The Great Lakes system contains about 20% of the world’s surface freshwater, but is “ecologically naive”: for thousands of years, its ecosystem has been isolated from the outside world as foreign fish can’t make it through the rapids and up Niagara Falls. This all changed in the 19th century as we opened several canals: the Erie Canal, the St. Lawrence Seaway to bypass the Lachine rapids in Montreal, and the Welland Canal to bypass Niagara Falls. The economic value of these canals were fairly small, but the boats travelling through them brought invasive species from across the world into the Great Lakes, usually stowed away in the ships’ ballast tanks.
Lake trout used to be abundant in the Great Lakes, until the introduction of sea lampreys in the 1920s. Sea lampreys preyed on the trout, which had no defense against it, causing the trout population to collapse. With the apex predator gone, another invasive species called alewives (a type of herring) grew to huge quantities in the 1960s. They were good at reproducing but were not good at surviving in the freshwater environment, and died in mass and covered the beaches of Lake Michigan.
Soon, sport fishers introduced salmon from the Pacific to Lake Michigan to feed on the alewives. This brought a few years of good fishing, until the alewife population collapsed due to overfeeding. At the same time, new invasive species of mussels were introduced; as they have no natural predators, they grew to huge numbers, almost covering the lakebed from shore to shore, driving out all other life in the lake. Mussels began to spread by recreational boaters across North America. Despite very strict regulations and fines and mandatory sterilization, it’s hopeless to control, since even if you stop 99.99% of them, the remaining 0.01% is enough to infect an ecosystem.
Most of the book is about invasive species, but it talks about a few other ecological issues as well. Runoff fertilizer caused algae blooms in Lake Erie, contaminating the water supply in Toledo in 2014. Threats of water diversion makes it so that water from the Great Lakes is only allowed to be used within its drainage basin; an exception was made for Waukesha, which lies just outside the drainage basin but is short on water. Overall, the Great Lakes ecosystems are a lot more fragile than we realize, and it’s easy to accidentally mess it up if we’re not careful.