Biodiversity simply means the sum of all of the variation in nature – the kind and number of species, their association into units (communities or ecosystems), or, at another level, the genes which are present in all of earth’s organisms and their arrangement, including genetic variation within species. The term includes functional diversity (such as nutrient capture and other ecological functions) as well as simply species diversity. However, most scientific studies of biodiversity have considered only species diversity and how it changes, particularly with regard to latitude. As a rule of thumb, there are many more species (often several times as many) in the tropical latitudes than there are in temperate zones. This is known as the “biodiversity gradient,” and occurs in both northern and southern hemispheres. Moving from higher latitudes to lower, one sees that biodiversity of both plants and animals (and presumably microbes and fungi) increases to a substantial degree. E.O. Wilson (1992, p. 196) illustrates this beautifully in his list of breeding bird species at various latitudes: Greenland, 56; Labrador, 81; New York State, 195; Guatemala, 469; Colombia, 1525. But this generalization must be modified because species richness in the tropics varies with longitude, altitude, soil type, topography, temperature, and rainfall, among other factors.