Pattern of biodiversity
- Latitudinal gradients-
- Species diversity decreases as we move away from the equator towards the poles.
- Tropics harbor more species than temperate or polar areas.
- Colombia located near the equator has nearly 1,400 species of birds while New York at 41° N has 105 species and Greenland at 71° N only 56 species. India, with much of its land area in the tropical latitudes, has more than 1,200 species of birds.
- The largely tropical Amazonian rain forest in South America has the greatest biodiversity on earth- it is home to more than 40,000 species of plants, 3,000 of fishes, 1,300 of birds, 427 of mammals, 427 of amphibians, 378 of reptiles and of more than 1,25,000 invertebrates.
- Tropics have greatest biodiversity because
- a) Unlike temperate regions subjected to frequent glaciations in the past, tropical latitudes have remained relatively undisturbed for millions of years.
- b) Tropical environments are less seasonal and more constant and predictable which promote niche specialization and lead to a greater species diversity.
- c) There is more solar energy available in the tropics, which contributes to higher productivity thus contributing indirectly to greater diversity.
Fig. amazon rain forest
- Species-Area relationships
- Alexander von Humboldt observed that within a region species richness increased with increasing explored area, but only up to a limit.
- The relation between species richness and area for a wide variety of taxa such as angiosperm plants, birds, bats, freshwater fishes 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 where,
S= Species richness A= Area Z = slope of the line (regression coefficient) C = Y-intercept.
- The species-area relationships among very large areas like the entire continents, the slope of the line is much steeper (Z values in the range of 0.6 to 1.2).
For example, for frugivorous (fruit-eating) birds and mammals in the tropical forests of different continents, the slope is found to be 1.15.