Latitudinal gradients

Diversity is spread unevenly over the Earth. A widely known pattern is the latitudinal gradient: species diversity generally falls from the equator towards the poles.

With very few exceptions, the tropics (23.5° N to 23.5° S) hold more species than temperate or polar regions.

Birds: Colombia, near the equator, has nearly 1,400 species; New York at 41° N has 105; Greenland at 71° N has only 56. India, mostly tropical, has more than 1,200.

A tropical forest in Ecuador can hold up to 10 times as many vascular plant species as an equal area of temperate forest in the Midwest of the USA.

The Amazonian rain forest, largely tropical, holds the greatest biodiversity on Earth. Its counts: plants over 40,000 species, fishes 3,000, birds 1,300, mammals 427, amphibians 427, reptiles 378, and invertebrates over 1,25,000. At least two million insect species there may still await discovery.

Hypothesis 1: speciation is a function of time. Temperate regions went through frequent glaciations, while the tropics stayed relatively undisturbed for millions of years, giving more time for diversification.

Hypothesis 2: tropical environments are less seasonal, more constant and predictable, which promotes niche specialisation and so greater species diversity.

Hypothesis 3: the tropics receive more solar energy, which raises productivity and may indirectly add to diversity.