Patterns of biodiversity distribution

Because organisms need a specific range of temperature, precipitation and other conditions for optimal growth and metabolism, and because those conditions are themselves set by latitude and altitude, biodiversity is distributed in clear, predictable patterns rather than randomly. The most important of these is the latitudinal gradient: diversity rises steadily moving from the poles toward the equator, peaking within the tropics — roughly between 23.5°N and 23.5°S, the belt between the Tropic of Cancer and the Tropic of Capricorn. Bird counts make the gradient concrete: Colombia, close to the equator, has nearly 1,400 bird species, New York at 41°N has only 105, and Greenland at 71°N has just 56; India, with most of its landmass in the tropical latitudes, is correspondingly home to more than 1,200 bird species. A parallel altitudinal gradient runs in the opposite direction — species diversity falls as one climbs a mountain, since temperature drops by roughly 6.5°C for every kilometre gained above mean sea level (the reference level, MSL, against which elevations such as Erode Junction's roughly 171 m are measured). The tropics owe their exceptional richness to several reinforcing factors: their warm, congenial habitats favour both speciation and the sheer number and variety of organisms an area can support; temperatures typically sit between 25°C and 35°C, the range in which most metabolic processes run most efficiently; average rainfall commonly exceeds 200 mm a year; climate, seasons, temperature, humidity …