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In 2010 two economists at Harvard and Yale attempted to account for such factors by comparing events in parts of Europe that were similar except for their suitability for potatoes; any systematic differences, they argued, would be due to the new crop. According to the two researchers’ “most conservative” estimate, S. tuberosum was responsible for about an eighth of Europe’s population increase. Put baldly, the figure may not seem high. But the continent’s long boom had many causes. One way to think of this calculation is to say that it suggests the introduction of the potato was as important to the modern era as, say, the invention of the steam engine.

THE GUANO AGE

It was said that the islands gave off a stench so intense that they were difficult to approach. They were a clutch of dry, granitic mounds thirteen miles off the Peruvian shore, about five hundred miles south of Lima on the west coast of South America. Almost nothing grew on them. Called the Chincha Islands, they were never inhabited by Indians—not for long, anyway. Their sole distinction is their population of seabirds, especially the Peruvian booby, the Peruvian cormorant, and the Peruvian pelican. The birds are attracted by the strong coastal current, which pulls cold water from the depths. Phytoplankton feast on the nutrients that rise with the water. Zooplankton eat the phytoplankton and in turn are the primary food of the anchoveta fish, a cousin to the familiar anchovy. Anchoveta live in vast schools that are preyed upon by other fish. Predators and prey both are preyed upon by the Peruvian booby, cormorant, and pelican. All three have nested on the Chincha Islands for millennia. Over time they have covered the islands with a layer of guano as much as 150 feet thick.

Guano makes excellent fertilizer. Fertilizer is, at base, a mechanism for providing nitrogen to plants. Plants need nitrogen to make chlorophyll, the green substance in their leaves that absorbs the sun’s energy for photosynthesis. Nitrogen is also a key building block for both DNA and the proteins assembled by DNA. Although more than three-quarters of the atmosphere is made up of nitrogen gas, from a plant’s point of view nitrogen is scarce—the gas is made from two nitrogen atoms that cling to each other so tightly that plants cannot split them apart for use. In consequence, plants seek nitrogen from the soil, where it can be found in forms that they can break down: ammonia (NH3, or one nitrogen atom and three hydrogen atoms), nitrites (compounds that include NO2, a group of one nitrogen atom and two oxygen atoms), and nitrates (compounds that include NO3, a group of one nitrogen atom and three oxygen atoms). All are in less supply than farmers would like, not least because bacteria in the soil constantly digest nitrates and nitrites, turning the nitrogen back into unusable nitrogen gas. Land that has been farmed repeatedly always risks nitrogen depletion.

Unlike mammalian urine, bird urine is a semisolid substance. Because of this difference, birds can build up reefs of urine in a way that mammals cannot (except, occasionally, for big colonies of bats in caves). Even among birds, though, Chincha-style guano deposits—heaps as big as a twelve-story building—are uncommon. To make them, the birds must be relatively large, form big flocks, and defecate where they live (gulls, for instance, release their droppings away from their breeding grounds). In addition, the area must be dry enough not to wash away the guano. The waters off the Peruvian coast receive less than an inch of rain a year. The Chinchas, the most important of Peru’s 147 guano islands, house hundreds of thousands of Peruvian cormorants, the most prolific guano producers. According to The Biogeochemistry of Vertebrate Excretion, a classic treatise by G. Evelyn Hutchinson, a cormorant’s annual output is about thirty-five pounds. Arithmetic suggests that the Chincha cormorants alone produce thousands of tons