Another reason why so many Indians succumbed to disease was that they suffered from malnutrition. This was because European plants and animals had disrupted the Indian ecosystem. Pigs and cattle, brought in the first Spanish ships, were commonly set loose in the Americas. Unchallenged by the predators and microbes that had thinned their populations in Europe and Asia, pigs reproduced rapidly and ate their way through fields of maize, beans, and squash. Rats, stowaways on most European ships, also proliferated in the Americas, infesting Indian crops. Europeans also brought plants to the New World, and in the process unknowingly introduced the seeds of hardy European weeds. Like the kudzu vines from Japan that have overrun much of the southeastern United States during the twentieth century, dandelions and other weeds from Europe choked Indian crops in the sixteenth century. Fewer ears of corn were harvested and Indians went hungry. When disease struck, many died.
The ships that brought Europeans to the Americas returned carrying more than gold and silver. European soldiers often contracted syphilis, a disease native to the Americas, and spread it to Europe and Asia. European ships also brought back maize and potato plants. These American crops yielded 50 percent more calories per acre than wheat, barley, and oats, the major European grains. Hungry European peasants swiftly shifted to maize and potato cultivation; the population of Europe rose sharply. Manioc (cassava), another Indian plant with a high caloric yield, did not grow in the colder climate of Europe, but it transformed tropical Africa. Population levels soared. As declining Indian populations proved insufficient to exploit the seemingly inexhaustible lands of the
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