Straddling the divide: invasive aquatic species in Illinois and movement between the Great Lakes and Mississippi basins

The Laurentian Great Lakes and the Mississippi River are two of the largest freshwater ecosystems in North America, and each contains large numbers of non-native species. In 1900 the Great Lakes and Mississippi were permanently connected with the opening of the Chicago Sanitary and Ship Canal in the US state of Illinois. More recently, movement of trailered boats, bait, and other overland vectors have increased and further enabled movement of non-native aquatic species. To investigate the role of Illinois in continent-wide species spread we assembled a comprehensive database of recorded occurrences of aquatic non-native species in Illinois inland waters. For each species we determined vector, location, current stage in the invasion sequence (introduced or established), and ecological impacts. The arrival of non-native species has accelerated over time. Sixty species are now established, and a further 39 have been recorded. The Great Lakes have been a stronger source of species to Illinois and the Mississippi River basin than the reverse. Over the last two decades the most important vectors delivering non-native aquatic species have been unintentional release and shipping. Through a survey of Illinois aquatic ecologists we found that established species exhibit a continuous range of ecological impacts and that almost a third have high or very high ecological impacts. Local, national, and continental approaches will be required to mitigate the threat of further invasive species spreading into Illinois and between the Great Lakes and Mississippi basins.