Biogeography: Major Terrestrial Biomes; Theory of Island Biogeography; Biogeographical Zones of India

Major Terrestrial Biomes

• Tropical Rainforest
  Tropical rainforests occur near the equator, characterized by exceptionally high rainfall (over 2,000 mm/year), stable warm temperatures, and immense species diversity. Canopy stratification supports myriad plants, insects, birds, and mammals.
  Example: The Amazon Rainforest hosts over 40,000 plant species and 2,500 fish species.

• Desert
  Deserts receive less than 250 mm of rain annually and exhibit extreme temperature fluctuations between day and night. Vegetation is sparse and water–conserving (e.g., succulents).
  Example: The Sahara Desert’s dark green patches around oases support date palms and migratory birds.

• Temperate Grassland
  Characterized by warm summers, cold winters, and moderate rainfall (500–900 mm/year), these biomes support deep-rooted grasses and seasonal wildflowers. Large grazing mammals and ground-burrowing rodents dominate.
  Example: The North American prairies sustain bison herds and prairie dogs.

• Taiga (Boreal Forest)
  Taiga spans the high northern latitudes with long, cold winters and short, moist summers. Coniferous trees (spruce, fir, pine) dominate, creating vast carbon sinks.
  Example: Siberian taiga is home to moose, Siberian tigers, and migratory birds.

• Tundra
  Found above the treeline in polar regions and at high altitudes. Permafrost limits root penetration; vegetation includes mosses, lichens, and dwarf shrubs.
  Example: Arctic tundra hosts caribou migrations and nesting grounds for geese.

World map of terrestrial biomes

Theory of Island Biogeography

• Species–Area Relationship

  Islands vary widely in size, and this variation has a direct impact on the number of species they can support. As island area increases, so does habitat diversity, offering more niches and resources. Larger islands reduce local extinction risks because small populations can find refuge in microhabitats and recover from disturbances. Mathematically, this relationship often follows a power‐law curve:
  S = cAᶻ
  where S is species richness, A is island area, c is a constant, and z typically ranges from 0.2 to 0.35 in nature (MacArthur & Wilson, 1967).

• Equilibrium Model of Immigration and Extinction

  MacArthur and Wilson proposed that the number of species on an island represents a dynamic balance between two opposing rates: immigration and extinction.

  • Immigration Rate
    Declines as more species colonize the island and available niches fill up.
  • Extinction Rate
    Increases with species richness due to intensified competition and smaller population sizes.

  At equilibrium, these two rates intersect, determining the island’s steady‐state species count. Near islands, high colonization keeps immigration rates elevated; small islands experience elevated extinction rates because of limited resources.

• Role of Isolation

  Distance from source populations (mainland or other islands) creates a barrier to dispersal. The further an island lies, the fewer propagules—seeds, spores, or animals—manage the journey.

  • Reduced Colonization: Fewer new arrivals slow down species turnover.
  • High Endemism: Isolated islands often harbor unique lineages that evolved without gene flow from continental relatives.

  Isolation thus fosters distinctive evolutionary pathways, leading to endemic flora and fauna. 

   Case Study: Hawaiian Honeycreepers

   A single ancestral finch that arrived in the Hawaiian archipelago roughly five million years ago diversified into over 50 honeycreeper species. Driven by ecological opportunity and geographic separation, these birds evolved a remarkable array of beak shapes—from nectar‐feeding tubes to sturdy seed crushers—demonstrating adaptive radiation in action.

• Applications and Empirical Examples

  • Krakatau Recolonization (Post‐1883)
    Following the 1883 volcanic eruption, barren Krakatau saw rapid influxes of plant and animal species. Over decades, early colonists like ferns and insects paved the way for more complex communities, illustrating stages of immigration, competition, and extinction.
  • Galápagos Finch Speciation
    Darwin’s finches exemplify how isolation and niche availability drive rapid speciation. Different islands fostered distinct beak morphologies suited to varied food sources, underscoring the predictive power of island biogeography principles.

Diagram of MacArthur & Wilson’s Island biogeography model

Biogeographical Zones of India

India’s unique geography—from Himalayas to tropical islands—hosts ten biogeographical zones as per Rodgers and Panwar (1988).

• Trans-Himalayan Zone
  Cold, arid high-altitude deserts with xerophytic shrubs and annuals.
  Example: Cold-desert flora of Ladakh.
• Himalayan Zone
  Ranges from subtropical foothills to alpine meadows. Supports oak-rhododendron forests, musk deer, and snow leopard.
• Indian Desert Zone
  Hot deserts of Rajasthan with drought-resistant grasses, acacia scrub, and desert wildlife like the blackbuck.
• Semi-Arid Zone
  Thorn forests and grasslands in central India; home to nilgai, chinkara, and peafowl.
• Deccan Peninsula Zone
  Tropical dry and moist deciduous forests; teak, sal, and mahua trees dominate. Tigers and elephants roam here.
• Gangetic Plain Zone
  Alluvial plains with moist deciduous forests and tall grasses. Key for Bengal tiger and gharial habitat.
• North-East India Zone
  One of the richest biodiversity pockets; tropical rainforests and cloud forests support numerous orchids, hornbills, and hoolock gibbons.
• Coastal Zone
  Mangroves (Sundarbans), littoral forests, and sand dunes; home to estuarine crocodiles and migratory shorebirds.
• Islands Zone
  Andaman & Nicobar and Lakshadweep archipelagos with high endemism—e.g., Andaman woodpecker and coconut crab.

Map of India’s biogeographical zones

Summery in video: Click the link to enjoy 

Biogeography explained Terrestrial Biomes, Island Theory & India’s Ecological Zones

References:

Champion, H. G., & Seth, S. K. (1968). A revised survey of the forest types of India. Government of India Press.

MacArthur, R. H., & Wilson, E. O. (1967). The theory of island biogeography. Princeton University Press.

Olson, D. M., Dinerstein, E., Wikramanayake, E. D., Burgess, N. D., Powell, G. V. N., Underwood, E. C., … & Kura, Y. (2001). Terrestrial ecoregions of the world: A new map of life on Earth. BioScience, 51(11), 933–938.

Rodgers, W. A., & Panwar, H. S. (1988). Planning a wildlife protected area network in India. Indian Institute of Public Administration.