The Response of Fish Diversity in a Highly Modified Large River System to Multiple Anthropogenic Stressors

Large riverine landscapes are among the most productive ecosystems on earth, and the most affected by humans. Few studies explicitly explore the effects of, or responses to, multiple stressors in highly modified large river systems. Changes in fish assemblages in response to multiple anthropogenic stressors were explored from a 60-year data set for the Illinois River (Illinois, USA). Despite a legacy of stressors, the highly modified Illinois River responded to additional multiple stressors, especially four distinct stressor phases, which included two policies to improve water quality and bigheaded carp invasion. The response in fish diversity (Shannon H) was complex in terms of the number of thresholds in the pattern of fish diversity, the rate and direction of change between thresholds, and the variance of response at two spatial scales and two ecological levels of organization. Overall, nine response trajectories were observed. Changes in fish diversity in the Illinois River since 1959 do not conform to current ecosystem response models. Without long-term and broad-scale data, elucidating complex responses in large river ecosystems is unlikely. Expanding the spatial and temporal scale of investigation in modified large rivers will increase our ability to understand how these ecosystems respond to multiple stressors.