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They began by listing the various ecosystems they could safely assume existed on the island, such as old-growth forests, wetlands, or plains, based on soil types, rainfall, and so on. Because it was located at the intersection of geographic regions, Manhattan probably had not only spruce trees from the northern forests but also magnolias from the southern forests, migratory birds from nearby flyways, and even tropical fish from the Gulf Stream during summer. In all, they identified 55 different ecological communities. "It was an incredibly diverse place," Sanderson said. "If the island had stayed the way it was back then, it could have become a national park like Yosemite or Yellowstone."

Once they identified the island's ecosystems, they could fill in the wildlife. But which animals lived where? To be as precise as possible, Sanderson's group took their research a step further. For each species they identified essential habitat requirements. A bog turtle, for example, needed a wet meadow, insects, and a sunny place to warm itself, while a bobcat needed rabbits and a den site in which to raise its young. "We just kept asking ourselves, What does this need? What does this need? What does this need?" Sanderson said. Then they compiled a list for each species. As they built their database, they discovered a dense network of relationships among species, habitats, and ecosystems on the island, not unlike the complex social networks that people create. Sanderson called this network a Muir web, after American naturalist John Muir, who once noted that "when we try to pick out anything by itself we find that it is bound fast by a thousand invisible cords that cannot be broken, to everything in the universe." Sanderson and his team, in a sense, were trying to make those thousands of cords visible.

Consider a beaver that lived at Times Square in 1609. If you grabbed him by the scruff of his neck and lifted him out of the web, you'd find lines connecting him to a slowly meandering stream, to the aspen trees he ate, and to the mud and twigs he used to build a lodge. Not only that, you'd also find lines to the bobcats, bears, and wolves that depended on him as prey and to the frogs, fish, and aquatic plants that lived in the pond he helped to create. "The beaver, it turns out, is a landscape architect, just like people," Sanderson said. "You need him to flood the forest, which kills the trees that attract the woodpeckers that knock out cavities that wood ducks use for shelter." Lifting a beaver out of the web disrupts scores of other residents, which demonstrates how important it can be to think about an ecosystem as a network.

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