UNIT 3 — BIODIVERSITY & CONSERVATION

CHAPTER 3.3 — BIODIVERSITY DISTRIBUTION AND PATTERNS: LATITUDINAL, ALTITUDINAL, SPECIES-AREA RELATIONSHIPS, AND BIOGEOGRAPHIC ZONES

1. Global Patterns and Gradients of Biodiversity

Biodiversity is not uniformly distributed across the biosphere; it exhibits pronounced spatial gradients dictated by climatic stability, evolutionary history, solar energy availability, and physical geography.

                    MASTER GRADIENTS OF BIODIVERSITY

     [ LATITUDINAL GRADIENT ]                   [ ALTITUDINAL GRADIENT ]

     High Latitude (Poles)                      High Elevation (Peaks)
             │                                          │
             │ Low Diversity                            │ Low Diversity
             ▼                                          ▼
     Low Latitude (Equator)                     Low Elevation (Foothills)

     (Maximum Species Richness)                 (Maximum Species Richness)

A. The Latitudinal Diversity Gradient (LDG)

• The Fundamental Pattern: Species richness is highest at the Equator (\(0^\circ\) latitude) and steadily decreases as one moves toward the temperate and polar regions. Tropics (latitudinal range of \(23.5^\circ \text{N}\) to \(23.5^\circ \text{S}\)) harbor far more species than temperate or polar zones.
• Quantitative Global Exemplars:
  • Avian Richness: Colombia (located near the Equator) harbors nearly 1,400 bird species. In contrast, New York (\(41^\circ \text{N}\)) has 105 species, and Greenland (\(71^\circ \text{N}\)) has only 56 bird species. India, situated largely in tropical latitudes, supports over 1,200 bird species.
  • Vascular Plants: A single hectare of tropical rainforest at the Equator can contain up to 10 times more species of vascular plants than a temperate forest in the Midwest of the USA. The Amazonian Rainforest in South America exhibits the greatest terrestrial biodiversity on Earth, housing over 40,000 plant species, 1,300 bird species, 427 mammal species, and 125,000 invertebrate species.
• Ecological & Evolutionary Hypotheses for Tropical Wealth:
  1. Evolutionary Time & Glacial Stability: Temperate and polar regions experienced repeated glaciations during the Pleistocene, wiping out biological communities. Tropical latitudes remained largely undisturbed for millions of years, allowing an uninterrupted timeframe for speciation and diversification.
  2. Climatic Predictability & Niche Specialization: Tropical environments are less seasonal, more constant, and highly predictable. This environmental stability promotes narrow ecological niches and high specialization, leading to greater species co-existence.
  3. Solar Energy & Primary Productivity: Tropics receive maximum direct solar insolation, driving high net primary productivity. This abundant energy base supports complex food webs and higher species densities.

B. The Altitudinal Diversity Gradient

• The Pattern: Species richness is highest at lower elevations (foothills and low valleys) and rapidly declines as altitude increases toward mountain peaks.
• The Environmental Lapse Rate: For every 1,000 meters (1 km) increase in elevation, ambient temperature drops by approximately \(6.5^\circ \text{C}\). This sharp temperature drop, combined with extreme seasonal variability, reduced atmospheric pressure, and strong winds, severely limits biological survival at high altitudes.
• Timber Line (Tree Line): Beyond an elevation of approximately 3,600 meters (12,000 feet) in the Himalayas, tree growth ceases entirely due to permafrost and extreme cold, marking the transition from sub-alpine forests to alpine tundra and cold desert vegetation.
• The Aspect of Slope Effect: Mountain slopes facing the Equator (e.g., southern slopes in the Northern Hemisphere) receive direct sunlight, higher precipitation, and warmer microclimates, supporting lush forests and higher species richness compared to colder, shaded northern slopes.

C. The Species-Area Relationship

• Humboldt’s Discovery: German naturalist and geographer Alexander von Humboldt observed that within a given region, species richness increases with increasing explored area, but only up to a specific limit.
• Mathematical Formulation: On a logarithmic scale, the species-area relationship forms a straight line represented by the linear equation:

\[\log S = \log C + Z \log A\]

\[\text{Or in non-logarithmic form: } S = C \cdot A^Z\]

Where:

• \(S =\) Species Richness
• \(A =\) Explored Area
• \(C =\) Y-intercept (constant)
• \(Z =\) Slope of the regression line (regression coefficient)

                  HUMBOLDT'S SPECIES-AREA CURVE

       Species
      Richness (S)
          │            Rectangular Hyperbola (S = C·Aᶻ)
          │               . - - - - - - - -
          │           . '
          │        . '   Log-Log Straight Line
          │     . '      (log S = log C + Z log A)
          │  . '
          └───────────────────────────────────► Area (A)

• The Significance of \(Z\)-values:
  • For small to medium-sized local regions (regardless of taxonomic group or continent), the value of \(Z\) consistently falls within the narrow range of \(0.1\) to \(0.2\).
  • When analyzing species-area relationships across entire continents or vast geographical units, the regression slope becomes much steeper, with \(Z\)-values ranging from \(0.6\) to \(1.2\) (e.g., frugivorous birds and mammals in tropical forests of different continents have a \(Z\)-value of \(1.15\)).

2. Biogeographic Classification & Global Realms

Biogeography is the study of the spatial distribution of biological species and ecosystems across geographic space and evolutionary time.

A. Global Biogeographic Realms

Tectonic plate movements split the ancient supercontinent Pangea approximately 70 million years ago into Gondwanaland (southern landmass) and Laurasia (northern landmass). This geographic isolation led to independent evolutionary radiation, dividing the biosphere into major Biogeographic Realms:

1. Palearctic Realm: Covers Eurasia (Europe, North Africa, and Northern Asia).
2. Nearctic Realm: Covers North America, Greenland, and northern Mexico.
3. Neotropical Realm: Covers South America, Central America, and the Caribbean (home to the Amazon).
4. Afrotropical (Ethiopian) Realm: Covers Sub-Saharan Africa and Madagascar.
5. Indo-Malayan (Oriental) Realm: Covers South and Southeast Asia, including India.
6. Australian Realm: Covers Australia, Tasmania, and New Guinea.
7. Oceanian & Antarctic Realms: Cover Pacific island chains and the Antarctic landmass.

B. India’s Evolutionary Assembly & Megadiverse Status

India's exceptional biological diversity is rooted in its unique geological journey:

• The Gondwana Heritage: Originally part of Gondwanaland alongside Africa, Australia, and Antarctica, India carried ancient southern floral and faunal lineages.
• The Tethys Collision: Tectonic drift pushed the Indian plate northward across the Equator to collide with the Eurasian continent, closing the shallow Tethys Sea and uplifting the Himalayas. This created a land bridge allowing European and Far-Eastern species to migrate into India.
• Ethiopian Influx: A later influx of African (Ethiopian) species adapted to savannas and semi-arid conditions crossed via temporary land bridges during low sea-level glacial periods.
• The Confluence: Situated at the junction of three major biogeographic realms (Palearctic, Afrotropical, and Indo-Malayan), India is recognized as one of the world's 17 Megadiverse Nations. With only 2.4% of the world's land area, India accounts for 7% to 8% of recorded global species.

3. Biogeographic Classification of India (Rodgers & Panwar, 1988)

To systematically plan conservation strategies and Protected Area Networks, Rodgers and Panwar (1988) of the Wildlife Institute of India (WII) classified India into 10 Biogeographic Zones and 26 Biotic Provinces based on climate, topography, vegetation, and faunal distribution:

                    INDIA'S 10 BIOGEOGRAPHIC ZONES

  1. Trans-Himalaya  ──► Cold desert plateau (Ladakh, Lahaul-Spiti)
  2. Himalaya        ──► Alpine, temperate, and sub-alpine conifer forests
  3. Desert          ──► Thar sand dunes and Salt flats of Kutch
  4. Semi-Arid       ──► Transitional thorn scrub (Aravallis, Gujarat)
  5. Western Ghats   ──► Evergreen rainforests & Shola montane grasslands
  6. Deccan Peninsula──► Largest zone (42% area); Sal & Teak dry deciduous
  7. Gangetic Plain  ──► Alluvial floodplains & Terai tall grasslands
  8. Coasts          ──► Mangroves, mudflats, and coral reefs
  9. North-East      ──► High endemism; tropical & subtropical forests
 10. Islands         ──► Tropical evergreen reefs (Andaman-Nicobar & Lakshadweep)

4. Specialized Spatial Formats: Hotspots, Coldspots, and Endemism

A. Biodiversity Hotspots

• Origin & Concept: The concept of "Biodiversity Hotspots" was developed by British ecologist Norman Myers in 1988 to identify priority geographical areas for intensive in-situ conservation.
• Conservation International (CI) Criteria: To qualify as a global biodiversity hotspot, a region must strictly satisfy two quantitative criteria:
  1. Endemic Vascular Plants: It must contain at least 1,500 species of vascular plants as endemics (\(>0.5%\) of the world's total), making it irreplaceable.
  2. Degree of Threat: It must have lost at least 70% of its primary native vegetation (i.e., retaining \(30%\) or less of its original natural habitat).

                     BIODIVERSITY HOTSPOT CRITERIA

     ┌────────────────────────────────────────────────────────┐
     │ 1. HIGH ENDEMISM                                       │
     │    Must harbor ≥ 1,500 endemic vascular plant species  │
     └───────────────────────────┬────────────────────────────┘
                                 │
                                 ┼──► QUALIFIES AS A HOTSPOT
                                 │    (36 Designated Globally)
     ┌───────────────────────────┴────────────────────────────┐
     │ 2. HIGH THREAT PERCEPTION                              │
     │    Must have lost ≥ 70% of original primary habitat    │
     └────────────────────────────────────────────────────────┘

• Global & Indian Hotspot Profile:
  • Globally, there are currently 36 recognized Biodiversity Hotspots. Although they cover only 2.5% of the Earth's land surface, they support over 50% of the world's endemic plant species and 43% of endemic terrestrial vertebrates.
  • India’s Four Biodiversity Hotspots:
    1. Himalaya: Encompasses the entire Indian Himalayan region (and contiguous parts of Nepal, Bhutan, and China).
    2. Indo-Burma: Covers Northeast India (south of the Brahmaputra River), the Andaman Islands, Myanmar, and parts of Southeast Asia.
    3. Western Ghats and Sri Lanka: Runs parallel to India's western coast and extends to Sri Lanka, featuring extraordinary amphibian endemism.
    4. Sundaland: Includes the Nicobar Group of Islands (along with Indonesia, Malaysia, Singapore, and Brunei). (Note: The Andaman group belongs to Indo-Burma, whereas the Nicobar group belongs to Sundaland).

B. Endemism in Indian Biodiversity

Endemic species are those restricted exclusively to a specific geographic region and found nowhere else on Earth.

• Plant Endemism: About 33% of flowering plants (angiosperms) in India are endemic, heavily concentrated in the Western Ghats, Northeast India, the Himalayas, and the Andaman & Nicobar Islands.
• Faunal Endemism: India displays very high endemism in lower vertebrates: 55.8% to 62% of Indian amphibians and 45.8% to 50% of Indian reptiles (lizards) are strictly endemic, with the Western Ghats serving as the primary center of endemic radiation.

C. Biodiversity Coldspots

• Definition: Coldspots are geographical regions characterized by relatively low species richness, but which harbor rare, evolutionary unique, highly specialized taxa or provide critical global ecosystem services (e.g., polar tundra, high-latitude boreal zones, or deep hydrothermal vents).
• Conservation Value: While hotspots prioritize sheer species richness and habitat threat, coldspots require protection to preserve unique evolutionary lineages and maintain global ecosystem stability.

🚀 QUICK REVISION

• Latitudinal Gradient: Species richness is maximum near the Equator and decreases toward the Poles.
• Altitudinal Gradient: Species richness drops with increasing elevation as temperature decreases by \(6.5^\circ \text{C}\) per 1,000 meters.
• Species-Area Relationship: Formulated by Alexander von Humboldt: \(\log S = \log C + Z \log A\). \(Z\)-value ranges between \(0.1–0.2\) for local areas and \(0.6–1.2\) for entire continents.
• Global Realms: 6 major terrestrial realms (Palearctic, Nearctic, Neotropical, Afrotropical, Indo-Malayan, Australian).
• Megadiverse Nations: India is one of 17 megadiverse countries, holding 7–8% of global species on 2.4% land area.
• 10 Biogeographic Zones of India: Trans-Himalaya, Himalaya, Desert, Semi-Arid, Western Ghats, Deccan Peninsula (largest, 42%), Gangetic Plain, Coasts, North-East, and Islands.
• Hotspot Criteria (Myers/CI): Must contain \(\ge 1,500\) endemic vascular plant species and have lost \(\ge 70%\) of original primary habitat.
• India’s 4 Hotspots: Himalayas, Indo-Burma, Western Ghats & Sri Lanka, Sundaland.
• Endemism: High in Western Ghats and Northeast India; over 60% of Indian amphibians are endemic.

🗺️ CONCEPTUAL MENTAL MAP

                    [ PATTERNS OF BIODIVERSITY ]
                                  │
         ┌────────────────────────┼────────────────────────┐
         ▼                        ▼                        ▼
 [GLOBAL GRADIENTS]      [SPECIES-AREA REL.]      [BIOGEOGRAPHY & REALMS]
 • Latitudinal (High     • Humboldt's Curve       • 6 Global Realms
   at Equator)             S = C·Aᶻ                 (Palearctic, Oriental, etc.)
 • Altitudinal (High     • Log Line:              • India: Gondwana origin +
   at Foothills)           log S = log C + Z log A  Tethys collision
 • Slope Aspect          • Z = 0.1-0.2 (Local)    • Megadiverse Nation (7-8%)
                           Z = 0.6-1.2 (Global)            │
                                                           ▼
                                                [INDIA'S 10 ZONES & 26 PROVINCES]
                                                • Deccan Peninsula (42% Largest)
                                                • Western Ghats & NE (Hotspots)
                                                           │
                                                           ▼
                                                [HOTSPOTS & ENDEMISM]
                                                • 36 Global Hotspots
                                                • 4 Indian Hotspots (Himalaya,
                                                  Indo-Burma, W. Ghats, Sundaland)
                                                • High Amphibian Endemism (>60%)

🔥 MUST REMEMBER (HIGH-YIELD POINTS)

1. Alexander von Humboldt formulated the Species-Area Relationship (\(\log S = \log C + Z \log A\)).
2. The \(Z\)-value for species-area relationships in small local areas is \(0.1\) to \(0.2\), whereas for entire continents it ranges from \(0.6\) to \(1.2\).
3. Environmental lapse rate causes a temperature drop of \(6.5^\circ \text{C}\) per 1,000 meters increase in altitude.
4. Rodgers and Panwar (1988) classified India into 10 Biogeographic Zones and 26 Biotic Provinces.
5. The Deccan Peninsula is the largest biogeographic zone in India, covering 42% of the landmass.
6. Norman Myers (1988) coined the term Biodiversity Hotspot.
7. Hotspots require harboring \(\ge 1,500\) endemic vascular plant species and losing \(\ge 70%\) of primary vegetation.
8. India contains 4 of the world’s 36 Biodiversity Hotspots: Himalayas, Indo-Burma, Western Ghats & Sri Lanka, and Sundaland.
9. The Andaman Islands belong to the Indo-Burma hotspot, whereas the Nicobar Islands belong to the Sundaland hotspot.
10. Over 60% of India’s amphibian species and 50% of lizard species are endemic, concentrated mainly in the Western Ghats.

📌 KEY TERMS

• Latitudinal Diversity Gradient (LDG): The global pattern where species richness increases from the poles toward the equator.
• Species-Area Relationship: The mathematical relationship showing that species richness increases with expanding geographical area up to a limit.
• Biogeographic Zone: A large regional unit representing distinctive ecological assemblages, climate, and endemic flora/fauna.
• Biotic Province: A secondary subdivision within a biogeographic zone defined by more detailed structural and climatic variations.
• Biogeographic Realm: A continent-scale terrestrial region defined by the shared evolutionary history of its fauna and flora.
• Megadiverse Country: One of 17-18 countries that house the majority of Earth's species and high levels of endemism.
• Biodiversity Hotspot: A biogeographic region with extreme species richness and endemism under severe habitat threat.
• Biodiversity Coldspot: A region of lower species richness that contains unique evolutionary lineages or critical ecological functions.
• Endemism: The status of a biological species being native and restricted exclusively to a defined geographical location.
• Timber Line: The high-altitude boundary on mountains beyond which environmental conditions prevent tree growth.

🎯 1-LINE BITS (OBJECTIVE EXAM ACCELERATORS)

• Alexander von Humboldt discovered the species-area relationship curve.
• The slope \(Z\) of the species-area curve on a log-log scale for small areas ranges between \(0.1\) and \(0.2\).
• Tropics (\(23.5^\circ \text{N}\) to \(23.5^\circ \text{S}\)) exhibit the highest species diversity on Earth.
• For every 1 km rise in altitude, atmospheric temperature drops by \(6.5^\circ \text{C}\).
• Rodgers and Panwar designed India's biogeographic classification of 10 zones and 26 provinces in 1988.
• The Deccan Peninsula is India's largest biogeographic zone, occupying 42% of the country.
• The Islands form the smallest biogeographic zone in India, covering 0.03% of the landmass.
• Norman Myers introduced the concept of Biodiversity Hotspots in the year 1988.
• To qualify as a hotspot, an area must have lost at least 70% of its original habitat.
• There are 36 designated global biodiversity hotspots today.
• India has 4 global biodiversity hotspots.
• The Nicobar Islands are part of the Sundaland hotspot.
• The Andaman Islands are part of the Indo-Burma hotspot.
• Over 60% of amphibians in India are endemic to the Western Ghats.
• India houses 7% to 8% of global recorded species on 2.4% of world land area.
• Colombia holds over 1,400 species of birds due to its equatorial position.
• The Amazonian Rainforest is known as the "Lungs of the Planet Earth".
• Coldspots are ecologically vital regions harboring rare lineages despite low overall species counts.
• Gondwanaland was the ancient southern supercontinent from which the Indian landmass split.
• The Tethys Sea closed during the tectonic collision that uplifted the Himalayas.