Rapid Expansion of Banded Killifish Fundulus diaphanus Across Northern Illinois: Dramatic Recovery or Invasive Species?

The distribution of the Illinois state-threatened Banded Killifish Fundulus diaphanus remained largely unchanged in Illinois from 1880 to 2000, being restricted mainly to the northeastern corner of the state. One population has remained stable in the glacial lakes region along the southeastern Wisconsin – northeastern Illinois border. Individuals from this population are identified as the Western Banded Killifish F. d. menona. Starting in 2001, a second population began to spread and become more common along the Lake Michigan shoreline. From there, they expanded through the Chicago Area Waterway System, into the lower Des Plaines River, and eventually into the Illinois River. Historical museum specimens from this area are identified as the Western subspecies, but recent specimens are identified as hybrids between the Western subspecies and the non-native Eastern subspecies F. d. diaphanus. A third population appeared in the Mississippi River near the mouth of the Rock River in 2009, and has spread from there, including downstream to the St. Louis area. These individuals are identified as the Western subspecies. The rapid expansion of Banded Killifish from Lake Michigan into the Illinois River appears to be an invasion of the Eastern subspecies and the subsequent hybridization with the native Western subspecies. It is unknown where the Banded Killifish in the Mississippi River came from, but they might have originated from populations 160+ kilometers upstream or through human introductions. As the Illinois River and Mississippi River populations continue to expand their ranges, their ecological impacts are unknown at this time. Future work includes a genetic analysis to help determine how the non-native Eastern subspecies invaded the Midwest from the Atlantic Slope.