As a general rule, the variety of life is most impoverished at the poles and richest at low latitudes. This pattern is referred to in the scientific literature as the “latitudinal diversity gradient,” or LDG, and it was noted already by the German naturalist Alexander von Humboldt, who was amazed by the biological splendors of the tropics, which offer “a spectacle as varied as the azure vault of the heavens.” “The verdant carpet which a luxuriant Flora spreads over the surface of the earth is not woven equally in all parts,” Humboldt wrote after returning from South America in 1804. “Organic
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