Build the (fish) wall!

A jumping silver carp (USGS)

Politics in both the United States and Europe has been roiled by the issue of immigration. Whether fleeing war, oppression, gangs, famine, or grinding poverty, large numbers of people are attempting to enter countries where they believe they can be safe or make a better life. This is not new; my own ancestors came to the United States before the first World War to escape Czarist Russia (and given what happened in Europe over the last century, I am glad they did). Then as, as now, immigration created turmoil and controversy within the resident populations, including dire warnings of the end of the cultural distinctness of societies. What I want to discuss here is another form of immigration, with a more certain loss of distinctness; that is, the introduction, whether planned or intentional, of invasive species.

Over geological time, species of plants and animals have always moved to new habitats. In some cases, they were able to become established and even (rarely) supplant natives. A large literature in both ecology and paleontology has examined the importance of species migration and assessed the factors that allow successful introductions. In one the most famous cases, known as the Great American Interchange, species of mammal in North and South America, long separated by oceans, intermingled when the Isthmus of Panama rose some 3 million years ago. Such familiar animals as porcupines, opossums, and armadillos are migrants from South America. Deer, wolves, and cougars are among the many species in South America with northern origins. In general, the northern invaders were far more successful, with many South American animals going extinct.

But as so much else in the world, the rise and spread of humans has profoundly changed the rules of the game. Species that could not cross oceans or mountains can now do so almost as readily as we can, as hitchhikers on our own movements. Being removed from their natural habitat and thus their competitors and enemies, they can spread virtually unchecked. As cogently described in Charles C. Mann’s 1493, the post-Columbus world has become more-and-more homogenized, with the loss of the biological distinctness that once characterized continents and regions. I need only to look at the window to see this; my birdfeeder is mobbed by English sparrows, aka house sparrows. These aggressive little birds were brought over from England and introduced in Brooklyn in 1850, in an effort to control an outbreak of the native Elm spanworm moth. Like many other early attempts at biological control, it went disastrously wrong. House sparrows are now one of the most abundant species of bird in the country, with major impacts on native species such as bluebirds (what is the last time you saw one of those?). Other familiar “not from here” species are starlings, pigeons (rock doves), and of course, brown and black rats.

The Great Lakes have particularly suffered from invasions. Since the opening of the St. Lawrence seaway, the lakes have been repeatedly invaded, by some 180 invasive and non-native species ( Many of the invasive species have hitchhiked on ocean going ships, often in their bilge water. Alewives, native to the East Coast, were able to bypass Niagara Falls when the Welland Canal was improved ca. 1900. The lack of native predators led to huge population booms and busts, with large die offs fouling the waters in the lakes. The absence of predators resulted from the introduction of another invasive species, the sea lamprey, which devastated the populations of large fish by the 1940’s. More recently, the zebra mussel, a native of Eastern Europe, arrived via ship ballast. Filter feeders, they have nearly removed all phytoplankton and other food particles from lake water. The lake water, now clear, may look cleaner, but it can no longer support the many other animals that relied on it.

And then there’s the looming threat of the Asian carp. These four large fish species, native to Asia, were deliberately introduced in the 1970’s to control weeds in aquaculture, such as catfish farms. Inevitably, they escaped and having been making their way north within the Mississippi and its tributaries. Voracious eaters, they have quickly become extremely abundant and have displaced numerous native species. The largest density in the world of one of the species, silver carp, is in the Illinois River.

And now they sit on the doorstep of the Great Lakes. For thousands of years, the Mississippi and Great Lakes drainage basins were separated. European settlement changes that; a canal connecting Lake Michigan to waters of the Mississippi basin was built in the 1840’s; and in 1900 the larger Chicago Sanitary and Ship Canal reversed the flow of Chicago River to prevent the city’s sewage from flowing into the lake. The unintended consequence of this is that aquatic species, such as the Asian carp, can readily move between the two watersheds. Molly Flanagan, of the Alliance for the Great Lakes, calls it an “invasive species superhighway” (

The fear is that if Asian carp do make into the lakes, it will be one more blow to the already stressed ecosystems and a severe impact to the $7 billion-a-year sport-fishing industry. In 2002, an electrical barrier was constructed across the Chicago Sanitary and Ship Canal. The Army Corps of Engineers released a draft plan in 2017 for a new $275 million barrier along the Des Plaines River below the canal; one year later, the estimate has ballooned to nearly $780 million How it will be funded, especially in light of the greatly increased costs, is in early stages of discussion. Maybe we can get Mexico to pay for it?

When the topic of global change is mentioned, most people’s thoughts immediately turn to climate change. But changes in life on Earth caused by the global movement of species is just as severe an issue. And just like the speed of climate change, the rate and magnitude of species movement is unprecedented in Earth history. In the long term, its impact will far outweigh the arrival of a few thousand desperate people at our border.



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Roy Plotnick

Roy Plotnick

Paleontologist, geologist, ecologist, educator. Professor at the University of Illinois at Chicago. Author of Explorers of Deep Time.