varied in the tropics than in temperate regions and more varied in temperate regions than at the poles. This is one of the earth’s most obvious features.

The possible causes are simple. Either species are more likely to evolve in the tropics or they are less likely to go extinct—or both. But working out what might speed up the birth of species or slow down their death has proved much more complicated. Two centuries after the first hypothesis was proposed, there is still no accepted explanation for what causes biodiversity to peak in the tropics. Besides being the oldest question in ecology, it is one of the toughest.

Discovering Diversity

The greater variety of tropical life must have been obvious to European travelers from the start. Science began to come to grips with tropical diversity in the eighteenth century, when expeditions added scientific inquiry to their missions of discovery and conquest. The model was the voyages of James Cook, whose ships observed, measured, and collected everywhere they went. In the process Joseph Banks, the naturalist on Cook’s first voyage, acquired the world’s largest herbarium.

Of course, besides containing more species, the tropics have a very different climate than the temperate regions. Natural philosophers immediately fastened on to this feature as the cause of tropical regions’ lushness. German botanist Carl Ludwig Willdenow discussed this issue in his book, The Principles of Botany and of Vegetable Physiology, published in 1792 and translated into English in 1805. Willdenow noted that warmer places had more plant species. He also gave a detailed measurement—it might be the original piece of macroecology—of what is now called the latitudinal gradient in diversity:

The Florae of different parts of the globe, with which botanists have favoured us, show indeed that vegetation increases with the degree of warmth. In Southern Georgia, according to credible accounts, only 2 wild growing plants are found; in Spitzbergen, 30; in Lapland, 534; in Iceland, 553; in Sweden, 1,299; in Brandenburg, 2,000; in Piedmont, 2,800; on the coast of Coromandel [the region of southeast India around Madras], about 4,000; in Jamaica as many; and in Madagascar nearly 5,000.

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