Native Great Lakes plants and animals have adaptations for toughing out long, harsh winters, but what about the region’s newer arrivals? Just like people who move to Chicago from different climates, some non-native species can thrive in the cold, while others struggle—not always successfully—to adjust.

Zebra and Quagga Mussels

Since they arrived about 25 years ago, these invasive invertebrates have had a huge impact on the Great Lakes. But how do the lakes’ seasonal changes affect them? If you’re a mussel, size matters. Scientists have found that larger mussels from both species are more likely to survive through winter, even though they react to the weather change in different ways. Quagga mussels keep on eating year-round, but zebra mussels eat less—or not at all—when winter arrives. Being mobile also has its advantages: Young larval mussels aren’t attached to surfaces yet, so they can migrate to deeper water to escape colder surface temperatures. In contrast, adult zebra mussels are permanently fixed to their locations. If they’re attached to docks or other surfaces near the water’s edge, a cold snap can freeze them to death.


The tale of the alewife is a reminder that not all invasive species thrive in their new habitats. Alewives came to the freshwater Great Lakes from the Atlantic Ocean, and their saltwater-adapted bodies cannot adjust fully to the difference in water chemistry. As a result, Great Lakes alewives experience constant physical stress, which makes them sensitive to environmental changes, including abrupt temperature swings. The food supply of alewives also dwindles in winter, so by the time spring arrives, the fish are in poor condition. Unfortunately for the weakened alewives, spring is when they travel from cold, deep winter habitat to spawn in warmer waters. The temperature change can be the last blow, triggering an event called a die-off, when many alewives perish at once. No Chicagoan of a certain age will forget the sight—and stench—on the lakefront after the great alewife die-off of 1967.

For us, it’s easier to handle winter weather: Put on a coat, turn up the heat, or even briefly escape to a warmer climate. For invasive species, however, winter can present a range of challenges that they aren’t prepared to handle.

—Meg Matthews, sustainability coordinator