Carved by glaciers and sealed off for millennia, the Great Lakes stayed ecologically naive—until canals and shipping lanes threw open the door. Drawing on thirteen years of reporting, Dan Egan traces how that opening let invasive species and pollution reshape the world's largest freshwater system.
Invading filter-feeders can strip nearly all the plankton out of a lake, rewiring how energy moves up the food chain. The water turns strikingly clear, but that clarity signals a lake drained of the food its other creatures depend on, not a healthy one.
An ecosystem sealed off from the outside world for thousands of years never develops defenses against newcomers, so the same isolation that makes it unique makes it fragile. Once a connection opens, it takes very little for arriving species to unravel it, much as a population never exposed to a disease is devastated when it finally arrives.
Canals and seaways built to move cargo also move living things, carrying a growing stream of invaders into waters that were once closed. A parasite that slipped through an early shipping channel was enough to collapse an entire fishery, and later shipping routes let in ever more harmful arrivals.
Linked lakes behave like one huge, slow-moving river, so a species that gets into one part eventually reaches all of them. An invader in such a system spreads like cancer cells in a bloodstream rather than staying local.
Lost native species won't come back and established invaders won't be removed, so the realistic goal is to stop new arrivals. Rules requiring ships to flush their ballast water have nearly halted new invasions, just as holding industries and cities accountable for their discharges once changed what went into the water.