Latitudinal and altitudinal gradients

With the exception of arid, semi-arid and some aquatic habitats, biodiversity across the globe generally follows both a latitudinal and an altitudinal gradient.

Latitudinally, ecological surveys show that diversity is not spread uniformly around the planet -- species richness for most plant and animal groups is highest at low latitudes, near the equator, and declines steadily as one moves towards the poles. Several factors are thought to explain this: tropical regions have remained climatically stable over millions of years with comparatively little seasonal variation, and this long-term stability -- rather than the periodic glaciations experienced at the poles -- appears to favour speciation. Reduced migration within tropical regions may also have limited gene flow between geographically separated populations, again favouring the formation of new species. Abundant and fairly constant sunlight, warmer temperatures and higher annual rainfall in the tropics are additional factors that support greater productivity and, in turn, richer food resources -- for example, because fruit is available year-round in rain forests, the variety of fruit-eating (frugivorous) animals there is far greater than in temperate regions. As a result, species richness is at its highest in tropical rain forests -- the Amazon rain forest alone is estimated to hold around 40,000 plant species, 1,300 bird species, 427 mammal species and around 3,000 fish species. …