0.1. Terms: 0.4 × 0.9163 = 0.3665; 0.3 × 1.2040 = 0.3612; 0.2 × 1.6094 = 0.3219; 0.1 × 2.3026 = 0.2303.
• H′ = 1.280. H′max = ln 4 = 1.386 → J′ = 1.280/1.386 = 0.92 (very even).
• D = 0.16 + 0.09 + 0.04 + 0.01 = 0.30 → 1 − D = 0.70, 1/D = 3.33 effective species.
• Margalef d = (4 − 1)/ln 100 = 3/4.605 = 0.65.

Example 2 — species loss from habitat loss. For a forest block, c = 5 and z = 0.30 (S in species, A in km²).

• Original patch 100 km²: S = 5 × 100^0.30 = 5 × 3.98 = 19.9 ≈ 20 species.
• After clearing to 10 km²: S = 5 × 10^0.30 = 5 × 1.995 = 9.97 ≈ 10 species.
• Losing 90% of the area costs about 50% of the species — the standard argument for large contiguous reserves rather than many tiny fragments.

Example 3 — population by mark-recapture. 60 chital are marked and released; a later sample of 80 contains 12 marked animals.

• N̂ = (60 × 80)/12 = 4,800/12 = 400 animals. Assumptions: closed population, no mark loss, equal catchability — violated assumptions bias the estimate downward or upward, so camera-trap studies use spatially explicit capture-recapture instead.

Example 4 — similarity and beta diversity of two forest patches. Patch A has 30 species, patch B has 24, with 18 shared.

• Jaccard = 18/(30 + 24 − 18) = 18/36 = 0.50; Sørensen = (2 × 18)/(30 + 24) = 36/54 = 0.67.
• S_total = 30 + 24 − 18 = 36; ᾱ = (30 + 24)/2 = 27 → βw = 36/27 − 1 = 0.33 (moderate turnover).

Example 5 — IVI from a quadrat survey. 20 quadrats of 10 × 10 m. Species A: present in 12 quadrats, 60 individuals, basal area 2.4 m². All species together: 300 individuals, sum of frequency percentages 240, total basal area 12 m².

• Density = 60/20 = 3 per quadrat = 300 per hectare; relative density = 60/300 = 20%.
• Frequency = 12/20 = 60%; relative frequency = 60/240 = 25%.
• Relative dominance = 2.4/12 = 20% → IVI = 20 + 25 + 20 = 65 out of 300.
• Abundance = 60/12 = 5; A/F = 5/60 = 0.083 (frequency in %) → above 0.05, so the distribution is contagious (clumped), typical of root-suckering or animal-dispersed species.

Where it is used

• Forest Department and FSI work: working plans, forest-type mapping, ISFR assessment, regeneration and phyto-sociological surveys using density, frequency and IVI.
• Wildlife management: tiger and elephant estimation under NTCA and WII protocols, management plans for Kaziranga, Ranthambhore, Periyar; Eco-Sensitive Zone demarcation; human-wildlife conflict mapping.
• EIA and biodiversity consultancy: flora-fauna baseline chapters, biodiversity impact assessment and Biodiversity Management Plans for highways, mines, transmission lines and ports — a standard junior-ecologist role in Indian EIA firms.
• Local governance: Biodiversity Management Committees preparing People's Biodiversity Registers in every panchayat and municipality under the Biological Diversity Act.
• Geospatial and research roles: NRSC/ISRO and IIRS vegetation-type and biomass mapping, species distribution modelling (MaxEnt), ZSI and BSI survey positions, ICFRE and WII research.
• Teaching, NGOs and ecotourism: school and college teaching, WWF-India, ATREE, Wildlife SOS, nature-guide and interpretation work in biosphere reserves and Ramsar sites.

Interview questions

• Q: What is the difference between ecological and historical biogeography? A: Ecological biogeography explains present ranges by current climate, soil and biotic interactions; historical biogeography explains them by past events such as continental drift, glaciation and speciation.
• Q: State the equilibrium theory of island biogeography. A: The species number on an island stabilises where the immigration rate equals the extinction rate; immigration falls with distance from the mainland and extinction falls with island area, so large near islands hold the most species.
• Q: Why is the Western Ghats a biodiversity hotspot? A: It has very high plant and amphibian endemism from long isolation and a steep rainfall gradient, together with heavy loss of primary vegetation to plantations, dams and settlement — the two criteria for hotspot status.
• Q: How can a small area lose 50% of its species when 90% of the habitat goes? A: Because richness scales as S = cA^z with z about 0.3, not linearly; 10^0.3 is about 2, so a tenfold area loss halves species richness.
• Q: Difference between Shannon and Simpson indices? A: Shannon combines richness and evenness and responds strongly to rare species; Simpson measures dominance, so it is driven by the commonest species and is less affected by rarities.
• Q: What is Wallace's Line and why does it matter? A: A faunal boundary between Bali and Lombok where Oriental and Australian faunas meet almost without overlap, kept apart by a deep strait that stayed open through Pleistocene sea-level falls — evidence that barriers, not climate, control these ranges.
• Q: Name the ten biogeographic zones of India. A: Trans-Himalaya, Himalaya, Desert, Semi-arid, Western Ghats, Deccan Peninsula, Gangetic Plain, Coasts, North-East and Islands (Rodgers and Panwar, 1988).
• Q: How many forest type groups did Champion and Seth recognise? A: Sixteen type groups within five major groups — tropical moist, tropical dry, montane subtropical, montane temperate and alpine.
• Q: What legal change did the Wild Life (Protection) Amendment Act, 2022 make to the schedules? A: It reduced and reorganised them — Schedules I and II for animals, III for plants and IV for CITES-listed scheduled specimens — and strengthened CITES implementation.
• Q: Who maintains the People's Biodiversity Register and under which law? A: The Biodiversity Management Committee of each local body, under the Biological Diversity Act, 2002 as amended in 2023.
• Q: Why is a corridor between two reserves ecologically valuable? A: It restores gene flow and allows recolonisation after local extinction, so two connected patches behave more like one large population than two isolated small ones.
• Q: Tricky — can an inverted biomass pyramid exist? A: Yes, in open-ocean systems where phytoplankton biomass at any instant is less than that of zooplankton, because the producers turn over very rapidly; the energy pyramid remains upright.

Common mistakes

• Using log base 10 in the Shannon formula while quoting the answer as if it were natural log — state the base, and note H′ cannot exceed ln S.
• Reporting Simpson's D as "diversity": D is dominance; diversity is 1 − D or 1/D.
• Confusing richness (number of species) with diversity (richness plus evenness), and quoting one for the other.
• Mixing up Wallace's realms (six zoogeographic divisions) with biomes (climate-vegetation units) and with India's ten biogeographic zones.
• Treating the species-area relation as linear, and so grossly over-estimating species loss from habitat loss.
• Quoting old legal positions — the Forest (Conservation) Act, 1980 is now the Van (Sanrakshan Evam Samvardhan) Adhiniyam after the 2023 amendment, and the WPA schedules were restructured in 2022; protected-area, Ramsar and tiger-reserve counts change, so always give the figure with its year and source.
• Forgetting that India's forest-cover figure (21.76%) is separate from forest-and-tree cover (25.17%), and comparing either loosely with the 33% policy target.

Related topics

Climatology: climatic classification and monsoons · Soil geography and soil erosion · Environmental geography and ecosystem management · Biodiversity conservation law and policy in India · Remote sensing and GIS for vegetation mapping · Population ecology and ecological succession