Essay: Chasing Invaders on a Water Planet

Water bodies on our planet form a web: scattered lakes link to branching rivers which link to oceans. Along this network, aquatic species migrate over evolutionary time. They meander to new habitats as needed or by accident. But in recent decades, the species Homo sapiens has dramatically changed and accelerated this process.

Ship traffic among ports worldwide is hastening biological invasion.AMNH

Ship traffic among ports worldwide is hastening biological invasion.


Humans have engineered a global network of transportation and trade, which allows water creatures to become high-speed, long-distance travelers. In the ballast water tank of a container ship, species can now cross an ocean in a few days and penetrate far inland through canals. In a bait bucket on a recreational boat, they can skip from lake to lake within hours.

Such human-aided species movements are chronic in the world’s largest freshwater system, the Great Lakes basin. The region is a hub for biological invasion: unfamiliar species arrive from waterways the world over and then disperse into the North American freshwater network. Out of the thousands of fishes, invertebrates, and plants that inhabit the Great Lakes region, at least 185 are considered nonindigenous—new migrants that did not evolve there. Many of the 185 are also considered invasive, meaning they spread easily and cause great harm.

“For the vast majority of these nonindigenous species, we have virtually no idea what they’re doing, no idea how they’re changing the ecosystems that they’ve invaded,” says McGill University researcher Anthony Ricciardi. “But we do know from case studies that these invaders are capable of changing the fundamental rules of existence for everything else that’s living in the system.” Invasive species can force out native species, consume valuable natural resources, and spread disease. They also affect humans by stressing important commercial species or becoming costly pests. 

Biological invasion is now one of the main drivers of global environmental change. “The changes that invasive species are causing proceed much more rapidly than climate change,” says David Lodge of the University of Notre Dame, “so there’s reason for urgency. The problem itself is increasing, and once a species becomes established, it’s largely irreversible.”

Ricciardi and Lodge are two of the pioneers of invasion science. Among the many questions raised about invasive species, one floats to the top: can researchers understand invaders faster than the invaders can spread? Since eliminating these species is generally not an option, prediction and prevention of future threats may be the only means to limit the harm done by today’s aquatic travelers.

A container ship cruises out of the Newark, New Jersey harbor.AMNH

A container ship cruises out of the Newark, New Jersey harbor.


Shipping Species

About two-thirds of the 185 nonindigenous species in the Great Lakes have arrived since 1959, when the St. Lawrence Seaway opened. The seaway’s canals allow large vessels to travel from the Atlantic Ocean to the Great Lakes.

Aquatic species hitch a ride to the region in ships’ ballast tanks, which contain a soup of seawater, adult organisms, larvae, and eggs. Ballast water stabilizes the ship in the absence of cargo. It is pumped in at one port and drained when the ship reaches its destination. “At any given moment,” says Ricciardi, “it’s estimated that 5,000 to 10,000 species are in motion around the world in ship ballast tanks.”

Not all aquatic species can live through the tumultuous trip, during which they may encounter dramatic changes in temperature and salinity. But so many ships ply the globe that, inevitably, the hardiest species will be repeatedly delivered to a new port. These survivors are usually tolerant of upheaval and able to adapt to new habitats.

The arrival and spread of aquatic invasive species often happens under the radar. Because these invaders are concealed underwater, biologists generally learn of them only when they’ve grown too numerous to be missed.

Invasive zebra mussels crowd Quebec's Soulanges Canal.AMNH

Invasive zebra mussels crowd Quebec's Soulanges Canal.


Zebra mussels, the most prominent invader of the Great Lakes, were delivered in ballast water from their birthplace in the Black and Caspian seas about 20 years ago. In 1991, Ricciardi discovered that zebra mussels had spread eastward into the St. Lawrence River. “It took me an hour to find about six specimens,” he says. “Now, you can lift up one rock and get a couple of thousand.”

The zebra mussel (Dreissena polymorpha) and its invasive cousin Dreissena bugensis, the quagga mussel, cling to hard substrates such as piers and water pipes, damaging them. They adhere to the shells of native mussels, overwhelming their ability to filter feed, which causes the native mussels to perish. They also stick to boat hulls and the vegetation caught on boat trailers. Small watercraft have now spread both species from the Great Lakes not only into the St. Lawrence River but also into an entire network of rivers that reach into the American interior, including the Illinois, the Mississippi, the Hudson, and others.

Recreational fishers have also propelled the invasive species Orconectes rusticus, or rusty crayfish, beyond its native waters of Ohio and Indiana. The crayfish are a favored fishing bait. They look nearly identical to several other American crayfish species but are measurably more aggressive and prolific. They now inhabit freshwater lakes in northern Wisconsin and southern Canada and are marching ever northward.

Rusty crayfish are native to the Great Lakes region, but are invading beyond their natural range.AMNH

Rusty crayfish are native to the Great Lakes region, but are invading beyond their natural range.


“Behind that invasion front,” says Brett Peters, one of David Lodge’s graduate students, “we see declines in plants, declines in insect larvae and snails that fish eat, and declines in fish that use these things as food and habitat.” These conclusions are the result of 20 years of research on just this one invasive species.

Prediction and Prevention

Scientists say, however, that decades of research are not necessary to take action on invasive species. Researchers are now predicting the arrival and spread of new species to get a step ahead. They are also helping municipalities orchestrate preventative measures.

Recently, Lodge and his colleagues forecast the spread of invasive mussels into the American West. The team analyzed recreational boating patterns and targeted habitats that the mussels would find suitable. They determined that the Colorado River and its dammed reservoir, Lake Mead, were at high risk of invasion. Between the time Lodge’s research paper was accepted and the time it was published, quaggas were indeed spotted there for the first time. A similar forecasting study by Lodge’s team proved true for invasive Chinese mitten crabs, Eriocheir sinensis, in Chesapeake Bay.

Ricciardi is using similar techniques to earmark high-impact invaders before they arrive in the Great Lakes. He focuses on “repeat-offender” source regions such as the zebra and quagga mussels’ native home, the Black and Caspian seas. “We’re looking for species that have a planktonic larval stage [and thus drift freely], that have a broad salinity tolerance, that have a broad temperature tolerance,” he says. He also focuses on species that have already shown an ability to invade elsewhere. One such species is Hemimysis anomala, a freshwater shrimp nicknamed the bloody-red mysid, which arrived in the Great Lakes in 2007. Ricciardi’s team had predicted its arrival back in 1998.

To eliminate designated species at arrival, Lodge’s colleagues at Notre Dame are developing rapid genetic tests to identify high-impact invaders in the ballast water of arriving ships. And engineers are exploring new technology to sterilize ballast water and design ballast tanks less likely to harbor invasives.

Recreational boaters help aquatic species invade.AMNH

Recreational boaters help aquatic species invade.


To prevent smaller craft from spreading species around North America, educational campaigns are now targeting high-risk invasion spots identified by forecasting studies. These campaigns encourage recreational fishers to wash their vessels and belongings (and even their dogs) and to pour out onboard water before boating back home. Lodge is experimenting to find the most effective boat-washing techniques for such campaigns.

On a national scale, invasive species legislation has been pending in the last few sessions of the U.S. Congress. The proposed laws require more effective treatment of ballast water before it is released. And since the pet trade is another pathway for invasives, the laws would require that any new species of exotic pet be screened for potential invasiveness before trade is allowed. Currently, nearly any animal may be imported into the United States without any consideration of the risks to ecosystems or humans.

While these proposed measures won’t return the Great Lakes to a preinvasion idyll, researchers are hopeful that a combination of science and management will prevent the situation from getting much worse. “The Great Lakes, in particular, are so radically changed by human activity that restoration to some pre-European baseline is simply impossible,” says Lodge. “So we must manage the lakes to get the best balance we can of native organisms, ecosystem function, and what humans want.”