Class 12 Biology

Patterns of Biodiversity


The term biodiversity was popularized by Edward Wilson. It includes diversity at all levels of biological organization. Following are the most important types of biodiversity.

  1. Genetic diversity: The diversity at genetic level in a single species is called genetic diversity. For example; there are more than 50,000 different strains of rice in India.
  2. Species diversity: This shows the diversity at the species level. For example; there is a greater diversity of amphibian species in the Western Ghats compared to in the Eastern Ghats.
  3. Ecological diversity: The ecological diversity shows the diversity at the ecosystem level. For example; India has diverse types of ecosystem, like deserts, rainforests, mangroves, wetlands, estuaries, etc.

Number of Species on Earth and in India

According to the IUCN (2004), the total number of plant and animal species described so far is slightly more than 1.5 million. A conservative estimate by Robert May says that there could be about 7 million species on the earth.

Patterns of Biodiversity

Latitudinal gradients

Species diversity generally decreases as we move away from the equator towards the poles. Tropics have more species than temperate or polar areas. Many hypotheses have been proposed by scientists to explain greater diversity in the tropics. They are as follows:

  1. Speciation is generally a function of time. Temperate regions had been subjected to frequent glaciations in the past. But tropical latitudes have remained relatively undisturbed for millions of years. This allowed a long evolutionary time for species diversification in the tropics.
  2. Tropical environments are less seasonal, relatively more constant and predictable. A constant environment promotes niche specialization, and leads to a greater species diversity.
  3. More solar energy is available in the tropics. It contributes to higher productivity. It may have indirectly contributed to greater diversity in the tropics.

Species-Area relationships

German naturalist and geographer Alexander von Humboldt observed that within a region species richness increased with increasing explored area, but only up to a limit. In fact, the relation between species richness and area for a wide variety of taxa turns out to be a rectangular hyperbola. On a logarithmic scale, the relationship is a straight line described by the 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 discovered that the value of Z lies in the range of 0.1 to 0.2, regardless of the taxonomic group or the region.