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“It might be you can create a certain soundscape that has an invigorating, growth-promoting effect,” he said — music to a forest’s ears.

Sailors balanced out the weight by leaving behind their ships' ballast: stones, gravel and soil. They swapped English dirt for Virginia tobacco. That dirt very likely contained the common nightcrawler and the red marsh worm. So, almost certainly, did the rootballs of plants that the colonists imported.

In worm-free woodlands, leaves pile up in drifts on the forest floor. Trees and shrubs in wormless places depend on litter for food. When earthworms arrive, they quickly consume the leaf litter, packing the nutrients deep in the soil in the form of castings (worm excrement). Suddenly, the plants can no longer feed themselves; their fine, surface-level root systems are in the wrong place. Wild sarsaparilla, wild oats, Solomon's seal and a host of understory plants die off; grass-like species such as Pennsylvania sedge take over. Sugar maples almost stop growing, and ash seedlings start to thrive.

Transported in the bodies of sailors, malaria may have crossed the ocean as early as Columbus's second voyage. Yellow fever, malaria's frequent companion, soon followed. By the 17th century, the zone where these diseases held sway—coastal areas roughly from Washington, D.C., to the Brazil-Ecuador border—was dangerous territory for European migrants, many of whom died within months of arrival

Initially, American planters preferred to pay to import European laborers—they spoke the same language and knew European farming methods. They also cost less than slaves bought from Africa, but they were far less hardy and thus a riskier investment. In purely economic terms, the historian Philip Curtin has calculated, the diseases of the Columbian Exchange made the enslaved worker "preferable at anything up to three times the price of the European."

At the time, England and Scotland shared a monarch but remained separate nations. England, the bigger partner, had been pushing a complete merger for decades. Scots had resisted, fearing a London-dominated economy, but now England promised to reimburse investors in the failed Panama project as part of a union agreement. As Mr. McNeill wrote, "Thus Great Britain was born, with assistance from the fevers of Panama."

Eighteenth-century farmers who planted potatoes reaped about four times as much dry food matter as they did from wheat or barley. Hunger was then a familiar presence in Europe. France had 40 nationwide food calamities between 1500 and 1800, more than one every decade, according to the French historian Fernand Braudel. England had still more. The continent simply could not sustain itself. The potato allowed most of Europe—a 2,000-mile band between Ireland and the Ukraine—to feed itself. (Corn, another American crop, played a similar role in Italy and Romania.) Political stability, higher incomes and a population boom were the result. Imported from Peru, the potato became the fuel for the rise of Europe.

The sweet potato played a similar role in China. Introduced (along with corn) from South America via the Pacific silver trade in the 1590s, it suddenly provided a way for Chinese farmers to cultivate upland areas that had been unusable for rice paddies. The nutritious new crop encouraged the fertility boom of the Qing dynasty, but the experiment soon went badly wrong. Because Chinese farmers had never cultivated their dry uplands, they made beginners' mistakes. An increase in erosion led to extraordinary levels of flooding, which in turn fed popular unrest and destabilized the government. The new crops that had helped to strengthen Europe were a key factor in weakening China.

European ships accidentally imported the fungus-like organism, native to Peru, that causes the potato disease known as late blight. First appearing in Flanders in June 1845, it was carried by winds to potato farms around Paris in August. Weeks later it wiped out fields in the Netherlands, Germany, Denmark and England. Blight appeared in Ireland on Sept. 13.

the Columbian Exchange, like a biological Internet, has put every part of the natural world in contact with every other, refashioning it, for better or worse, at a staggering rate.

Mars won't just be a harsh environment for humans, but the fungi as well. The fungi will need cyanobacteria to survive. Cyanobacteria uses solar energy to convert dioxide and water in oxygen and food.