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By Root 2003 0

impassable, six-foot thickets on the Peruvian coast; thousands of feet higher, mint overwhelmed Andean valleys. In the pampas of Argentina and Uruguay, the voyaging Charles Darwin discovered hundreds of square miles strangled by feral artichoke. “Over the undulating plains, where these great beds occur, nothing else can now live,” he observed. Wild peach was rampant in South America, too. Peachwood, Darwin discovered, had become “the main supply of firewood to the city of Buenos Ayres.” Some invasions cancel each other out. Peru’s plague of endive may have been checked by a simultaneous plague of rats, which the sixteenth-century writer Garcilaso de la Vega reported “bred in infinite numbers, overran the land, and destroyed the crops.”

A phenomenon much like ecological release can occur when a species suddenly loses its burden of predators. The advent of mechanized fishing in the 1920s drastically reduced the number of cod from the Gulf of Maine to the Grand Banks. With the cod gone, the sea urchins on which they fed had no enemies left. Soon a spiny carpet covered the bottom of the gulf. Sea urchins feed on kelp. As their populations boomed, they destroyed the area’s kelp beds, creating what icthyologists call a “sea urchin barrens.”

In this region, cod was the species that governed the overall composition of the ecosystem. The fish was, in ecological jargon, a “keystone” species: one “that affects the survival and abundance of many other species,” in the definition of Harvard biologist Edward O. Wilson. Keystone species have disproportionate impact on their ecosystems. Removing them, Wilson explained, “results in a relatively significant shift in the composition of the [ecological] community.”

Until Columbus, Indians were a keystone species in most of the hemisphere. Annually burning undergrowth, clearing and replanting forests, building canals and raising fields, hunting bison and netting salmon, growing maize, manioc, and the Eastern Agricultural Complex, Native Americans had been managing their environment for thousands of years. As Cahokia shows, they made mistakes. But by and large they modified their landscapes in stable, supple, resilient ways. Some milpa areas have been farmed for thousands of years—time in which farmers in Mesopotamia and North Africa and parts of India ruined their land. Even the wholesale transformation seen in places like Peru, where irrigated terraces cover huge areas, were exceptionally well done. But all of these efforts required close, continual oversight. In the sixteenth century, epidemics removed the boss.

American landscapes after 1492 were emptied—“widowed,” in the historian Francis Jennings’s term. Suddenly deregulated, ecosystems shook and sloshed like a cup of tea in an earthquake. Not only did invading endive and rats beset them, but native species, too, burst and blasted, freed from constraints by the disappearance of Native Americans. The forest that the first New England colonists thought was primeval and enduring was actually in the midst of violent change and demographic collapse. So catastrophic and irrevocable were the changes that it is tempting to think that almost nothing survived from the past. This is wrong: landscape and people remain, though greatly altered. And they have lessons to heed, both about the earth on which we all live, and about the mental frames we bring to it.

ONE OUT OF EVERY FOUR BIRDS

When passenger pigeons drank, they stuck their heads beneath the surface of the water until they were eye deep. When they walked, their heads bobbed awkwardly and they looked around from side to side. Passenger pigeons were greedy eaters with terrible manners; if they found some food they liked just after finishing a meal, they would vomit what they had previously eaten and dig in. Gobbling their chow, they sometimes twittered in tones musical enough that people mistook them for little girls. They gorged on so many beechnuts and acorns that they sometimes fell off their perches and burst apart when they hit the ground. But in flight they were angelic: they