Earth. They cover about 55 million square kilometers, three times more than the next largest zone, the tundra. Likewise in the sea, tropical oceans cover more of the earth’s area than any other land or water mass.
What’s more, all the world’s tropical habitat is in one continuous swath lying on either side of the equator. Other zones, such as temperate or subtropical, are found in separate bands in the north and south. A grass seed or a ground squirrel trying to migrate from the North American prairie to the South American pampas has got to cross thousands of miles of hostile tropical habitat. But a monkey could, in theory, travel from Mexico to Argentina without entering alien territory. And within the tropics, between about 25° latitude north and south, the climate is relatively constant. The annual average temperature is about 28°C throughout this zone. As you leave the tropics, the temperature drops by about 8°C for each 10°N you travel. So species adapted to tropical climates have much more land or water to play with compared with their nontropical counterparts.
It’s obvious that larger areas will contain more species. They can harbor more individuals and a greater diversity of habitats. The relationship between a place’s area and the number of species found there follows a power law. The exponent in the species-area power law—the bit that is 3/4 in Kleiber’s rule—varies from place to place but is usually between 0.1 and 0.25. So the rate at which you find new species slows down as the area you search expands. Measuring the form of this power law also allows conservation biologists to make predictions about the effects of habitat destruction. For example, the species-area curve for the forests of Eastern America suggests that removing half of the forest will drive about one in every seven bird species to extinction. Sure enough, half of the forest has been lost since 1870, and in that time four of the 30 bird species found there and nowhere else have disappeared.
Less obvious, from this line of reasoning, is why the tropics should also contain, as they do, proportionately more species than the temperate regions, a greater number for each square kilometer of area. John Terborgh and Michael Rosenzweig argue that the tropics’ large area creates a high diversity because tropical species, given more land, have larger ranges—and that this makes them both less likely to go extinct and more likely to give rise to new species. Tropical species are less