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1.2 Patterns of Biodiversity

(i) Latitudinal Gradients

• The diversity of plants and animals is unevenly distributed across the globe, with a notable pattern known as the latitudinal gradient in diversity.

• Generally, species diversity declines as one moves from the equator towards the poles.

• The tropics (latitudinal range of 23.5° N to 23.5° S) host a greater number of species compared to temperate or polar regions.

• For instance:

  • Colombia (near the equator) has nearly 1,400 species of birds.

  • New York (41° N) has 105 bird species.

  • Greenland (71° N) has only 56 species.

  • India, primarily in tropical latitudes, is home to over 1,200 bird species.

• Tropical forests, such as those in Ecuador, have significantly more vascular plant species than temperate regions; for example, a tropical forest may have up to 10 times more species than a temperate forest of equal area.

• The Amazon rainforest is the most biodiverse region on Earth, with:

  • Over 40,000 species of plants,

  • 3,000 species of fishes,

  • 1,300 species of birds,

  • 427 species of mammals,

  • 427 species of amphibians,

  • 378 species of reptiles,

  • More than 125,000 invertebrates.

• It is estimated that at least two million insect species in these rain forests are yet to be discovered.

Reasons for Greater Biodiversity in Tropics:

1. Evolutionary Time: Tropical latitudes have remained relatively undisturbed for millions of years. This stability has allowed for more time for species diversification compared to temperate zones that have experienced glaciations.

2. Environmental Stability: Tropical environments tend to be less seasonal, providing constant and predictable conditions, which encourages niche specialization and enhances species diversity.

3. Solar Energy Availability: The tropics receive more solar energy, which contributes to higher productivity, potentially fostering greater biodiversity.

(ii) Species-Area Relationships

• Alexander von Humboldt observed that species richness increases with area explored, but only up to a limit.

• The relationship between species richness (S) and area (A) can be expressed mathematically as a rectangular hyperbola:

  Equation: log S = log C + Z log A

  • S: Species richness

  • A: Area

  • Z: Slope of the line (regression coefficient)

  • C: Y-intercept

• Ecologists have found that the value of Z typically ranges from 0.1 to 0.2 across various taxa (e.g., plants, birds, bats, freshwater fishes).

• However, when analyzing species-area relationships over very large areas (like entire continents), the slope can be much steeper (Z values from 0.6 to 1.2).

• For example, in tropical forests, the slope for frugivorous birds and mammals is 1.15.

Interpretation of Steeper Slopes

• Steeper slopes in the species-area relationship indicate a more significant increase in species richness with area, suggesting that larger areas tend to support disproportionately more species. This phenomenon emphasizes the importance of habitat area in biodiversity conservation.