ill suited to our notion of an easy life—hot springs, salt lakes, deserts—have few other species living in them. Yet to a thermophilic bacterium, a hot spring is home. To a Stenocara beetle, which can collect the water from fog on its wings, the Namib Desert is just fine. If these species can adapt to their environment, why haven’t more? Hot springs are rich in nutrients, after all, and there is no shortage of solar energy in the Namib.

There are ways to save arguments about environmental quality from circularity. One is to replace harshness with commonness. Some environments, such as the climates and conditions that support forests, are widespread. Others, such as hot springs, are usually small and isolated. We would expect lots of species to adapt to life in a common environment, but few of them would then be equipped to colonize a very different one. Yet some environments with relatively few species, such as deserts and tundra, are neither small nor isolated. Their environments, however, are hostile to life by other measures than the number of species found there. Here, it might be the physical and chemical limits on living processes that limit diversity. Life’s chemistry needs water, so living in places where water is absent or frozen will be difficult and expensive, requiring resources to be diverted from growth and reproduction. But the question of why, given that a few species can adapt to life on the tundra, more cannot still applies.

Evelyn Hutchinson addressed this problem in his Homage to Santa Rosalia. Maybe, he suggested, as well as placing physical limits on what can live where, climate controls diversity by controlling life’s fuel supply. Hot and sunny—high-energy—environments contain more species than cold, gray places. Tropical forests also get more rain, another essential ingredient for plant life. In fact, a place’s temperature and rainfall more accurately reflect its species diversity than does its latitude—the diversity gradient is climatic. The match between energy and diversity stares us in the face. Perhaps this is why more species survive in the tropics than temperate or cold climates.

In other words, the reason there is only one species of polar bear is not that only one bear species has evolved to hunt on ice floes and swim in the Arctic Ocean, but that there isn’t enough food in the Arctic to support two bear species. Every species must maintain a certain



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